ABSTRACT: Cyclostratigraphy of Dolomitized Ramp Deposits in the Middle Devonian Simonson Formation, Eastern Great Basin

ELRICK, MAYA, University of New Mexico, Albuquerque, NM

The Middle Devonian Simonson Formation and its temporal equivalents of the eastern Great Basin (Utah-Nevada) were deposited on a westward-dipping ramp. Inner through outer ramp deposits are completely dolomitized and are composed of meter-scale upward-shallowing peritidal and subtidal cycles (104-105 yr durations). Peritidal cycles consist of dark gray, subtidal burrowed dolosiltite/dolomudstone, burrowed skeletal dolowackestone, or LLH stromatolites, which are overlain by light gray, intertidal/supratidal thick laminites, and cryptalgal laminites with some regolith-brecciated tops. Up to 40% of these cycles are symmetrical; i.e., cycle tops are overlain by thin intervals (<30 cm) of tidal-flat facies that grade upward into subtidal facies, then shallow back into cycle-capping tida -flat laminites. Subtidal facies within peritidal cycles may contain replaced evaporite nodules or intraformational solution-collapse breccias with fitted fabrics; the intensity of collapse brecciation within individual cycles decreases upsection into undisturbed tidal-flat facies. Subtidal cycles are composed of nodular-bedded skeletal dolowackestone/packstones with multiple hardgrounds, overlain by domal stromatoporoidal biostromes, which may be capped by well-sorted skeletal to peloidal dolopackstone/grainstones.

Systematic changes in the thickness of meter-scale cycles have been utilized along with regional trangressive-regressive facies patterns to define several third-order (1-3 m.y.; 50-150 m thick) depositional sequences that can be correlated across the platform. Relatively thick subtidal cycles characterize transgressive phases of sequence deposition, whereas thin peritidal cycles occur during third-order regressions and lowstands. Utilizing the vertical changes in meter-scale cycle thickness and cycle types to identify and correlate larger scale sequences is particularly useful for platform successions that lack diagnostic fossils for biostratigraphic correlations.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)