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The AAPG/Datapages Combined Publications Database

AAPG Bulletin


Volume: 68 (1984)

Issue: 7. (July)

First Page: 954

Last Page: 954

Title: Structural Complexities that Control Localization of Mississippian Shale-Generated Oil Prospects in Eastern Great Basin, Utah and Nevada: ABSTRACT

Author(s): John E. Welsh, Sr.


Mesozoic anticlinoria and synclinoria determine gross regional surface and subsurface distribution of Mississippian source rock shales. The original thicknesses of the shales are preserved only in the synclinoria. Over anticlinoria and on their flanks, shales were not deposited or were thinned by Late Jurassic to Early Cretaceous younger-over-older denudation thrusting. The Chainman decollement is a widespread stratigraphic zone of shearing, which, in extreme cases, caused the Triassic, Permian, and Pennsylvanian sequences to be rotated and dropped down in a systematic "chaos" upon the Devonian Guilmette Limestone (e.g., Ferguson Flat, Ferber Flat, Copper Flat). The Pilot, Joana, Chainman, and lower Ely Formations were eliminated either entirely or partly by shearing. The Chainman decollement zone is intruded by 110 m.y.-old quartz monzonite and 35 m.y.-old quartz latites within many mining district areas.

In the synclinoria areas, post-Oligocene block faulting has produced a mosaic of horsts and grabens. In many places on the horst blocks, the Oligocene volcanics and the late Paleozoic sequence are removed by erosion, but in the graben blocks the entire Paleozoic and volcanic sequences are preserved in the subsurface. These graben blocks still contain complete subsurface sections of the Mississippian source rock shales. The block-faulted mosaic is outlined by an older northeast fault set that displaced the sub-Miocene volcanic terrane in the valleys and refracted through the Paleozoic bed rock of the ranges. A younger set of normal faults outlines the geomorphology of the mountain blocks, bajadas, and playas. Geomorphology, gravity, and magnetic surveys help define the suballuvial patt rn of the fault mosaic.

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Copyright 1997 American Association of Petroleum Geologists