Identification of Sequence Boundaries in Lower to Middle Devonian Platform-to-Basin Carbonates of the Eastern Great Basin
Lower to Middle Devonian carbonates (270-400 m thick) in the eastern Great Basin were deposited along a low-energy, westward-deepening carbonate platform. Four 3rd-order depositional sequences are recognized across the platform-to-basin transition based on deepening and shallowing trends in regional facies, stacking patterns of meter-scale cycles, and the stratigraphic distribution and intensity of subaerial exposure features. Sequence boundary zones (SBZs) are 2-16 m-thick along the shallow platform and are composed of exposure-capped cycles that exhibit upsection increases in the intensity of subaerial exposure features and the proportion of tidal-flat subfacies. The cycle-capping exposure features indicate that the shallow platform was affected by multiple episodes of ubaerial exposure rather than a single, long-lived event.
Along the basin and slope, the transition over tens of meters from fine-grained hemipelagic deposits to coarser grained slope turbidites, which may be capped by platform- margin grainstones, then back into hemipelagic fines indicates a decrease, then increase in 3rd-order accommodation space. No meter-scale cycles or evidence of subaerial exposure can be discerned indicating these deposits represent downdip correlative conformities. These basin/slope successions can be interpreted in several ways: 1) the SBZ or correlative conformity lies at the base of the slope turbidites, the turbidites and platform-margin grainstones (if present) represent the shelf margin wedge (SMW), and overlying hemipelagic fines represent the basal transgressive systems tract, or 2) minimum 3rd-order accommodation (SBZ development) is reflected by the deposition of slope turbidites, or if present, platform- margin grainstones and overlying hemipelagic fines represent an increase in 3rd-order accommodation related to SMW development. These different interpretations underscore the difficulties involved in sequence boundary identification in basin and slope environments particularly in outcrop studies of carbonate systems deposited when the combined rates of 5th through 3rd-order sea-level tall were less than basin/slope tectonic subsidence rates.
AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California