Abstract: A Mid-Mississippian Episode of Flexural Loading in the Antler Foreland, Eastern Great Basin

N. J. Silberling, K. M. Nichols

Among the Mississippian strata of the Antler foreland in the eastern Great Basin, sequence-stratigraphic analysis shows that only those of Osagean to early Meramecian age are consistent with the depositional patterns predicted for an episode of flexural loading. Older discrete episodes of flexural loading may be represented by the Upper Devonian to lowermost Mississippian Pilot Shale, but much of the record for this has been obliterated by subsequent tectonism.

Mid-Mississippian foredeep fill constitutes the Diamond Range sequence (DRS) of Trexler and Cashman (1991). The hemipelagic initial deposits of the DRS formed during the period of flexure-induced subsidence before the encroaching allochthon became the source for most of the clastic foredeep fill. Imbricates of the Roberts Mountains thrust system were active throughout filling of the foredeep by pelitic rocks and chert-clast sandstones and conglomerates. Hypothetical flexural-extension fault blocks on the distal flank of the foredeep (or proximal flank of the peripheral bulge) are called upon as the source of coarse debris flows of pre-foredeep rock types (lower Joana limestone, Pilot Shale, and Devonian carbonate rocks) that occur locally in the hemipelagic lower part of the DRS.

Backbulge deposits correlative with the foredeep fill belong to the Sadlick sequence (SA) and the third-order lower part of the Maughan sequence (LMA) in easternmost Nevada and western Utah (Silberling, et. al., 1993; 1995). The tectonically controlled lowstand systems tract of SA is composed of peritidal carbonate rocks that aggraded the subsiding backbulge area. Maximum flooding of the backbulge initiated the Delle phosphatic event, and highstand SA deposits onlapped westward onto the peripheral bulge. The LMA grades upward from below wave base black shales to shoal-water crinoidal grainstone.

Strata immediately above the DRS and LMA sequences, on either side of the peripheral bulge, have been assigned to endothyrid zone 16 of late Meramecian age. Although not always so interpreted, the biostratigraphic record is consistent with all lateral parts of this mid-Mississippian flexural-loading system having been formed during the same time span.

AAPG Search and Discovery Article #90959©1995 AAPG Rocky Mountain Section Meeting, Reno, Nevada