Print this pageAbstract: Devonian Sequence Stratigraphic Framework, Southeastern Nevada
John E. Warme, Alan K. Chamberlain
Outcrop sequence-stratigraphic analysis, coupled with surface gamma-ray profiles, provide significant tools for interpreting Nevada's complex subsurface geology. Vertical changes, such as abrupt facies shifts, regional unconformities, and surface gamma-ray intensities, mark significant sequence boundaries in Devonian rocks. Sharp transgressions typically mark the base of sequences, and relative sea level lowstands (accompanied in some cases by deep karst) mark the tops. Six major sequences generally conform to recognized formations: (1) Lower Devonian Sevy Dolomite and (2) Oxyoke Formation, (3) Middle Devonian Simonson Dolomite. (4) Upper Devonian Guilmette Formation and (5) West Range Limestone, and (6) Mississippian-Devonian Pilot Formation.
Stratigraphic breaks, recognized as sequence boundaries, help refine natural formation limits. For instance, a basal transgression with rip-up clasts, hummocky cross-stratification and burrows distinguishes the transgressive base of the Oxyoke Formation from the underlying laminated Sevy Dolomite. Some locations show another transgression separating Oxyoke quartzose carbonates from the relatively pure quartz Oxyoke Sandstone, a regional marker. Lowstand erosion with significant karst separates the coarsely crystalline upper Simonson Dolomite from the overlying fine-grained Fox Mountain Member of the Guilmette Formation. Some formations, notably the Simonson and Guilmette, internally exhibit bundles of upward-shallowing smaller-scale sequences that correlate over long distances.
Changes in surface gamma-ray intensities mark all scales of sequence boundaries. Typically, sequence bases show sharp decreases, then increase gradually upward to capping supratidal dolomites, probably reflecting an upward increase in wind-blown siliciclastics. Surface gamma-ray profiles, calibrated by parallel outcrop sequence-stratigraphic analysis, can be compared to gamma logs from wells in adjoining basins, and applied toward problems of formation identification, paleoenvironmental analysis and hydrocarbon potential.
AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994