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Description
Approximately 4,300 feet of Ordovician, Devonian, and Mississippian rocks crop out in the Portuguese Mountain area of the Pancake Range. The Ordovician is represented by mature sandstones, the Devonian by carbonate rocks, and the Mississippian by clastic rocks with minor carbonates. Most of the contacts between formations are not exposed. The structure of most of the area is interpreted as a gently folded syncline. This structure is inferred from outcrops of Devonian rocks which dip east in the western part of the area and dip west in the eastern part of the area. An alternative explanation is that the Devonian rocks have been emplaced by younger over older thrusting. Field relationships do not preclude either hypothesis. In the west limb of the suggested syncline the section is repeated by high-angle faulting. In the northwest part of the area rocks of Mississippian age are exposed in a possible eastward-overturned syncline which is thought to be related to a similar fold known in the area to the north. The Ordovician quartzite is exposed above a Devonian limestone in a fault block in the central part of the area as a result of older over younger thrusting. The area was in the Cordilleran miogeosyncline during Paleozoic time. Carbonate deposition was pre-dominant through the Devonian. In Mississippian time the Antler Orogenic Belt, to the west of the Portuguese Mountain area, shed a clastic wedge eastward into the miogeosyncline. Sedimentation is known to have been continuous locally in the Great Basin through Triassic or earliest Jurassic time, however in the Portuguese Mountain area no Paleozoic rocks younger than Mississippian are exposed. A Mesozoic deformation folded the Paleozoic rocks in the area. In Oligocene time the area received deposits of pyroclastic rocks, primarily welded rhyodacitic to dacitic tuffs of nuée ardente origin. A late Tertiary deformation produced the north-south trending ranges which now characterize the Basin and Range Province.
