Monday, May 25, 2015

Anatomy of a Cinder Cone Roadcut and Its Tale of Farallon Plate Geo-Gymnastics - Part I: The San Bernardino Volcanic Field

"Nothing in geology makes sense except in terms of plate tectonic theory."
Lynn S. Fichter, Ph.D., Department of Geology, James Madison University (here)

In spite of their global ubiquity and profusion, cinder cones don't typically reveal their internal plumbing, even when degraded by erosion or mined for the production of aggregate. Their simplicity of construction likely explains why they haven't attracted the attention of geoscientists in the literature. For these reasons, geologist Wayne Ranney and I couldn't resist the allure of an anatomy-exposing roadcut through a cinder cone catalogued as V2009, while on a geological journey through southeast Arizona.  

Facing northeast on Arizona State Road 80, the highway slices through the northwest apron of cinder cone V2009.

Our rather ordinary and diminutive cinder cone is situated on the San Bernardino volcanic field in the northern third of a valley with the same name. Launch Google Earth, paste the following coordinates into search, and it will take you there: 31°32'07.35" N, 109°17'21.40" W. Fortuitously, the cone is transected by Arizona State Route 80, which runs from northeast to southwest across the San Bernardino Valley. Not to be confused with the one in southern California, the valley is situated in the extreme southeastern corner of Arizona's Cochise County, the adjoining corner of southwestern New Mexico's Hidalgo County, and across the international border into northeast Sonora, Mexico.

The San Bernardino volcanic field lies in the northern third of the San Bernardino Valley in the extreme southeast corner of Arizona. Clockwise, it is bordered by the mountain ranges of Chiricahua, Peloncillo, San Luis, Perilla and Pedregosa. Nearby are other similarly-situated basins and ranges such as the Animas, Sulfur Springs, San Simon and San Pedro. Modified from Shawn Blissett thesis, 2010 (left) and Drewes and Thorman, 1978.

The San Bernardino Valley is also within the Basin and Range physiographic province that extends across southern and western Arizona, northwest through the entire state of Nevada into eastern California, up to southern Oregon and Idaho, southeast into New Mexico and across the border into central Mexico. The geologic province differs from neighboring provinces in terms of topography, elevation, climate, population demographics, water availability, agriculture, industry and mineral resources, yet they share a commonality of evolution. Suffice it to say for now, the region is typified by crustal extension, which is responsible for the repetitive basins and ranges on the landscape, grabens and horsts in proper geological parlance.

Notice the locale of the San Bernardino volcanic field (arrows) within the Basin and Range province tucked into the extreme corner of Arizona. Modified from Wikipedia

The shrubby desert and grass covered plains of San Bernardino Valley are bordered by forested mountain ranges, while beyond in every direction, there are similar juxtapositions. The landscape is typical of the Basin and Range province. The ranges, called Sky Islands, have distinctive Spanish and Apache names - Chiricahua, Peloncillo, San Luis, Perilla and Pedregosa - that hint at the rich history of the region in this, the Land of Cochise and homeland of the Chiricahua Apache. 

Their story, like that of so many other Native Americans, is one of intrusion, oppression and subjugation. First came the Spanish, followed by the governments of Mexico and the United States. It's a story of bloody confrontation and retaliation between Apache Chief Cochise and warrior Geronimo, and the U.S. military stationed at Fort Bowie within Apache Pass between the Dos Cabezas and Chiricahua ranges. This extreme southeast corner of Arizona also tells a tale of the "Old West", about the gunfight at the O.K. Corral between Wyatt Earp, Doc Holliday and the outlaw Cowboys, and about Tombstone's Boothill Graveyard and a rush for Late Cretaceous silver, gold and copper. Geology and history. They're forever inseparable, wherever you go.

From the Geronimo Surrender Monument (below) just off highway 80 between the San Simon and San Bernardino Valleys, we're facing the Peloncillo Mountain range to the southeast. Nestled in the hills is Skeleton Canyon, the surrender site of Geronimo to the U.S. Army in 1886 and the pass to Animas Valley of New Mexico, another basin. Drive just a few kilometers south, and you enter the San Bernardino volcanic field, where we're headed. The terrain looks the same, but there are cinder cones and lava flows everywhere.

Why is the San Bernardino volcanic field peppered with cinder cones? If tectonics explains the location of magmatic systems along plate boundaries, what is the explanation for intraplate magmatism on the field? What did the cinder cone road-exposure reveal about its construction and emplacement mechanism?

On a grander scale, does a geologic relationship exist between the cinder cone, the volcanic field, and the sedimentary basin within which it emplaced, the physiographic province where it's situated, and even neighboring provinces? Is there a commonality of evolution that exists among these seemingly disparate entities? If so, how did it affect the geological development of the American Southwest? Lastly, is volcanism and deformation on the field active, dormant or extinct?

In this post, I address the details of V2009's construction and emplacement, and its relationship to the San Bernardino volcanic field. In Part II, which will follow, I'll discuss the tectonic big picture of the American Southwest and its relationship to V2009 and the volcanic field. 

Let's investigate.

Wayne Ranney rejoices in his position atop the west slice of cinder cone V2009. Notice the contour of the cone's slope across the road and three or four eroded cinder cones off to the northeast on the volcanic field.

It's a steep, straight-sided, low-profile, symmetrical volcano built of pyroclastic fragments. It's the most common type of volcano, the least destructive and the most simply constructed. It's found in a variety of tectonic regimes worldwide, both singly and in groups. Cinder cones range in diameter from .3 to 2.5 km, and vary from tens to 300 meters (1,000 feet) in height with a few exceeding 700 meters.

Their glassy, furnace-like "cinders", referred to as scoria (and hence the alternative name of scoria cone), contain voids from gas bubbles (1-2%) entrapped as molten magma forcefully explodes into the air. In contrast, pumice, along with ash, is the vesicular, light-colored, light-weight ejecta generated when magmas of intermediate (andesite) and felsic (rhyolite) composition erupt explosively. 

Pu'u 'O'o is an example of a large, active cinder cone with voluminous outpourings of magma that erupted from vents on the eastern flank of shield volcano Kilauea on the Island of Hawaii, the Big Island, in 1983. During its prolonged history, it has fed lava flows from crater-rim spillovers, vents along its flanks and from subterranean lava tubes that extend far downslope. Many flows have reached the sea to the south and recently advanced into the village of Pahoa to the southeast.

Cinder cone eruption occurs when pressure builds in magma reservoirs and ascends as a melt within a cylindrical conduit that feeds the developing cinder cone at a vent or eruptive center on the surface. Ascending magma may forcefully eject from cinder cones as a spectacular fiery fountain in a series of pulses or a continuous jet. The gas-charged magma violently blasts ejecta airborne, which quickly fragments, cools and falls back to earth solidified. 

The combination of short-lived, non-sustained spurts and prolonged eruptive events - mostly within a month but often up to a year and on occasion, several years - conspire to built a well-defined, and exquisitely symmetrical cone of tephra around and downwind of the central vent. Anointing the summit, a bowl-shaped crater represents the area above the vent from which material was explosively ejected.  

Cross-section of an idealized cinder cone. Modified from a Wikipedia image

As fallen pyroclasts avalanche downward around the vent, they form a conical apron of deposits on the flanks of the growing cone, some welded and others not. "Loose" cinders can't tolerate a slope greater than 30 to 40 degrees without slumping, called the angle of repose. The angle also varies with clast size and angularity. It's an example of how cinder cones, as with all volcanoes, are characterized by their compositional material, which also dictates their behavior. The following cross-section of an idealized cinder cone illustrates its external and internal architecture.

Schematic cross-section through a typical cinder cone showing the volcano-sedimentary processes and geomorphologic structures. Modified from Kereszturi and Nemeth

Classified by size, tephra (Greek for "ash") ranges from meter-sized, aerodynamic bombs, that form blocks when hardened, down to pea- and walnut-sized lapilli, and even fine-grained, millimeter-sized ash. The unconsolidated, pyroclastic fragments may weld together into an agglutinate and/or become compacted and cemented into a coherent volcaniclastic mass of agglomerate, which is mostly bombs (75%). 

The "cinders", as they are commonly called, are typically vesicular (pitted with cavities of "frozen" gas bubbles) and dark gray to black in color, due to a high iron content, which may oxidize to a deep reddish-brown. As the tephra rains down, the cinder cone becomes centrifugally layered into strata that reveal the history of their emplacement. Typically, fine-grained ejecta in buoyant plumes is transported by the wind, while coarser fractions are mostly ejected along ballistic paths.

Generally basaltic in composition, a potpourri of tephra conspire to build the cinder cone.

Molten lava may eject from a cinder cone's crater in a tall, fiery fountain or spillover from a breach in the crater, but typically exudes from a vent located at the base of the cone or an independent vent on the field. Following a path of least resistance, basaltic lava follows the topography of the landscape downslope in broad, thin sheets or stream-like ribbons in a manner that reflects its high fluidity (low viscosity), which in turn is related to eruption temperature (in excess of 950° C) and mafic chemistry (high ferromagnesium content). Individual flows associated with volcanic fields tend to be ~1-10 km long and several tens of meters thick. 

Against a backdrop of the San Luis range in Mexico, the eroded cinder cone (below) on the San Bernardino field possesses an amphitheater-like morphology largely attributable to agglomerate within the rim of the crater (Arizona Geological Survey map-verified here). Slumping, a type of post-eruptive cone degradation, can also contribute to the horseshoe shape, especially if an associated flow rafts pyroclastic material from the base of the cone. What appears to be an elongate, abruptly-terminating tongue of lava emanating from a breach in the crater or from its lower flank is in reality a large lava platform that surrounds the cone and its neighbors. The San Bernardino volcanic field was gradually built over a few million years from intersecting flows that have interbedded with Quaternary alluvium and colluvium. Notice the height of the flow front above the valley floor, an indication of the mass that has added to the field. The subtle concavity on the steep southeast (left) flank represents human excavation into the slope for aggregate. 

A few kilometers west of V2009 is this eroded cinder cone and its thick tongue of lava. 

Once the eruption of a cinder cone has ceased, surficial processes gradually begin to degrade the cone. Unconsolidated and highly permeable pyroclastic deposits are susceptible to erosion, which is highly contingent on rainfall, temperature and climate. Morphological variations of cinder cones are not caused so much by erosion but by eruption characteristics such as the nature of the pyroclasts that blanket the cone. Retardation is contingent on the degree of welding, agglutination, and cover of compacted ash and lava. A resistant rim of agglutinate around the crater may delay erosion and a lessening of the slope angle. 

On a larger scale, tectonics is a factor if post-orogenic chemical weathering decreases atmospheric carbon dioxide concentrations. Wind direction during construction affects cone symmetry by distributing ejected ash downwind, but once eruption has ceased wind deflation may degrade the edifice by stripping the windward side. During the lifespan of a volcanic field, which can last millions of years, erosive processes on cones may vary as the climate changes. Cinder cones on the San Bernardino field are in a relatively good state of preservation, which attests to the youthful age of the field. They emplaced during the wetter, erosion-inducing climate of the Pleistocene, but today are experiencing degradation at a seasonally-intermittent rate due to the Southwest's semi-aridity.

On the San Bernardino field, an eroded cinder cone languishes against a backdrop of the Peloncillo Mountains and San Luis range across the International Border. Animas Valley is interposed between the two ranges and has extensive lava flow with a chemical signature and age similar to the San Bernardino field, hence, is related tectonically. The grass-covered field is constructed of many inter-layered flows, which have been partially identified by drilling for water into the aquifer. Flank flows along the range are interbedded with eroded deposits from the mountains and interbed with valley flows on the field.

Young cinder cones are generally steep with little scoria oxidation. Older cones lack large-scale erosive features and exhibit sparse vegetation but begin to demonstrate clay formation as silicate-bearing (feldspar and pyroxene), basaltic sediments are reworked. More aged cones are vegetated and support rills and gullies that begin to reveal internal dikes and ridges. Although vegetation retards erosion by anchoring the soil, it hastens rill and gully development. Degradation diminishes the slope angle and cone height, but the ratio of crater diameter to basal diameter doesn't change appreciably. Large cones, which are associated with more volatile-rich magmas, more intensive eruptions and finer particles, appear to erode more quickly than small ones; whereas, smaller cones tend to be welded by hotter, erosion-resistant particulates. Thus, erosivity is also related to cone size and explosivity. The final stage of erosion may expose the cone's innermost plug, but only a roadcut can provide an architecture-preserving transect through the body of a cinder cone for direct observation.

In all of Arizona, I can't think of a more pristine cinder cone and associated lava flow than 71,000-year old SP Crater on the San Francisco volcanic field. Located 25 miles north of Flagstaff, this spectacular photo was captured by well-known geologist and author Wayne Ranney with glider pilot and photographer Ted Grussing at the controls. Its unweathered appearance is due to its youth, the semi-arid climate of the Colorado Plateau and the erosion-resistant agglutinate in the rim. Symmetricality implies minimal prevailing winds resided during emplacement. Notice older, eroded cones on the field.

SP is composed of basaltic andesite, while its flow bears a somewhat different chemical signature. That's not unusual, although not well understood, considering that lava generation is generally a late-stage development during cinder cone construction. Flow direction is dictated by the slope of the landscape, which dips slightly to the northeast. There are over 600 volcanoes on the Miocene to Holocene-age volcanic field. Please read my post about the San Francisco volcanic field here.

Does a genetic relationship exist between the San Francisco volcanic field in northern Arizona and the San Bernardino field in southeastern Arizona?

Photograph of SP Crater and associated 4-mile lava flow on the San Francisco volcanic field north of Flagstaff, Arizona. Courtesy of geologist and author Wayne Ranney and photographer and glider pilot Ted Grussing.

In addition to cinder cones and lava flows, the San Bernardino field contains at least eight maar craters and associated tuff rings. They are the hydrovolcanic or phreatomagmatic (Greek for "well of magma") equivalent of cinder cones that erupt when ascending magma interacts with aquifers within basin-fill sediments or fracture-controlled groundwater. Molten contents are explosively evacuated via steam-blast eruptions leaving the funnel-shaped maar crater and tephra ring cut into the landscape, and a diatreme (Greek for "through perforation") as the substructure. When the rapidly expanding, superheated water contacts the confined space of the country rock, it breaks into fragments forming a microbreccia.

Schematic cross-section through a maar-diatreme (top) and a tuff ring (bottom) showing the typical volcano-sedimentary processes and geomorphic features. The left side of the diagram represents the characteristics of the volcaniform formed in a hard substrate, while the right side is a soft rock environment. Modified from Keresturzi and Nemeth

Seven miles due east of V2009 is horseshoe-shaped Paramore, the largest maar crater on the San Bernardino field at 1.5 km in diameter. Surrounded by a fine-grained, light-colored ring of laminar tuff beds, its depressed crater is covered with Holocene playa deposits. The steam-blast explosion has lifted fragments of older basalt flows along with unconsolidated detrital material out of the crater. As the conduit and fissure system gradually cooled down, post-eruptive subsidence of the crater occurs due to diagenetic compaction and lithification. That places the crater below the level of the surrounding bedrock, which accommodates subsequent playa formation and deposition. Interestingly, the initial phase of cinder cone emplacement may involve an unsorted, xenolith-rich basal phreatomagmatic layer associated with initial magma-water interaction.  

Surrounded by younger eruptions that somewhat obscure its surface morphology, the phreatomagmatic crater of Paramore is situated below the level of the bedrock. The Pedregosa (left) and Chiricahua (right) Mountain ranges frame the horizon.

Cinder cones typically reside: 1.) as satellites or parasitic cones on the flanks of shield volcanoes located over hotspots and rift zones; 2.) on the flanks of composite (strato-) volcanoes, their back-arc spreading regions and calderas in subduction zones, and 3.) isolated or in clusters of 10 to 100 within flat-lying volcanic fields in intraplate, continental settings, such as the San Bernardino field. Their global abundance makes cinder cones the most frequent volcaniform, while phreatomagmatic volcanoes reign second. Conical structures that resemble terrestrial, basaltic cinder cones have been tentatively identified on the Moon, Mars and Venus. They may provide valuable information regarding volcanic processes and planetary evolution, which has ironically spurned an interest in cinder cones back on Earth.

An example of a satellite cinder cone is Pu'u ka Pele, on the flanks of Mauna Kea, one of five shield volcanoes that comprise the Island of Hawaii, the "Big Island." Cinder cones and lava flows typically erupt from vents on the flanks of the parent volcano in Hawaii. Beyond the cone is a young flow distinguishable by its dark color that emanated from a vent on nearby Mauna Loa, the world's largest volcano measuring from the ocean floor. Vegetated, older flows are in abundance on the shield's almost imperceptible slope, which is, flow-upon-flow, how the Hawaiian Islands were built from beneath the sea. 

Pu'u  ka Pele, on the southeast flank of Mauna Kea, is 95 m in height and 400 m in diameter. Over 300 cinder cones pepper the upper slopes of Maina Kea largely along three principal rift zones. Hawaiian cinder cones generally don't emit lava flows, which emanate from vents on the flanks of the parent shield volcano.

Still an unclear process, magma is generated at great depth within the Earth's convecting mantle. In order to reach the surface at the continental crust, it must pass through the lithospheric mantle. Partial melting (in that only a fraction of the available mass forms a melt while the remainder stays solid) in the upper mantle occurs and forms molten material with a mafic composition (described below) that buoyantly rises toward the surface through the lithosphere and ponds forming a magma chamber. 

On the flanks of shield and stratovolcanoes...
Cinder cones form when the supply of magma within the upper mantle begins to diminish or cease, and the magma chamber begins to cool and crystallize. First-formed minerals are high-temperature, olivine-rich mafics, which are mantle-abundant, rich in magnesium and iron, and silica-poor. Depleted magma minerals remain in the chamber and endow it with silica, which makes it viscous. Eventually, back-pressure forces a mafic eruption that emplaces cinder cones as satellites on the flanks of its parent volcano, which in turn may fuel mafic lava flows downslope.

On volcanic fields...
Investigations of mineral composition and thermodynamic calculations indicate the source region of volcanic fields at a depth of 67 km at a temperature of 1400°C, and any local magma chambers are at a depth of 33 km beneath the presumed crust-mantle boundary. Volcanic fields are characterized by a thin crust and lithosphere created by extension above an anomalously shallow asthenosphere with high heat flow. Volcanic fields can be formed by products of every composition, although they are most commonly basaltic. On the San Bernardino field, following Basin and Range uplift and extension, widespread basaltic volcanism formed pockets of melts. 

As pressure increased, magma began its buoyant ascent through the crust, dissecting to the surface along faults, structural weaknesses and sub-surface dike complexes. Episodic extension, variations in the geochemical, temperature and pressure states of the mantle, shifts in the locus of volcanism and magma supplies can add to the complexity of vent distribution. Thus, the emplacement of cinder cones on the field can form in a variety of distributions and geometries.

Cinder cones can form in a variety of geometries along faults or their intersections (A), offset laterally from the fault as a function of dip (B), and in an en echelon array along the fault trace. Modified from 

In northern Arizona, at 11,820 feet on the saddle between Mounts Humphreys and Agassiz of the San Francisco Peaks north of Flagstaff, we're facing the eastern portion of the San Francisco volcanic field through the Peak's caldera. It contains many linearly-distributed and clustered cones including the O'Leary Peak lava dome (left) and historically recent Sunset Crater (right) cinder cone.

Temperature, gas content and chemical composition of magma directly influences the size, shape and activity of all volcaniforms. To varying degrees, these factors affect the magma's mobility or viscosity. In regards to cinder cones, its magma is "thin and runny" with a low resistance to flow, since it's very hot, gas volatile-rich (1-6% by weight of water vapor, carbon dioxide and others such as sulfur dioxide) and silica-poor (largely of basalt but even some andesite). Thus, cinder cones tend to erupt effusively and are constructed with a symmetrical, low-profile, layered architecture about a central vent. When present, lavas flow readily in thin, broad sheets for considerable distances. 

The behavior of lava depends primarily on its viscosity (resistance to flow), the slope of the ground cover and the rate of lava eruption. Because basalt contains the least amount of silica and erupts at the highest temperature compared to the other types of lava, it has the lowest viscosity (the least resistance to flow). Thus, basaltic lava moves over the ground easily, even down gentle slopes. Dacite and rhyolite lava, however, tend to pile up around a vent to form short, stubby flows or mound-shaped domes. Modified from USGS and J. Johnson illustration

Geoscientists, true masters at categorization, distinguish types of volcanoes by their eruption behavior, which are named after volcanoes where the behavior has been observed. Icelandic eruptions are typified by effusive eruptions of basaltic lava from long, parallel fissures. Hawaiian eruptions are similar, but lava exudes from the summits of shield volcanoes and from radial fissures along the flanks. Strombolian eruptions consist of initial moderate bursts of expanding gases with later continuous small eruptions. Vulcanian involves moderate eruptions of gas laden with ash in dark clouds that rapidly ascend and expand. Pelean are explosive outbursts with pyroclastic flows and dense mixtures of hot fragments and gas that pour down slopes with great velocity. Plinian are intensely violent eruptions of gas-rich magma that rocket gases and fragments into the stratosphere often generating lightning.

Types of Eruptions Based on Behavior (Explosiveness)
Modified from Encyclopedia Britannica 2006

Based on their explosivity, plume height, frequency of eruption, and volume, cinder cones on the San Bernardino volcanic field are thought to have been Hawaiian style eruptions, that is "calm" (relatively speaking) from vents and fissures, or low-level Strombolian eruptions, short-lived but more explosive with increased plume height and with magmas of intermediate viscosity. It is conceivable that during construction of a cone, as magma fractions, temperatures and chemistries evolve, a combination of eruption types may occur. In the V2009 roadcut (discussed below) upward increases in the abundance of coarse blocks and bombs, and sequences of welded agglutinate imply an evolution in the growth process of the cone. Strombolian eruptions, as opposed to Hawaiian, tend to produce more sustained fountains of lava and more extensive welded facies, also seen.

San Bernardino Valley is a 21-mile long and 18-mile wide, northeast by southwest-trending, gently-sloping, sediment-filled intermontane basin. The surface extent encompasses about 1,000 square kilometers in Arizona and about 90 in Mexico. The topographic gradient averages 49 feet per mile. Geologically, the valley, which is divided into northern and southern portions, is classified as an asymmetric, down-dropped block of crust called a graben. The northern portion is divided into smaller half-grabens by four transfer faults that strike NW-SE, aligned with the structural lineament (more on that later). Geomorphically, the valley has also been described as a semi-bolson, which is a wide desert basin with ephemeral playa drained by intermittent streams, and, in the case of San Bernardino, that cumulatively funnel south into Mexico.

Specifically, the valley is bounded by roughly-parallel mountain ranges of the Perilla, Pedregosa and Chiricahuas to the west, and Peloncillo and Sierra San Luis to the east. The ranges are riddled with Oligocene-age calderas, which are likely buried beneath the intervening valleys as well. All this overlies a basement of Paleozoic and Mesozoic sedimentary rocks. Excluding the deeply-buried Proterozoic foundation, the landscape is comprised of these three distinctive elements - sedimentary rocks, Mid-Tertiary eruptive centers, and Tertiary to Quaternary basalt lava flows and alluvial deposits. 

Sketch map of southeastern Arizona and southwestern New Mexico showing structural features most recently from Basin and Range movement, although the region has been deformed many times as early as the Precambrian. For orientation, Tucson is 100 miles to the northwest. Modified from Drewes and Thorman, 1978

Our geo-journey through the San Bernardino volcanic field began from camp in the Turkey Creek caldera of the Chiricahua range. Facing east, the valley is the San Simon that abuts the San Bernardino Valley basin to the south and is half its width and separated from it by an ill-defined hydrologic divide. Beyond is the Peloncillo range and further back in New Mexico, the Animas Valley and the Pyramid range. Repeating basins and ranges - grabens and horsts - can be identified on the landscape in this, the Basin and Range geologic province of southern Arizona.  

Facing east from the Turkey Creek caldera within the Chiricahua Mountains, the San Simon Valley basin is backed by the Peloncillo Mountain range, and it by the Animas Valley and the Pryamid Range.

The valleys are alluvial basins filled with volcanic and sedimentary deposits shed from the surrounding mountains, and volcanic rocks that have erupted within the valleys. They formed from extension on the landscape during the Miocene-Pliocene period of high-angle faulting during the Basin and Range Disturbance starting at least 15 million years ago and continues to the present. The details are discussed in my post Part II.

And yes! There are turkeys in Turkey Creek.

The major geomorphic feature in the San Bernardino Valley is the San Bernardino volcanic field (named by Lynch in 1972), which dominates its northern third. In older literature, it was referred to as the Geronimo volcanic field. Measuring some 850 square kilometers, the field possesses over 130 separate basaltic volcanic vents, associated lava flows and pyroclastic deposits. Four radiometric dates for basalts exist for the volcanic field. Older basalts are dated at ~3.3 to ~4.7 Ma, while younger flows fall between ~750,000 and ~274,000 years ago of the late Pliocene to Pleistocene.

Lavas within the valley are distinguished by their location, age and composition. Flank lavas, which originate along the fronts of bordering ranges, are older and composed primarily of alkali-olivine basalts and have low ratios of magnesium to iron; whereas, valley lavas are predominantly basanites and fall within the younger range with relatively high magnesium to iron ratios. Overall, their composition suggests an anomalous mantle, whose properties are consistent with the presence of areas of partial melting.

Most volcanic activity occurs between converging and diverging tectonic plates or over hotspots, often far from plate boundaries. The latter is an example of intraplate magmatism, which is found at regions of lithospheric extension where mantle-derived asthenospheric melts are permitted to passively rise. This scenario is typical of continental rifting and may develop into a divergent tectonic regime. In a region that defies generalities, this is found within the Basin and Range Province of western and southwestern North America and what we see within the San Bernardino volcanic field. 

 Being bounded by mountain ranges, the San Bernardino volcanic field is located in the extreme southeastern corner of Arizona and dominates the northern third of the San Bernardino Valley. Cinder cone V2009 on SR 80 and well-known Paramore maar crater are labelled. Captured from Google Earth

Scattered across the field and onto the flanks of the neighboring ranges, monogenetic (short-lived, single-eruption, small-volume) cinder cones are the most common volcaniform. Volcanic fields typically consist of volcanic clusters and/or alignments along fissures, which are dictated by structural influences and tectonic regimes. Vents on the field are aligned along a NNE trend, which cuts across the dominant N-S Basin and Range tectonic trend. Taking the cumulative volume of pyroclastics generated by the entire field into account over periods of hundreds of thousands to millions of years, monogenetic activity can exceed that of individual composite volcanoes.

Facing southeast from atop V2009, notice its layered apron across the road, and many cinder cones and lava flows on the San Bernardino volcanic field. The backdrop is formed by the closer Pedregosa Mountains (right) and further Mule Mountains (left). Our evening destination for this day's geo-journey was the copper-mining town of Bisbee up in the Mules.

Multiple (at least seven encountered in drilling for water), thin (5-20 meters) alkaline basaltic lava flows lie beneath volcanic field's grass covered plain. The flows interbed and are partially covered by a veneer of middle and late Pleistocene alluvium, aeolian material, and near the mountain fronts, colluvium. In the center of the field, such as around V2009, it consists of dark to reddish brown soils and clay-rich vertisols, and is littered with subangular to angular boulders and cobbles of vesicular basalt (below). Closer to the fronts, rocks of newer alluvial deposits are interbedded with flank lava flows and include basalts, rhyolites and sedimentary rocks.

Concordant remnants of oldest lava flows along the flanks of the bordering mountains on both sides of the volcanic field indicate that the basalt flowed onto pediments or alluvial embayments in mountain fronts of much lower relief relative to the valley floor than today. Subsidence in the valley, which lowered the floor relative to the mountains, continued (or was renewed) not long after volcanism initiated, which left older flows as remnants flanking the valley.

On the Google Earth image below, Highway 80 slices through a thin portion of V2009's northwest flank, thereby creating two exposures, herein designated "east" and "west". Wayne and I pondered as to why highway 80 didn't circumvent the cone altogether rather than dissect through it. Apparently, the now-empty railbed of the Arizona and Southwestern Railroad (built in 1888 to transport copper mined in nearby Bisbee to El Paso and beyond) "forced" the highway to transect the cone rather than make a circuitous detour around it. 

On Google Earth, the cone's diameter measures ~1,371 (west to east) by ~1,342 feet (north to south), while its summit is skewed slightly to the south of center. This fact is confirmed within the east cut and is suggestive of a southwest prevailing wind during eruption. Its eroded crater is revealed on the 3x vertically-exaggerated image. It is located at 4,641 feet and rises some 86 feet above the field. By comparison, the highest elevation in the valley is 5,135 feet on a cone to the east, and the lowest point is 3,700 feet in Black Draw that drains the valley across the International Border into Mexico. 

A few gullies (see image below) have dissected the flanks of the cone and formed a small debris apron of colluvium around it, while an ephemeral stream has carried alluvial debris to the southeast and converged with one from another cone forming a small playa (far right of center on the Google image and the map below). No lava flows appear to have emanated from vents associated with V2009, although the entire edifice resides on a multi-layered platform of interbedded flows that constitute the volcanic field.

View of cinder cone V2009 sliced by SR 80 as it cuts through a small portion of its northwest flank. Notice the empty railbed that parallels the highway and the other vertically-exaggerated cinder cones on the field.

Using Google Earth's Elevation and Profile tool, I ran a 1,574-foot, SW-NE linear transect through V2009 just south of its degraded summit crater. The average elevation gain/loss above the field was 73.7 feet, and the maximum slope was 21.1%. As with our observations in the field and on the Arizona Geological Survey map, the Google Earth profile of V2009 attests to its displacement to the northeast. 

In addition to the chemistry of its lavas, the life history of a volcano is preserved in its stratigraphy, structure and form provided by the "window" of the roadcut. The west exposure contains only pyroclastics, while the east exposure (below) exposes not one but two lava vents or possibly feeder dikes consisting of a dark-gray basalt. That fact is substantiated on Plate 3 of the Arizona Geological Survey map (here) and is designated as Qbvl, basaltic vent lava and agglutinate proximal to vent. It would be interesting to know if the cone's innermost plumbing are conjoined, the assumed emplacement architecture. The degraded summit is situated between the conduits, as anticipated geologically, since they both would have contributed to the rain of tephra that built the edifice. 

A few map observations, as previously mentioned, the pyroclastics are skewed to the northeast suggesting prevailing wind direction during emplacement. In addition, many of the valley lava flows progress in a northerly direction, while contemporary streams and alluvial deposits from the cones progress southerly, such as the serpentine drainage emanating from V2009, Qy on the map (arrow). This suggests, requiring substantiation in the field, that the basin has progressively tilted to the south following the cessation of volcanism, which directed drainage internally or externally to Black Draw across the border into Mexico. Recent tectonic activity could have renewed stream downcutting, while uplift would have entrenched older streams. Thus, drainage patterns are good indicators of recent and past tectonic activity. The implication is that Basin and Range deformation is on-going, whether or not volcanism is dormant or extinct on the field.

This AGS map shows V2009 transected by highway SR 80. Two vents are exposed within the roadcut on the east face. Also note the Quaternary alluvial deposits emanating from the apron of the cone to the southwest.  Relevant Map Units: Qbpc (Basaltic cinder deposits proximal to vent); Qbvl (Basaltic vent lava and agglutinate proximal to vent); and Qcb (Basalt-derived hillslope colluvium); Qy (Alluvial deposits in active drainages). Modified from the Geology and Geomorphology of the San Bernardino Valley, Southeastern Arizona, Arizona Geological Survey (map three of three) by Thomas Hi. Biggs et al 1999

Distal to the vents, and therefore circumferential to them, massive reddish-brown, oxidized agglomerate forms the internal bulk of the cinder cone. The agglomerate is highly brecciated and includes scoriaceous deposits of cinders, volcanic bombs of varying size and welded agglutinate. Progressing outward, the cinder cone possesses well-defined facies and a bedded, unsorted pyroclastic stratigraphy of brownish-red lapilli of varying size (2 to 64 mm by definition), which is layered and dips centrifugally (far left), reflective of the manner of emplacement. Lapilli are also somewhat interbedded with the agglomerate centrally. Large volcanic blocks of varying sizes are scattered on the surface along with various woody plants, forbs and grass typical of the field.

East face of the roadcut

Closeup of the northernmost vent in the east face of the roadcut

This view of the east exposure reveals dark gray to dark reddish brown basaltic vent lava and massive clumps of agglomerate proximal to it. The second vent (unseen and less well exposed) is off to the right in the photo. The flanks are composed of reddish-brown, indurated pyroclasts distal to and surround the vents. 

View of the east face of the roadcut

From atop the smaller west roadcut (below), angled beds of pyroclastics are visible across the highway on the cone's apron and a small portion of the second vent. Numerous volcaniforms and associated flows are scattered across the volcanic field. In the distance, the Pedregosa range is to the right, while the Mule Mountains, the location of copper mining town Bisbee (our destination), are to the left. Many of the lava flows on the field, if not all, contain sub-rounded to well-rounded, ultramafic xenoliths (rocks not from the parent magma but from the upper crust and mantle, and introduced during emplacement). 

Where might these foreign inclusions have been derived? Think "commonality of evolution" of the landscape!

Facing highway 80 south from atop the west slice of the roadcut

Having investigated V2009's construction, emplacement and relationship to the San Bernardino volcanic field, my post Part II will address the grander question. Does a geologic relationship exist between the cinder cone, the volcanic field and sedimentary basin on which it emplaced, the physiographic province where it's situated, and even neighboring provinces in the American Southwest? Is there a commonality of evolution that exists among these seemingly disparate features? 

• Ancient Landscapes of the Colorado Plateau by Ron Blakey and Wayne Ranney, Grand Canyon Association, 2008.
Arizona Water Atlas by Herbert Guenther et al, Volume 3, 2008.
• Basaltic Volcanic Fields by C.B. Conway and F.M. Conway, Encyclopedia of Volcanoes, 2000.
Basin and Range Volcanism as a Passive Response to Extensional Tectonics by Keith Putirka and Bryant Platt, Geosphere, 2012.
• Compositional Variations Within Scoria Cones by Mel Strong and John Wolff, GSA, 2003.
Fate of the Subducted Farallon Plate Inferred From Eclogite Xenoliths in the Colorado Plateau by Tomohiro et al, GSA, Geology, 2003.
Geological Causes of the Hydrogeology of Southern Arizona's Basin and Range Province by Jan C. Wilt and Gary L. Hix, source and date unknown.
• Geological Evolution of the Colorado Plateau of Eastern Utah and Western Colorado by Robert Fillmore, The University of Utah Press, 2011.
• Geologic Map of the Southern Peloncillo Mountains, Cochise County, Arizona, and Hidlago County, New Mexico, by Scott J. Skotnicki, Arizona Geological Survey, Digital Map DGM-24, 2002. 
• Geology and Geomorphology of the San Bernardino Valley, Southeastern Arizona by Thomas H. Biggs et al, Arizona Geological Survey, 2010.
Hiking Arizona's Geology by Ivo Lucchita, The Mountaineers Books, 2001.
Plate Tectonics: Continental Drift and Mountain Building by Wolfgang Frisch et al, 2011.
Major Geologic Structures Between Lordsburg, New Mexico and Douglas and Tucson, Arizona by Harald Drewes and C.H. Thorman, USGS, New Mexico Guidebook, Land of Cochise, 1978.
• Monogenetic Basalt Volcanoes: Genetic Classification, Growth, Geomorphology and Degradation by Gabor Kereszturi and Karoly Nemeth, 2012.
Monogenetic Volcanic Fields: Origin, Sedimentary Record, and Relationship with Polygenetic Volcanism by Karoly Nemeth, GSA, Special Paper 470, 2010.
Morphometric Analysis of Cinder Cone Degradation by Charles A. Wood, Journal of Volcanology and Thermal Research, 1980.
• Petrogenesis of Xenolith-Bearing Basalts From Southeastern Arizona by Stanley Evans, Jr. and W.P. Nash, American Minerologist, Volume 64, 1979.
Quaternary Mafic Lava Xenoliths from Southeastern Arizona by S.H. Evans and W.P. Nash, GSA abstract Vol. 10, 1978.
• Reconnaissance Assessment of the Geothermal Potential of San Bernardino Valley, Cochise County, Arizona by Claudia Stone and James Witcher, AGS Report 05-A, 2005.
Study of Volcanic Cinder Cone Evolution by Means of High Resolution DEMs by Jean-Francois Parrot, Geographical Institute, Mexico, date unknown.
• The Structure and Emplacement of Cinder Cone Fields by Mark Settle, American Journal of Science, Vol. 279, 1979.
Tectonically-Controlled, Time-Predictable Basaltic Volcanisn from a Lithospheric Mantle Source by Greg A. Valentine and Frank V. Perry, Earth and Planetary Science Letters 261, 2007.
The 1887 Sonoran Earthquake: It Wasn't Our Fault by Thomas G. McGarvin, Arizona Bureau of Geology and Mineral Technology, Summer 1987.
• The San Bernardino Volcanic Field of Southeastern Arizona by D.J. Lynch, New Mexico Geologic Society Guidebook, 29th Field Conference, 1978.
Volcanic History of Arizona by Stephen J. Reynolds et al, Field Notes, Arizona Bureau of Geology and Mineral Technology, Summer 1986.

Sunday, January 4, 2015

Big Brook - New Jersey's Classic Late Cretaceous, Fossil-Collecting Locality

"At first it may seem to be a piddly little dribble through the farmlands and forests of rural New Jersey, 
but careful observation shows Monmouth County's Big Brook 
to be glass-bottomed boat sailing through a Late Cretaceous sea busy with life.” 
From the New York Paleontological Society Field Guide, 2002

Big Brook at a slightly high water level as seen from the Hillsdale Road bridge facing east

Step into the waters of this very ordinary-looking brook, and you’ll go back in time to North America’s continental shelf only a few million years before the Great Extinction that ended the Age of the Dinosaurs. In its lazy and short course to the Atlantic, Big Brook has carved a shallow, curvy trough through New Jersey's Inner Coastal Plain through the upper sediments of a Late Cretaceous paleoshelf. In so doing, weathering of the streambank provides a steady supply of fossils that are washed into the streambed.

Big Brook is one of the East Coast’s classic fossil-collecting localities with both amateurs and professionals alike. As early as 1863, the Smithsonian Institute in New York sent an expedition to explore and gather fossils from the brook and others within Monmouth County. Less widely appreciated amongst amateurs is that the strata through which Big Brook transects preserves an outstanding sedimentological record of the transition from an inner to an outer shelf environment during the Late Cretaceous as sea level rose.  

Although rare, be on the lookout for fragmented, water-worn dinosaur bones and teeth - hadrosaurs, theropods, ankylosaurs and ornithomimids - from the Late Cretaceous terrestrial shoreline to the west. Add to the mix, Pleistocene mammalian remains of mastodon, sloth, beaver and horse. From the Holocene, there's even an occasional Native American Lenni-Lenape arrowhead from the surrounding countryside and a coral from Paleozoic tropical seas entrapped within the Appalachian orogen that was transported to the area fluvially or via glacial outwash.

A Late Cretaceous terrestrial fauna similar in some respects to eastern North America
From National Geographic

But by far, the big attraction is teeth from Late Cretaceous chondrichthyans (shark, rays and skates) and, less often, osteichthyans (bony fish) and large marine reptiles (mosasaurs, turtles and plesiosaurs). 

The Late Cretaceous marine ecosystem teemed with life. 

In addition, abundant macro-invertebrate remains include brachiopods, bryozoans and molluscs (bivalves such as oysters, snails, belemnites and ammonites) and disarticulated arthropod carapaces and claws (lobster, crab and shrimp), all from the Late Cretaceous shelf ecosystem. 

The shelf's benthic and demersal zone was rich and diverse with brachiopods, bryozoans, molluscs and arthropods.
From Matthew McCullough on Flickr and license here.

Big Brook is barely 50 miles south of New York City via the Garden State Parkway off exit 109 west. The brook winds its way through the rural New Jersey hamlets of Colts Neck and Marlboro to the Navesink River near the borough of Red Bank, and ultimately the Atlantic Ocean. Besides Big Brook, other nearby fossil-bearing tributaries include Poricy Brook in Middletown Township, Ramanessin Brook in Holmdel and Shark River (Eocene and Miocene) in Neptune and Wall Townships. They're all located in gentrified and well-healed Monmouth County, which is in the top 1.2% of counties by wealth in the United States. The County's website advertises itself as the "Gateway to the Jersey Shore", while locals know it as Springsteen country (“…sprung from cages on highway nine…”). 

The arrow points to the location of Big Brook within Monmouth County.
Modified from Roadside Geology of New Jersey

Some sixty-seven million years ago, this lazy, oak-shaded stream and the surrounding countryside were a tiny submerged section of the newly-formed, Atlantic continental shelf. During the Late Cretaceous, global high seas drowned the shelf that now represents the broad, low relief of the Atlantic Coastal Plain through which Big Brook flows. But the geological story of the plain begins well before the Cretaceous. 

Paleozoic tectonic convergence...
Beginning in the early Paleozoic, Laurentia - the rifted megacontinental sibling of the Late Proterozoic supercontinent of Rodinia - was converged upon by a procession of magmatic arcs, micro-continents and megacontinents and their intervening ocean basins. 

Birth of Pangaea...
In a parade of orogenic events, they accreted to Laurentia's growing ancestral core - building mountains and adding crust with each collision. By the Pennsylvanian Period of the Paleozoic (below), their cumulative convergence had constructed the supercontinent of Pangaea and built a massive, centrally-located mountainous spine. By the Late Permian at the close of the Paleozoic, the mountains had been ravaged by erosion.

Subsequent to the collision of Gondwana (the other megacontinental sibling of Rodinia) with equatorially-positioned Laurentia in the Middle Pennsylvanian, Monmouth County is nestled somewhere within the lofty peaks of the Appalachians. The supercontinent of Pangaea is fully formed and awaits its imminent fragmentation.
Modified from Ron Blakey and Colorado Plateau Geosystems. Inc.

Demise of Pangaea...
In the Late Triassic, Pangaea’s fragmentation began. As the Atlantic Ocean began to open within the schism, the remnants of Pangaea's central mountainous spine began to fragment as well. A portion remained astride the Atlantic Coast in newly-formed eastern North America - today's Appalachians - while other remnants were carried across the globe on the backs of Pangaea's rifted siblings. Pangaea's breakup also endowed North America (and of course New Jersey) with a new passive shoreline characterized by seismic and volcanic inactivity, and most importantly, subsidence and sedimentation. 

Subsidence and sedimentation of the Atlantic margin...
Beginning probably during Jurassic time, lithospheric cooling of North America's newly-formed passive margin, in concert with the weight of voluminous sedimentation, promoted rapid subsidence and provided a vast accommodation space for the accumulation of clastic erosive products. With subsidence, the Atlantic margin was broken into a series of faulted-blocks, which experienced differential movements. 

Downward movement created embayments - deep indentations of the ancient shoreline in which sediments accumulated in greater thicknesses in greater water depths. Upward movement created structural highs, arches or uplifts - with thinner sequences and even the absence of deposition. One such coastal geo-indentation that would become a portion of the Coastal Plain - the Raritan embayment - influenced sedimentation in New Jersey between Staten Island at the western end of Long Island and Jersey's northern Coastal Plain.  

Map showing the outline of the Atlantic Coastal Plain and major structural elements that persist on North America's modern coast line. In particular, the Raritan Embayment between is encircled.
Modified from Summary of Lithostratigraphy and Biostratigraphy of the Atlantic Coast by Ollson

Formation of the Atlantic Coastal Plain...
Concomitant with crustal cooling and subsidence, deposition in the coastal plain began in earnest in the Early Cretaceous with fluvial sedimentation from the highlands of the Appalachians. However, in the Late Cretaceous (below), marine incursions representative of global high seas flooded low-lying regions of the world that included the newly-formed, low-lying Atlantic coast.

Progressing from the shoreline seaward, gravel and sand on the inner continental shelf gave way to silt and clay, and, in progressively deeper water, glauconitic sand and silt. The deposits record a progressive but discontinuous and fluctuating rise in sea level - perhaps four in Late Cretaceous time and three in the Cenozoic. Thus, the landform of the Atlantic Coastal Plain gradually developed, representative of some 150 million years of sedimentation.

Of course, the flood waters of the Cretaceous have receded exposing the broad Coastal Plain on the eastern seaboard. Although global seas continued to vacillate, erosion became the dominant geological process through the Tertiary. Ice Age glaciers made it to northern New Jersey but not to the south, while the Coastal Plain continued to receive a thin and varied veneer of colluvial and alluvial Quaternary and Holocene debris. Today, the modern shoreline is 10 miles to the east of Big Brook as it lethargically dissects its way to the sea through Late Cretaceous sediments. As for Monmouth County, the Jersey Shore and the entire East Coast, they await the sea's return, like it always does. 

With Pangaea fragmented apart, the Late Cretaceous witnessed the initiation of the development of the ecosystem of Monmouth County (red dot) on the submerged Atlantic Coastal Plain (light blue). The Atlantic Ocean has opened between the north and south, and is actively spreading. Note the Mid-Atlantic spreading center (light blue) along the line of tectonic divergence. The Western Interior Seaway in central North America is about to become confluent between the waters of the Arctic and the Gulf of Mexico.
Modified from Ron Blakey and Colorado Plateau Geosystems. Inc.

The Paleozoic collisions that assembled North America formed geomorphic provinces that are seen in the colorful mosaic of the Tapestry of Terrain and Time map by the USGS here. The yellow and tan Cretaceous and Cenozoic deposits of New Jersey (within the ellipse) illustrates the extent of the Coastal Plain within the state, which is continuous with that of the entire Atlantic and Gulf Coasts from Cape Cod and Long Island, through northern Jersey at Sandy Hook to southern New Jersey at Cape May, and down to Florida and around to the Gulf of Mexico. Let's take a closer look at the provinces of New Jersey.

The geomorphic provinces of Northeastern and Mid-Atlantic North America with New Jersey encircled. 
Modified from the USGS Tapestry of Time bedrock map located here.

For an area its size, New Jersey has a diverse geological history. From west to east, from the mountains to the sea, and across the multitude of orogens that formed eastern North America, New Jersey’s main geological subdivisions or provinces are the Valley and Ridge, the Highlands (equivalent to the familiar Blue Ridge down south), the Piedmont and the Coastal Plain

By definition, the four geomorphic or physiographic regions are each unique as to relief, landforms and geology. Being inherited subsequent to the tectonic collisions that occurred throughout the Paleozoic, they're on strike from northeast to southwest in accordance with the direction of tectonic convergence. A fifth smaller province - in accordance with tectonic divergence - is the Newark Basin (green and red), which lies interposed within the Piedmont. It's a sediment-filled rift-basin, one of many along the east coast that formed during the initial stages of Atlantic opening in the Late Triassic and Jurassic.

Geologic Bedrock Map and Physiographic Provinces of New Jersey
The region of Big Brook within the Atlantic's Inner Coastal Plain is located at the red dot.
Modified from the Department of Environmental Protection, Division of Science, Geological Survey, 1999

The Coastal Plain Province is relatively featureless save a few gently undulating hills and overlapping rocks of the Piedmont Province to the west. It covers the entire lower half of New Jersey (see above map), dipping seaward from 10 to 60 feet per mile to the the southeast and extending beneath the Atlantic Ocean to the edge of the Continental Shelf at the Baltimore Canyon Trough. Its unconsolidated and compacted (but not cemented) sediments range in age from the Cretaceous to the Miocene. The composition of its bedrock and fossils confirms that it was submerged by Late Cretaceous high seas. 

This west to east cross-section through the modern Coastal Plain and continental shelf illustrates the increasingly deep seaward-facing wedge of sediments that extends from a feather edge at the Fall Line of the Piedmont to the sea, where it's over a mile thick.
From the USGS and the Roadside Geology of New Jersey

The Coastal Plain is further subdivided into two regions. Because it was uplifted, weathered and dissected, the Inner Coastal Plain is higher in altitude than the Outer Plain but not by much. Its composition is largely a mix of quartz sand, glauconitic sand, silt and clay. This fertile agricultural zone gave rise to New Jersey’s nickname as the Garden State. It's also the location of Big Brook within Monmouth County. The Outer Plain is a region of lower altitude where low-relief terraces are bounded by subtle erosional scarps. It consists of Tertiary and Quaternary sand, and being acidic and less fertile, is the location of Jersey’s heavily forested cedar swamps and pine-scrub oak of the Pine Barrens. 

As mentioned, in the Early Cretaceous the Coastal Plain region of New Jersey received deltaic and floodplain-derived (non-marine) sediments from the Appalachian highlands to the west. In the Late Cretaceous and into the early Paleocene, sea levels rose and flooded the coastal region in a series of transgressions over land and regressions back again. Layer after layer, sequence after sequence (packages of strata deposited during a single cycle of sea level rise and fall), sediments of the sea were laid down beginning with the earliest Late Cretaceous Raritan Bass River Formation upon the latest Early Cretaceous fluvial Potomac Formation (chart below). 

In the late Late Cretaceous - from the Campanian into the Maastrichtian - the Monmouth Group, the state's youngest Cretaceous package, was laid down - a unit that is compacted but unlithified. New Jersey's Monmouth Group includes the basal Mount Laurel sand (5-60 feet thick), the transgressive marl of the Navesink (25-60 feet thick), the regressive silt and sand of the Redbank Formation (thin film to 100 feet thick) and the Tinton Formation's coarse quartzose and glaucontic sand (20-40 feet thick). 

As the sea transgressed and regressed, shorelines moved accordingly. Existing sediments were eroded, reworked and redeposited, leaving behind unconformities. Breaks between sequences were punctuated by lag deposits or "shell beds." The great majority of fossils at Big Brook such as within the Navesink Formation, which we will visit, were eroded from lag deposits and released into the streambed. They were deposited within the neritic zone - the relatively shallow waters of the ocean from the littoral zone (closest to the shore) to the drop-off at the edge of the continental shelf. 

Big Brook's journey to the sea, has excavated a channel into the Inner Coastal Plain. It cut through the Red Bank Formation's Sandy Hook Member (Krsh), through the Navesink Formation (Kns) and underlying Mount Laurel Formation (Kml), and in some areas, into the Wenonah Formation (Kw) - all deposits of the vacillating Late Cretaceous sea. You can find the complete map of the Freehold and Marlboro Quadrangles here. For orientation, the red arrow marks the location of the car park on the north side of the Hillsdale Road bridge. 

This map depicts the channel of Big Brook. The red arrow points to the car park on the north side of the Hillsdale Road bridge.
Modified from the Bedrock Geologic Map of the Freehold and Marlboro Quadrangles, New Jersey, 1996.

Two unmarked bridges are your portals to Big Brook- one on Boundary Road and the other on neighboring Hillsdale Road just east. On the north side of the Hillsdale Road bridge is a designated car park, while parking is on the street just south of the Boundary Road bridge (as of this writing). The photographs taken and fossils displayed in this post were from four visits to Big Brook on the east side of the Hillsdale Road Bridge. 

The countryside through which Big Brook flows is peppered with residential settlements, horse farms and wineries that are either private or post no trespassing. Big Brook's banks are private as well and off limits to excavation. They're dangerous too, since they are unconsolidated and slump and collapse with little provocation, especially by overzealous excavators. But the streambed is fair game. The only caveat is that on a nice summer day you may have to share it with a paleontologist, a geologist, a scout troop and a fossil club - all after the same piece of time.

The bucolic Hillsdale Road bridge over Big Brook facing south

To extract the fossil bounty at Big Brook Preserve, no geological hammers and chisels are necessary owing to the unconsolidated nature of the bedrock. In fact, they're prohibited by a posted Colts Neck Township ordinance in order to preserve the resource and minimize over-collecting. All you'll need are a pair of Wellies or suitable waders (there's some broken glass so don't go barefoot), and equip yourself with a garden trowel (with a maximum blade length of 6") and a small, homemade sifting-screen (no greater than 18” x 18”). I borrowed a kitchen colander from home. 

The Department of Recreation and Parks of the Township of Colts Neck has determined that "there is an increasing need for the preservation of the many natural resources located within Big Brook Preserve. It has been observed that natural resources such as fossils have been taken from the park in large quantities. It has also been observed that certain other dangerous conditions continue to threaten the natural beauty, assets and environmental resources within Big Brook Park." 

Therefore, "Fossil extraction is prohibited from the walls of the streambed above the stream waterline", and "No person may harvest more than five fossils per day." With all that in mind, you’re ready to “beachcomb” at Big Brook, panning and sifting for treasures buried within the streambed.

After parking your car in the designated area, assemble your regalia, and follow the short footpath through the woods on Late Cretaceous Redbank soil. You can make out the brook's shallow, shady trough running from right to left. I've been to Big Brook many times over the years and haven't seen one mosquito or tick. Having said that, come prepared! 

The short path through the woods to Big Brook

Slide down the vegetated slope to the brook, and step back in time into the continental shelf. The deposition rate of the Navesink has been estimated to be about a meter in a million years, so from the footpath to the streamline, you're back about 2 million years. You can wade through the brook upstream (west) to the Boundary Road bridge about a half-mile or go downstream for a quarter-mile or so.

The Hillsdale Road bridge seen from Big Brook. 
The water level is somewhat high here, so the mudflats would be the best option for fossil-foraging.

Within the brook's oak-shaded world, things become quiet and peaceful with gurgling waters, chirping birds, occasional hawks cruising overhead and gentle breezes wafting down from above. Only the occasional car flying across the bridge will remind you of the civilization that surrounds you. 

Many collectors choose to visit Big Brook in the Spring or after a heavy rain, thinking that new runoff refreshes the fossils that weather in from the banks. Others say it makes no difference. Cobbles and fossils tend to aggregate in horizons, so many collectors focus on sieving there. 

If the brook is at high water and the bed is totally flooded, it's safest not to enter, besides, hunting for fossils will be extremely difficult and unproductive. Trees and large limbs that have fallen across the brook add a measure of challenge to negotiating the stream, especially if the current is swift and you're forced to the center of the channel.

Seen here at high water, the brook's bed, gravel bars and mudflats are less accessible for foraging. 

Wade and trudge around until you've found a "good" spot, be it a mudflat or gravel bar, or simply excavate directly into the streambed. Just don't disturb off-limit streambanks. Although highly fossiliferous and tempting, once again, they are unstable. Groundwater near the base of exposures is under artesian pressure and continually discharging from small seeps and springs that undermine the cliff-face provoked by the slightest excavation - historically a potentially fatal mistake.

The stretch of brook east of the Hillsdale Road bridge is largely within the Navesink Formation, while portions of the bed are in the Mount Laurel and deeper Wenonah. The age of the Navesink has been estimated to range from about 70 million years at the base of the formation to about 66 million years at the top, almost at the end of the Mesozoic. 

By the way, above the Tinton Formation of the Monmouth Group, the K-T boundary between the end of the Cretaceous and the beginning of the Cenozoic ("T" stands for Tertiary) has been identified in test borings beneath the Outer Coastal Plain and at Inversand Mine in the town of Sewell within the Inner Coastal Plain on the western part of the state across the Delaware River from Philadelphia.

The Navesink is a transgressive interval in the last of six depositional cycles of changing sea levels coupled with subsidence that includes the overlying Red Bank formation. An idealized cycle includes a basal glauconitic unit (of massive flooding and maxiumum faunal diversity), a superjacent clay or silt surface (representing the highstand tract deposited in shallower water than the previous tract), and a sandy unit (that may contain a lowstand tract at the top). The sequence of lower glauconite sand, middle clay-silt and an upper quartz sand was repeated some four or five times on the plain's inner to mid-shelf in the Late Cretaceous.

Glauconite sands of New Jersey...
The Navesink is a massively bedded, olive-gray, olive-black and dark greenish-black clayey, glauconite sand unit - also called greensand or marl - that is compacted but unlithified. Glauconite is an iron-rich mica (iron potassium phyllosilicate) that forms diagenetically at the sediment-water interface on the continental shelf from clay minerals during prolonged intervals of sediment starvation. Glauconite is not confined solely to the Navesink but is found in most of the Late Cretaceous formations within the Inner Coastal Plain. 

Geologic bedrock map of Late Cretaceous and Paleogene Formations of New Jersey's Inner Coastal Plain
Modified from Zehdra Allen-Lafayette

The coastal plain's glauconite beds are not only highly fossiliferous but, being nutrient-rich and holding water, were widely mined as fertilizer in the 19th century. In fact, the duck-billed dinosaur Hadrosaurus foulkii was discovered in an old marl fertilizer pit in Haddonfield, New Jersey, in 1858. It was the first almost complete dinosaur skeleton discovered in the United States and is now the New Jersey state fossil. The hadrosaur, being terrestrial as all dinosaurs, was thought to have been carried to the plain's former marine environment via a "bloat and bloat" or "fluvial-flood carried" scenario. The time-frame and curious marine burial are reminiscent of the therizinosaur Nothronychus graffami in Big Water, Utah. You can read about it in my post here.  

Highly fossiliferous and bioturbinated...
Big Brook's banks provide an excellent opportunity to inspect a portion of the Navesink in cross-section down to the streamline. Above the Navesink are fossil-depauperate sands of the Red Bank Formation, stained red by iron oxide. Within the Navesink are quartz-rich sand layers and sand-filled burrows containing granules, black phosphate pebbles and small lignite fragments. Unseen are planktonic microfossils (such as Globotrucana gansseri and Lithraphidites quadratus), and, to the unaided eye, disarticulated macrofossil horizons of bivalves

The latter form lag deposits exposed within the streamcut that are traceable for some distance. Lag deposits are common in the Late Cretaceous of North America and represent complex taphonomic histories that include multiple episodes of exhumation and reburial associated with sea level cyclicity. Although a work in progress, four or more distinct facies within the Navesink have been identified using these litho- and biofacies horizons that correlate to sequence boundaries and unconformities.  

Standing directly in Big Brook's stream, this cut bank is within the Navesink Formation with a portion of the overlying Red Bank Formation. The voids are where large clusters of bivalves have avulsed (or were excavated) from the glauconitic matrix. 

Iron-rich mineral seeps help to identify bedding interfaces. Note the myriad of overlapping vertical and horizontal burrows exposed within the bank. The extensive infaunal bioturbination is very evident. Not one visitor to Big Brook that I've seen has taken the time to study the streamcuts through the Navesink. There's a great story to be told in the banks, not just by what's washed into the bed!

Close-up of a heavily bioturbinated and water-saturated Navesink bank with an iron-rich mineral seep.

The fossil fauna of Big Brook is in keeping with a thriving shelf environment. The following is a small sample of its bounty discovered on four visits to the brook east of the Hillsdale Bridge. 

It's easy to overlook small fossils from the brook's mud and gravel bed, especially tiny shark and fish teeth, some of which are a barely 2 mm in diameter. The screen affords an opportunity to patiently inspect your excavated sample. Note the camouflaged remnants of a Gyrphaed scallop shell, a Cephalopal belemnite rostrum and a Squalicorax shark tooth below.

A gravel bar alongside the streambed of Big Brook

Unquestionably, the major attraction at Big Brook is shark teeth, and there are many varieties to be found. To the geologically-uninitiated, they appear incongruous to the modern landscape. Their abundance is a testimony to the richness of the Late Cretaceous ecosystem. 

Shark teeth are well preserved, whereas their skeletal remains being cartilaginous are not, with the exception of an occasional vertebral centrum. All the dental specimens, particularly the radicular structures, being less calcified and more porous, are stained by iron derived from the host sediment. Some permineralization of the teeth has occurred during burial, fossilization and diagenesis (alteration induced by chemical and physical processes mediated by water and stopping short of metamorphism). 

Scapanorhynchus ("spade-snout") is an extinct genus of shark from the Cretaceous. Often referred to as the anatomically similar goblin shark, which is distinct enough to have been placed within its own genus, Scapanorhychus had an elongated and flattened snout with awl-shaped teeth suited for tearing flesh and seizing fish. 

S. texanus is the species that frequented the Atlantic shelf in the Late Cretaceous. The 35 mm long anterior tooth (below) is sigmoidal in shape viewed from a lateral aspect with prominent striations running from the root to the apex of the crown. It has a bulbous lingual cingulum (facing the viewer) between the furcation of the two roots, which have a prominent length. Two small, opposing cusplets are variably found on anteriors at the cemento-enamel junction (where the crown meets the root), while posterior teeth exhibit heterodontic variability, although shark teeth are generally homodontic - of the same or similar morphology.

A 35 mm long Scapanorhynchus anterior tooth

Some degree of difficulty can exist in identifying shark teeth from Archaeolamna kopingensis from Cretlamna appendiculata (lower left below). Likely the latter, both genera have teeth that are robust with heavy, triangular side cusps and a thick, bi-lobate root with a deep U-shaped furcation.

Squalicorax (lower right) is a genus of an extinct Cretaceous lamniform shark related to the Great White and Goblin sharks. Its blade-like teeth possess a distinctively curved and serrated crown with a prominent notch on the mesial aspect. Its bi-lobate roots are separated by a shallow furca. Squalicorax ("crow-shark") was both a coastal predator and scavanger, as evidenced by teeth having been found embedded within the metatarsal of a hadrosaur, obviously non-marine indigenous. The species is likely S. kaupi.

Shark teeth from Archaeolamna-Cretolamna (?) and Squalicorax

Here are a few more examples of the many shark and fish teeth found at Big Brook. The abbreviated radicular length assists in the exfoliation of teeth under stress, which are replaced within a week by a seemingly endless supply of unerupted teeth. With the assumedly large number of sharks feeding on the shelf in the Late Cretaceous, this accounts for the large number and diversity of teeth recovered from Big Brook. Many of the bones recovered from the seafloor show fossil evidence of shark predation and scavenging and bear the distinctive teeth marks of Squalicorax's serrations. Likewise, many serrations and cusp tips of shark teeth exhibit signs of wear and chipping related to lifestyle and behavior.

Top Row: Squalicorax, Odontaspis, Archaeolamna and Scapanorhynchus.
Bottom Row: Two Squalicorax, Four unidentified and Enchodus.  

On the left is one-third of a vertebral centrum of a shark with its characteristic concentric rings and saucer-like depressed center. On the right is possibly a vertebral centrum of a ray, also a chondrichthyan. 

Vertebral centra from a shark and a ray

Enchodus (upper right tooth above) is an extinct genus of small to medium-size bony fish in the Late Cretaceous. Thought to be a highly predatory species, it possessed fang-like teeth in the anterior, more conventional posterior teeth and a compliment of palatine teeth as well.

First appearing in the Jurassic, belemnites are Mesozoic molluscs and members of an extinct order of the class Cephalopoda ("head-foot") that superficially appear squid-like. They possessed 10 equi-length arms studded with small inward-curving hooks used for grasping prey but lacked the pair of specialized tentacles present in modern squid. Uniquely, they possessed hard internal skeletons (below) - not hydroxylapatite of phosphatic bone - composed of calcium carbonate (calcite) in the form of a bullet-shaped rostrum or "guard." 

Located on the posterior aspect and often mistakenly assumed to be anterior for propulsion through water, the rostrum (diagram) was attached to a chambered, conical shell called a phragmocone, and that to the tentacular head of the cephalopod. Based on the behavior of extant lifeforms, it is assumed that belemnites were powerful swimmers and active predators. 

The rostra found at Big Brook are plentiful and easy to spot but generally fragmented. Close inspection of a rostrum in cross-section shows its internal structure to be of non-uniform, radiating concentric crystallites that are interpreted as growth rings. Early colonists suspected they formed when lighting bolts struck the ground, hence they are referred to as "thunderbolts." Locals refer to them as "bullets."

Calcitic rostra from belemnites

Diagram of a belemnite from

Exogyra is an extinct genus of saltwater oyster, a common marine bivalve mollusc, that lived in great abundance within the benthic zone (just above, at and below the sediment surface) of the warm Cretaceous sea. Five species have been reported from New Jersey (C. cancellata, C. costata, C. erraticostata, C. spinifera and C. ponderosa), some of which are found at Big Brook. Exogyra and pycnodonte oysters are preserved in great numbers within the Navesink Formation's muddy glauconitic sands of Big Brook, typical of an outer shelf environment. Assemblages of the bivalves Exogyra, Pycnodonte and Agerostrea form biofacies horizons within the Navesink.  


Another extinct Cretaceous saltwater oyster in the same family as exogyra, Pycnodonte is also a bivalve that is well represented at Big Brook. Many of the upper valves (referred to as "left") preserve the original shell coloration in the form of reddish brown radial bands, which are often discontinuous or offset indicating growth lines. The upper valve is strongly convex with concentric growth rings. Pycnodonte can reach up to 10 cm across.

Many modern oysters fall victim to predation from crustaceans such as lobsters and crabs, and gastropods. The oyster might survive the invasive attempts by continually accreting new shell layers. Back in the Cretaceous, the predatory sponge Cliona cretacica created trace holes by boring into Pycnodonte's shell (lower left). The shell on the right is the lower (or "right") valve of Exogyra.

Also an extinct genus of Late Cretaceous fossil oyster, this bivalve was prominent within the Navesink beds. It's semilunar shape and highly recognizable scalloped edge are characteristic. Along with Pycnodonte, it served as a biofacies assemblage horizon. 

Inoceramus ("strong-pot") is also an extinct species of bivalve, a saltwater clam that resembles an extant oyster. It had a worldwide distribution during the Cretaceous that included the Western Interior Sea way as well as coastal regions, the Atlantic included. Its prisms of calcite confirmed it with its typical pearly luster. Inoceramus, along with the bivalves previously mentioned, are found in lag deposits that weather into the brook and provide biostratigraphic facies recognition.

Common to the Late Cretaceous shelf's ecosystem were various arthropodal crustaceans - lobsters, crabs and shrimp - that left fragmented remains of their dorsal exoskeletal carapaces, pincers and claws. Many of the specimens may be molts rather than the remains of the parent lifeform.

The claws of the callianassid crustacean Callianassa are often preserved within infaunal burrows and, to the astute observer, can occasionally be spotted in situ within the stream banks. Many coprolites (fecal pellets) bear a striking resemblance to exoskeletal remains in terms of glossiness, black color and similarity in shape. Exoskeletons often possess a marked symmetry, have sutures between fused segments and have surface rugosities distinctive of arthropods (bottom row, far right). 

Top row are artifacts; bottom row are remnants of crustacean carapaces and claws.

Typical of a coastal shelf ecosystem, many invertebrates such as worms, digging bivalves and shrimp plied the seafloor and burrowed into the shelf's sand and mud seeking food and protection from predators. Callianassa is a genus of "mud" or "ghost" shrimp common to the shelf fauna that reinforced their burrows with fecal pellets to prevent collapse. In time, the burrows filled in with sand and became iron-cemented, which is what I believe are demonstrated below. The pointed specimen at the right is a belemnite guard. 

Trace fossils such as this - also called ichnofossils - are geological records of biological activity. They are impressions created on the surface and tunneled into the substrate of the seafloor. Ophiomorpha is a trace fossil classification or ichnotaxon of a burrowing organism in a near-shore environment. Callianassa is considered to be the best-known modern analog for this burrow. Trace fossils also include the organic digestive fecal remains or coprolites left behind by lifeforms, which are also found at Big Brook. 

I suspect that the following specimen, displayed from three perspectives, is a cross-section of a small sand-lined burrow - a remnant of a marine organism that lived on and within the seafloor. On the left, a small circular entry on the seafloor leads to the burrow; the middle photo shows the lobate burrow from below; and on the right, the sandy substrate and burrow are visible from a lateral perspective. Other interpretations of this specimen are welcomed.

Artifact or ichnofossil? The following specimens were extracted from Big Brook's streambed. The specimen on the left appears to contain an anastomosing network of iron-cemented burrows enveloping an oyster shell. On the right, a small section of cemented quart sand is enveloped by a similar burrow with extensive, irregular branching. Other interpretations?

Note the morphological similarities of Ophiomorpha from the Upper Cretaceous Blackhawk Formation of Utah to the burrows found at Big Brook.

There are many specimens or artifacts at Big Brook that defy any attempt at identification. Iron within the Navesink can cement the clayey and sandy substrate together into strangely shaped concretions. Often the result is a "fossil" that is totally inexplicable, many with curious symmetrical holes running entirely through them. Some resemble vertebral centra with small foramina, and others appear as if man made. Again, any other interpretations?

Here's the belemnite rostrum (pictured above) that has begun to acquire a surface coating of gravelly iron-cemented material. One can envision, that once totally encrusted, it might defy identification unless visualized in cross-section.

The tracks of a deer on the mudflats likely go unnoticed by the majority of visitors that come to the brook as do the criss-crossing maze of horizontal burrows. The latter is reminiscent of the coastal shelf some 70 million years ago. 

Burrowing of marine substrates was not unique to the Cretaceous. Horizontal and vertical burrowing has been going on throughout the Phanerozoic beginning in the Cambrian with the Burgess Shale type-biota. In the latest Precambrian, largely horizontal mining of benthic surfaces has been identified amongst the Ediacaran biota. It is believed that vertical subsurface excavation (whether for protection or to feed) in the Cambrian reworked the seafloor to the extent that it disrupted the cyanobacterial mat to which the Ediacara biota attached and thrived. With the coming of the "substrate" or "agronomic revolution", it is thought that three-dimensional bioturbination might have led to their demise.  

A heavily bioturbinated and deer-trampled mudflat alongside Big Brook

Most assuredly, there's a lot to experience and comprehend in the "piddly little dribble" of Big Brook.

Bedrock Geologic Map of Central and Southern New Jersey by James P. Owens et al, 1998.
Bedrock Geologic Map of of the Freehold and Marlboro Quadrangles, Middlesex and Monmouth Counties, New Jersey by Peter J. Sugarman and James P. Owens, 1996.
• Big and Ramanessin Brooks by the New York Paleontological Society, Field Trip 2002. 
Callainassid, Burrowing Bivalve, and Gryphaeid Oyster Biofacies in the Upper Cretaceous Navesink Formation, Central New Jersey: Paleoecological Implications and Sedimentological Implications by J.B. Bennington et al, Department of Biology, Hofstra University.
• Cretaceous Fossils of New Jersey - Part I by Horace G. Richards et al, 1958.
Cretaceous Stratigraphy of the Atlantic Coastal Plain, Atlantic Highlands of New Jersey by Richard L. Ollson, Department of Geological Sciences, Rutgers University, GSA Centenial Field Guide-Northeastern Section, 1987.
Geology Map of New Jersey, Department of Environmental Protection, Geological Survey, 1999.
• Greensand and Greensand Soils of New Jersey: A Review by J.C.F. Tedrow, 2002.
• New York Paleontological Society. Established in 1970, individual and family memberships are open to all, regardless of education or previous experience, It's a fantastic way to visit Big Brook and many other fossil-collecting localities in the Northeast with an enthusiastic and well-informed group of amateurs and professionals. Meetings are held in the American Museum of Natural History in New York City and includes an outstanding newsletter. Visit them here to join. Yes, I am a member.
• Paleocommunities and Depositional Environments of the Upper Cretaceous Navesink Formation by J. Bret Bennington et al, Department of Geology, Hofstra University, 1999.
• Paleontology and Sequence Stratigraphy of the Upper Cretaceous Navesink Formation, New Jersey by J. Bret Bennington, Hofstra University, Long Island Geologists Field Trip, 2003.
Pictorial Guide to Fossils by Gerard R. Case, 1992.
Roadside Geology of New Jersey by David P. Garper, Mountain Press Publishing Company, 2013.
Shell Color and Predation in the Cretaceous Oyster Pycnodonte Convexa from New Jersey by J. Bret Bennington. Hofstra University.
Summary of Lithostratigraphy and Biostratigraphy of the Atlantic Coastal Plain by Richard K. Ollson, Rutgers University.
Uppermost Campanian-Maestrichian Strontium Isotopic, Biostratigraphic and Sequence Stratigraphic Framework of the New Jersey Coastal Plain by Peter J. Sugarman et al, GSA Bulletin, 1995.