Origin of "Fall-in" Bed Geometry, Capitan Reef Margin - Ideas from Stratigraphic Computer Modeling
Paul M. Harris and James M. Borer
Stratigraphic computer modeling of the Yates and Capitan formations (Guadalupian, Permian Basin) provides insight to a possible origin for the "fall-in" bed geometry that is characteristic of outer shelf equivalents of the Capitan reef. Modeling suggests that "fall-in" beds occur at the shelf margin as a natural consequence of sea-level fluctuations across an accommodation gradient that is produced by a unique depositional profile (flat platform, submerged shelf-edge reef, steep margin), low hinged subsidence (combination of tectonic subsidence, compaction, and loading), and a "keep-up" carbonate factory. From the modeling, the "fall-in" cycles correspond with 4th-order, progradation-aggradation couplets in which the progradation distance is on the order of 0.25 to 0.5 km and the aggradation approximately 30-38 m, not unlike the pattern observed in the Guadalupe Mountains.
"Fall-in" occurs in the model during 4th-order, sea-level lowstands when accommodation space shifts seaward and off the previous carbonate bank. Littoral sandstones deposited during the lowstand mark the shoreline and updip limit of accommodation and inherit a strong seaward dip. Progradation is limited because the carbonate factory is decreased and lowstand siliciclastics could not prograde across the steep carbonate margin. As the next 4th-order sea-level rise slowly shifts accommodation back up the profile, there is a period of decreasing "fall-in" angle as the area in front of the previous carbonate bank is filled during aggradation and apparent onlap with each successive cycle exhibiting less of a "fall-in" angle. During the subsequent 4th-order fall, the shoreline jumps seaward f the aggrading margin and sets up the next "fall-in" scenario. Periods of greatest "fall-in" coincide with longer-term (3rd-order) lowstands.
Although the true origin of "fall-in" beds will no doubt remain in question, the results described herein do suggest the pattern of increasing then decreasing "fall-in" angle is at least a partly related to depositional topography and sea level fluctuations. Hinged subsidence continued to rotate beds seaward, but "fall-in" geometry was not simply a function of post-depositional rotation alone since there would be monotonically decreasing dips up section. Modeling also highlights variations in reef bathymetry due to sea level fluctuations. Although no reef exposure is predicted, there are times when the reef is in very shallow water and reef topography may have resulted in a restricted backreef setting.
AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California