Sequence stratigraphic analysis of Mississippian limestones in the San Andres and Sacramento Mountains, south-central New Mexico

Abstract

Mississippian (Kinderhookian-Chesterian) strata in the San Andres and Sacramento Mountains of south-central New Mexico comprise a series of vertically stacked, gently basinward-dipping carbonate ramp packages. Exposed strata are comprised of mainly outer ramp-to-basin lithofacies; inner ramp facies have mostly been removed by post-Mississippian erosion. Spectacular cliff-face exposures provide a three-dimensional view of stratal relationships and lithofacies distribution along the carbonate ramp-to-basin profiles. The Mississippian strata (Caballero, Lake Valley, and Rancheria Formations) can be subdivided into four depositional sequences (Sequences 1-4). Sequences 1-3 form three basinward-thinning wedges comprised of muddy transgressive systems tracts (TST) and coarse-grained, progradational systems tracts (PST). Lowstand systems tract (LST) strata in Sequences 1-3 are confined to local, discontinuous carbonate sand sheets or channels of Sequence 3. The basinward-thinning geometry of Sequences 1-3 is suggestive of an accretionary platform prograding into a starved basin. Sequence 4 is a basinward-thickening wedge comprised of very fine- to coarse-grained peloidal-skeletal strata (LST), muddy TST strata, and fine-grained peloidal and skeletal limestone (PST). The onlapping, basinward-thickening geometry of Sequence 4 is indicative of basin-fill strata deposited by sediment bypass onto the outer margin of an erosional platform. The Mississippian sequences in south-central New Mexico provide an outcrop analog for slope-readjustment along a relatively low-gradient carbonate ramp. Sequences 1-3 are interpreted as progressively basinward-prograding sequences that built depositional topography with each successive relative sea-level change. The cumulative effects of this progressive progradation caused the ramp margin to steepen until an equilibrium gradient was surpassed at the end of Sequence 3. Concurrently with subsequent sea-level fall, slope erosion and sediment bypass occurred and a submarine erosional surface formed (i.e., the slope-readjustment surface) on top of Sequence 3. Sequence 4 was then deposited as onlapping basin-fill stratal packages that returned the platform to its equilibrium grade and allowed progradation to resume by the end of Sequence 4. The updip part of the platform was later incised by Pennsylvanian fluvial channels during post-Mississippian sea-level fall.