Depositional Cycles as a Shelf-to-Basin correlation Tool, Capitan Reef Margin - Ideas from Stratigraphic Computer Modeling
James M. Borer and Paul M. Harris
Correlations from the Yates shelf through the Capitan reef margin and into the Bell Canyon of the deep Delaware Basin (Guadalupian, Permian Basin) are not known with great detail due to limited biostratigraphic control and the inability to trace beds or time lines from the cyclic shelf deposits, through the massive reef and foreslope, and into basinal siliciclastics. The presence of a strong hierarchy of depositional cycles on the shelf and in the basin suggests that cycles may be useful as a correlation tool.
A Yates shelf-to-basin correlation is proposed based on matching 3rd- and 4th-order cycles recognized with a stratigraphic computer model and on cores and outcrop. We believe thick, sand-dominated cycles were deposited in the basin during long-term lowstands; whereas, thin, organic-rich lime mudstone-dominated cycles were deposited during long-term highstands. The correlation scheme hinges on equating 3rd-order condensed intervals of organic mudstone in the basin with thick carbonate "banks" deposited at the lower and upper Yates boundaries. Within this tentative large-scale framework, there is a reasonably good match between 4th-order shelf and basin cycles. Some ambiguity remains, however, due to the possibility of missed cycle beats in both the shelf (thin or no cycles, erosion) and basin (no or minor condensed mudstones resulting in sand on sand, erosion) during long-term lowstands.
Computer modeling suggests siliciclastics bypassed the Yates shelf and were delivered to the basin as turbidites during high-frequency (5th-order) lowstands, but the amount of bypass during any lowstand was controlled by longer-term cycles. The large-scale basinal cycles are slightly more symmetrical than their shelf counterparts; typically showing a gradual increase in clean sandstone and decrease in siltstone and organic mudstone away from a condensed interval, and the converse towards the next condensed interval. This symmetry is considered to represent bypass deposition during first an increase and then decrease in the rates of relative sea-level fall. Fourth-order (400-k.y.) "condensed intervals" within the Yates basinal equivalent contain multiple organic mudstones separated by turbidite sandstones deposited during 5th-order (100-k.y.) lowstands. Stratigraphically above and below the Yates equivalent interval, long-term highstands limit high-frequency bypassing and result in stacked organic mudstones that represent major 3rd-order condensed intervals.
AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California