WHERE ARE WE?
The norm rather than the exception is that successive strata do not always form in an uninterrupted manner. Interruptions or cessations in deposition result in a geologic contact or inter-stratal boundary between rock layers. It's a "temporal gap" between an older surface that has been subjected to erosion and an overlying, younger one that experienced a hiatus in deposition. Thus, an unconformity is a buried surface of missing strata, unseen until exposed.
They vary in type depending on composition of the juxtaposed layers and occur globally on our ever-evolving planet. They can be thousands or even millions of years in duration, if not more. In Grand Canyon, there are so many "time-gaps" (at least 14 disconformities) in the mile-high Paleozoic record that the sum of missing time is greater than that preserved, although there is one 'great' unconformity that exceeds all others.
It's a profound break in the Earth's rock record. Typically, it resides above a denuded (eroded and exposed) Precambrian basement of igneous and metamorphic rocks and below flat-lying sedimentary rocks deposited at sea level during the Paleozoic (often at the base of the Cambrian system) or Mesozoic. The temporal magnitude of the "buried erosion surface" is governed by the age of the juxtaposed strata, which can vary greatly.
MISSING TIME WITHIN THE GRANITE GORGE
With walls comprised of slabs of Tapeats Sandstone, the Great Unconformity resides just above the side canyon's river cobbled floor. Go there: 36°14'24.43"N, 112°28'21.10"W
|West-East Cross-Sectional View of B&R Tilted Frenchman Mountain|
Modified from geoscience/unlv.edu/pub/rowland/virtual/geology.html
|Facing North Toward the Painted Wall in Colorado's Black Canyon of the Gunnison|
Go there: 38°34'32.71"N, 107°45'6.94"W
|Great Unconformity of the Saint Lawrence Lowlands|
Here it is again northwest of the Adirondack dome. Go there: 44°28'2.29"N, 75°45'47.64"W. It's also in the Lake Champlain Lowlands here: 3°25'44.05"N, 73°28'10.22"W
|Mt. Moran and Jackson Lake in Grand Teton National Park|
Go there: 43°50'1.08"N, 110°46'36.77"W
|Wayne Photographs the Great Unconformity on the South Side of the Highway|
|Cross-Section of an Idealized Marine Trangressive Sequence onto a Continental Shelf|
Modified from marlimiller.com
|Changes in Seafloor Biota across the Precambrian-Cambrian Boundary of the Great Unconformity|
Modified from The Rise of Animals by Mikhail A. Fedonkin et al, 2007.
|Julia daintily perched on the Great Unconformity|
Higher in the four-member, beach to offshore Flathead sequence bedding appears thicker and increasingly interbedded and cross-bedded perhaps as longitudinal bars migrated their locale.
|The Archean-age Crystalline Walls of Shoshone Canyon and Greenish Shoshone River|
It's appearance below the Great Unconformity has a familiarity that can be globally observed.
|Shoshone Canyon Facing West and the 'old' Cody Road|
With permission of Mark Fisher from GeoWyo.com (here).
|Generalized Bedrock Geologic Map of "Cody Country"|
The Eocene-age volcanic Absaroka Range dominants the landscape to the west of the Laramide compression-formed Rattlesnake Mountain anticline.
Modified from Wyoming State Geological Survey
|Confluence of North and South Forks in the Region of the Future Reservoir around 1906|
|Distinguished Portrait of Buffalo Bill from 1911|
From the U.S. Library of Congress in the Public Domain
|Poster of Buffalo's Bills Wild West Shows from 1908|
|Dizzying View into the Narrowest Aspect of Shoshone Canyon|
The dam's spillway consists of a rock weir, an intake channel and a tunnel. It allows the controlled release of water into the Shoshone River. An original powerplant , addition 1992
|Overturned Forelimb of the Western Rattlesnake Anticline|
|Fluvial Processes of Antecedence and Superposition|
In both cases, the river flows from right to left across a subdued landscape and maintains it s course.
With permission, image from Carving Grand Canyon by Wayne Ranney, 2012. See references.
|Buffalo Bill Center of the West in Cody|
Me, Buffalo Bill and Wayne
Go there: 44°31'28.71"N, 109° 4'26.00"W