The Great Unconformity

Unconformities and basement nonconformities associated with the Great Unconformity as exposed in the Western US

The Ignacio Sandstone (of contested age, though likely Cambrian) unconformably overlies the Mesoproterozoic (1.433 Ga) Eolus Granite in the Needle Mountains, Colorado.


A closer vew of the nonconformable contact between the Ignacio Sandstone and the Eolus Granite in the Needle Mountains, Colorado. Note hematitic coloration in both basement and overlying sandstone.


Burrowed, mat-like siltstone and sandstone beds in the Ignacio Sandstone shortly above the Great Unconformity.


Spectacular curved burrow casts, possibly Gordia marina in the Ignacio Sandstone shortly above the Great Unconformity. Note the sharp contrast between dark purple hematitic sandstone and surrounding beige iron-free sandstone, suggestive of reductive bleaching and leaching of Fe3+ by chemically reduced aqueous fluids prior to exposure.


Numerous overlapping burrow casts, possibly Gordia marina in float shortly above the Great Unconformity, Needle Mountains, CO.


Rusophycus sp. (trilobite resting trace) in float shortly above the Great Unconformity in the Needle Mountains, CO. Together with the rest of the local trace fossil assemblage, suggestive of an early Cambrian ichnofauna.


Nearly orthogonal unconformable contact between the metamorphosed Paleoproterozoic sandstones and siltstones of the Uncompaghre Formation and the flat-lying Devonian Elbert Formation near Ouray, CO.


The Cambrian Sawatch Sandstone unconformably overlies Paleoproterozoic crystalline basement in a tributary of Glenwood Canyon, CO.


A thin layer of slope-forming red silststone of the Triassic Chine Formation, topped by cliff-forming Wingate Sandstone, unconformably overlies a Proterozoic basement of quartz-mica-kspar pegmatite, Ladder Canyon, CO.


Dartmouth graduate student Gailin Pease examines meter-scale books of mica in the Proterozoic basement quartz-mica-kspar pegmatite, Ladder Canyon, CO.


Cambrian Flathead Sandstone unconformably ovleries deeply weathered Archean basement near the mouth of Clarks Fork Canyon, Wyoming.


Cambrian Flathead Sandstone unconformably ovleries the Archean “Oldest Gneiss Complex” of the Wyoming Craton in a roadcut in the canyon of the Shoshone River near Cody, WY.


Cambrian Flathead Sandstone unconformably ovleries a deeply weathered outcrop of 2950 ± 5 Ma granite of the Archean Bighorn Batholith, wherein a thick grus develops around incipient boulders. Shell Creek, Bighorn Mountains, WY.