The "Forgotten Dolomites" of Southern Nevada and Southeastern California: Vital Clue to Sequence Stratigraphic Interpretation of the Lower-Middle Ordovician Succession, Southern Great Basin

Martin Keller and John D. Cooper

Lower and Middle Ordovician carbonates (Pogonip Group and equivalents) of the southern Great Basin have received insufficient attention mainly because of pervasive, dolomitization and lack of calcareous body fossils, rendering them difficult to correlate with their better studied counterparts in central Nevada and western Utah. However, these rocks, which were deposited in inner miogeocline to craton-margin paleotectonic settings, reveal a variety of sedimentary textures typical of shallow subtidal to supratidal depositional conditions. Additionally, the stratal interval can be subdivided into 9 lithologic units that can be correlated regionally. Finally, different kinds of breccias, sand-filled grikes, and other karst-related features have been used to identify 12 major xposure surfaces within the succession. These surfaces are interpreted as type-1 sequence boundaries and are the basic elements of our proposed sequence stratigraphic framework.

A stratigraphic transect from the miogeocline (e.g. Nopah Range, CA) towards the craton margin (northern and eastern Spring Mountains, NV) shows a dramatic reduction in thickness of Ordovician section. The major loss of section occurs beneath a single surface, which separates units 8 and 9 in the miogeoclinal sections. This surface, which is the uppermost of a succession of surfaces within the lower Antelope Valley Formation, is characterized by paleotopographic relief, grikes, deep karst pipes (cutting down as much as 150 m), and regional truncation of underlying strata from the top downward. This surface is interpreted to be the Sauk-Tippecanoe megasequence boundary in this region and correlates well with the late Whiterockian lowest sea-level stand on the global Ordovician sea-leve curve.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California