Thursday, January 20, 2011

Lithified Sand Dunes of the Ancient Bahamian Landscape


On a recent vacation to the Bahamas
, Paradise Island in particular, while the rest of my crew was swimming, reading and kicking back, I did some exploring down beach and out onto a narrow "rocky" spit of land. I was surprised to find that the spit was a platform composed of sand dunes. Not only were they lithified but cross-bedded, reminiscent of the eolian Coconino and Wingate Sandstones on the Colorado Plateau, but on a vastly smaller scale.

A LITTLE BACKGROUND ON THE BAHAMIAN ISLANDS
Positioned a mere 50 miles off the coast of Florida at its nearest point, the Bahamian Islands, of which there are 700, form a northwest-southeast trending archipelago. The climate of the region is sub-tropical with hot summers, warm temperate winters and an average yearly rainfall of about 30 inches. The islands of the Bahamas rest on a shallow carbonate platform, which during the Pleistocene, had been intermittently exposed and submerged in conjunction with glacially-induced high and low sea level-stands. Glacial maxima favored lower sea levels that exposed bank sediment. In turn, this favored eolianite deposition which possessed the capacity for lithification under the right circumstances.

This is a Google Earth image taken from about a 15,000 foot-altitude showing the location of the lithified dunes on Paradise Island, and showing the relationship to much larger New Providence Island and its populated capital city of Nassau. The total length of the platform measured about 1/5 of a mile and the greatest width was 150 feet.
It's highest elevation above sea level is perhaps 20-25 feet. 

Location of the lithified dunes on Paradise Island. The spit is connected to the main body
of Paradise Island by a narrow neck of a sandy beach.


This extreme close-up is taken from a distance of about one foot. It provides a good view of the lithified dune's macroscopic structure. Although the surface of the dune is severely weathered, you can clearly make out its bedding planes and its oolitic composition.

INTRIGUING QUESTIONS
Interestingly and totally unanticipated (as an avocational geologist), the dune’s composition wasn’t the typical silica-sand variety (in the form of quartz) but instead a carbonate (a limestone). Upon close inspection, the sand grains had an oolitic (egg-shaped), spherical shape, like fish roe. Indeed, silica sand-dunes are typical of inland continental and non-tropical coastal settings, while tropical coastal settings possess sands of eroded limestone. How did the dunes lithify, while above ground (subaerially) or did they? And, how did the sand acquire its oolitic shape? Here’s the intriguing answer.

GENESIS OF THE LITHIFIED DUNES
The Bahamas are not of volcanic origin, typical of many of the Caribbean islands. There are no igneous and metamorphic rocks to be found. Shallow-water carbonates are ubiquitous, having formed near the surface for 200 million years. The Bahamas are a vast “carbonate factory,” producing sediment at a fairly rapid rate on a slowly subsiding crustal platform (keeping the water deep enough for the process to continue). Oolitic limestone is precipitated directly from sea water, although containing carbonate forms from other sources such as skeletal remains.

The sand dunes formed on land when global sea level fell during the Pleistocene Ice Age. As sea level rose and fell during each of four glacial-interglacial periods, new sediments washed up onto new beaches forming a new line of dunes with classic bedding planes and erosive bounding surfaces. Cementation of the dunes with calcium carbonate occurred both during interglacial-period, marine submergence and glacial-period, rainwater exposure by both crystallization and recrystallization. The process of converting the sediments to the rocks is called diagenesis.

Looking down the coast, it appears that several “fossil platforms” are on higher ground. During the Ice Ages, continental glaciers tied-up great quantities of water making global sea levels lower. This exposed more shoreline to undercutting-erosion. During interglacials, the melted glaciers freed-up water making global sea levels rise. This created wave-cut platforms above the normal level of the sea. Since the region exhibits no folding, tilting or faulting, we can safely assume that glaciation-induced subsidence rather than geological uplift is the only causative explanation for the “elevated” platforms.


 A fossilized tree and root structure preserved within the lithified dune
adds testimony to its origin as a terrestrial sand dune.
PHYSICAL AND CHEMICAL WEATHERING
Weathering is the breaking down of the Earth's rocks, soils and minerals through direct contact with the atmosphere. Weathering occurs in situ without "movement" and is not to be confused with erosion, which involves the movement of rocks and minerals by agents such as water, ice, wind and gravity. Physical weathering involves "breakdown through direct contact with atmospheric conditions such as heat, water, ice and pressure," whereas, chemical weathering involves the direct effect of atmospheric chemicals or biologically produced chemicals (Wikipedia).

The spit is essentially a narrow, rocky carbonate platform forming a small portion of the coast. It is evident here, in contrast to the neighboring beach itself, that morphologic change is a slow and gradual process dominated by physical,  biologic and chemical weathering processes. Tide, current and wave processes all yield change but not on temporal scales of hours and days compared to the beach. Both types of weathering can be found on the coastal carbonate platform but in varying degrees and at differing locations. The mechanisms yielding the various morphologies appear to be controlled by factors such as the position relative to sea level, the interface-distance between water and land, and the porosity and degree of cementation of the rock (which is undoubtedly directly proportional to its age) .  

PHYSICAL WEATHERING
On the oceanic margin of the spit, it has been eroded into cliffs which have been undercut everywhere by wave action forming wave-cut platforms that extend outward toward the sea. The most highly-dissected terrain was to be found in a zone that developed closest to the sea. In fact both physical and chemical weathering decreased as a function of distance from the edge of the platform.

An additional type of physical weathering includes haloclasty or salt crystallization which causes the disintegration of rocks when saline solutions seep into cracks and joints in the limestone. When the water evaporates, it leaves a residue of salt crystals behind. The salt crystals can expand up to 3 times their volume when they become heated, exerting pressure on the confining rock. It's reminiscent of the 9% expansion of water when it freezes. Salt crystallization can also occur when solutions decompose rocks, which likewise leaves a salt residue that can expand. This phenomenon is common in arid climates and along coasts.

It can readily be seen that physical and chemical weathering go hand-in-hand. On the platform, the delicately etched textures of the rocks were seen to develop within reach of frequent salt spray and are absent amongst identical rocks further away from the influence of the sea.  


Undercutting of the platform by wave action. The surface exhibits solution weathering.

 Further evidence of physical weathering and chemical dissolution contributes to the dramatic beauty.
The Pleistocene and Holocene-age limestones of the supratidal, coastal platform
are undergoing surficial meteoric diagenesis from weathering yielding "eogenetic" karst.


Watch the waves relentlessly breaking and eroding the coastal platform on the video below.

video

CHEMICAL WEATHERING
Rainfall is inherently acidic because of atmospheric carbon dioxide (although other atmospheric gases can be absorbed which may increase the acidity additionally). This produces a weak carbonic acid which leads to solution weathering on highly-susceptible rocks such as limestone. In addition, coastal platforms such as these are in the spray-zone. Over considerable time, the limestone undergoes chemical dissolution to the extent that its appearance becomes sharply-jagged with numerous voids, small excavations and holes, and razor-sharp edges. The holes tend to link up and gradually enlarge which gives the surface a pitted, honey-combed and drilled-out appearance. Also, kamenitza or solution pans tend to form which are shallow, rounded relatively flat-bottomed basins on exposed surfaces that develop via dissolution of limestone by standing water. These surface phenomena are generically known as karst. Subterranean karstic landforms (not the subject of this post) exist in the Bahamian tropics but  differ somewhat from traditional karstic landscapes formed in temperate climates.

Digressing briefly, classical karst terrains have distinctive landforms and drainage arising from greater rock solubility in natural water that is derived elsewhere. They are characterized by numerous caves, subterranean caverns, sinkholes, solution valleys, fissures and underground rivers and streams. Karst topography usually forms in regions of plentiful rainfall (cold and humid mid-latitude, temperate climates) where the bedrock consists of carbonate-rich rock such as limestone (CaCo3)  and dolomite (MgCaCO3), which is easily dissolved. Examples of classical karst terrains are the Dinaric Kras region (the type locality) of the Adriatic between Slovenia and Italy, and the Appalachian mountainous regions of the Mid-Atlantic States).

Most karstic features are created by carbonic acid (carbonation) which forms from the absorption of carbon dioxide (CO2) by rain (meteoric) water. Biological activity (such as plants, algae and lichen) can secrete acids that dissolve soluble bedrock. In addition, blue-green algae can produce a plant-generated surface karst (called phytokarst)  characterized by pitting and a sharp-edged, spongy lattice of ridges and pinnacles.    
The following is the main mechanism of calcium carbonate dissolution in limestone: Rain passes through the atmosphere picking up CO2 which dissolves in water. Once on the ground, the water containing the weak carbonic acid in solution passes through the bedrock and dissolves calcium carbonate.


Further attacks on the landscape occur as a result of fossilized plant-roots called rhizomorphs that once grew in the dunes long ago. Their roots may harden the soil around them via their secretions. Upon weathering, the resistant limestone can form thin, jagged edges. Biological weathering (biokarst or bioerosion) can further add to the jagged, etched and honey-combed effect from boring blue-green bacteria and invertebrate grazers (mainly molluscs such as gastropods), especially along the regions that are regularly wetted by waves and sea spray. Such plants produce acids and their filaments penetrate the rock promoting its disintegration.

It appears that distinct geomorphic zones exist on the platform that are discernible by their color, degree of  weathering and proximity to the land-marine interface.

 An extreme close-up of the razor-sharp, jagged and honey-combed surface on the platform.
This surface was virtually impossible to traverse safely in bare feet.
LET THERE BE LIFE
Not surprisingly, many creatures make their home in the intertidal zone of the platform which was teeming with life. Here are just a few inhabitants that I stumbled upon.

On the platform, a female polyplacophora (chiton) with 8-articulating shelly plates and her associated eggs are attached to the bottom of a shallow pool. Interestingly, the male chiton releases his sperm into the sea which finds (hopefully) a receptive egg-release. Various colorful gastropods (marine snails) were everywhere. Both chitons and snails are members of the Mollusc Phylum along with clams, mussels, oysters, squid and octopus.
A prickly-looking sea urchin, a member of the Echinoderm phylum (eg. starfish, sand dollars and brittle-stars),
bides its time.

A crab, a crustacean and member of the Arthropod Phylum (along with insects, spiders and extinct trilobites)

In summary, the Bahamas are largely a depositional landscape, unlike the more common, eroded landscapes of the continents, with their own unique carbonate signature. Both physical and chemical weathering can be observed on the platform that appear unique to marine coastal environments.

My casual stroll down the beach at Paradise Island turned out to be an unanticipated lesson for me in Bahamian dune composition, formation, lithification and weathering.  

P.S. Bahamian Landscapes by Neil Sealey is a great introduction to the geology and geography of the Bahamas with tremendous photos and illustrations!

2 comments:

  1. Jack: Great pictures! Thanks for your expert work today. Eric

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  2. Excellent observations and literature research for a stroll along the beach. I just returned from the Bahamas, and was intrigued by the intricately weathered limestone at or near sea level. Some of the locals call it "razor rock." I couldn't see how such dramatic textures could form in pure limestone, and presumed there had to be some content of silica - but there isn't. I think your comments about biologic activity and salt crystallization must be the correct explanation. Good pictures.

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