ELRICK, MAYA, Earth and Planetary Sciences, University of New Mexico, Albuquerque, NM

ABSTRACT: Development of Subaerially Exposed, Transgressive-Prone, Meter-Scale Cycles in the Middle Devonian Simonson Formation of the Eastern Great Basin

The Middle Devonian Simonson formation (220-530 m) of Nevada and Utah was deposited along a westward-facing distally-steepened ramp, four 3rd-order depositional sequences have been recognized from regional transgressive-regressive facies patterns, thickness variations of component meter-scale upward-shallowing cycles, and by variations of subfacies withincycles. Sequence boundaries, systems tracts, and maximum flooding intervals can be correlated (approx.) 225 km along depositional strike and >60 km downdip.

The majority of meter-scale upward-shallowing cycles (50-120 k.y. durations) within the sequences are peritidal in nature; subtidal cycles (composed wholly of subtidal facies) occur only in transgressive systems tracts. Peritidal cycles are composed of poorly fossiliferous, shallow subtidal facies overlain by intertidal/supratidal dolomitized algal laminites. Cycle tops commonly display subaerial exposure features such as karsting, brecciation, and laminated argillaceous sediment infilling dissolution voids which extend up to 1 meter below cycle caps. Subtidal cycles are composed of deeper subtidal, nodular dolowackestone-mudstones overlain by stromatoporoid boundstones; no evidence of subaerial exposure is observed. The occurrence of karsted tidal-flat caps indicates relative falls s a level below the platform surface. This combined with the occurrence of subtidal cycles precludes autocyclic processes as a mechanism generating the upward-shallowing cycles, instead short-term sea-level fluctuations bestexplain cycle development.

Approximately 50% of the peritidal cycles are transgressive prone; i.e., they display .05-1.5 m-thick deepening-upward trends at cycle bases, followed by shallowing-upward trends to cycle tops (symmetric cycles). Transgressive-prone peritidal cycles occur in all portions of transgressive and highstand systems tracts, indicating their development was not controlled by changes in 3rd-order accommodation space. The abundance of basal transgressive facies is unique in that it indicates minimal to absent lagtimes/depths before the onset of carbonate accumulation after platform emergence. It is likely that the preservation of the transgressive facies was due to low wave/current activity which allowed sediment to accumulate during short-term sea-level rises.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.