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