BACHTEL, STEVEN L., and STEVEN L. DOROBEK, Department of Geology, Texas A&M University, College Station, TX

ABSTRACT: Facies Transitions, Depositional Processes, and Sequence Stratigraphic Relationships along a Carbonate Ramp Slope-to-Basin Profile: Mississippian Lake Valley Formation, Sacramento Mountains, Southern New Mexico

Exposures of the Mississippian Lake Valley Formation in the Sacramento Mountains of southern New Mexico provide a three-dimensional view of slope-to-basin facies transitions along a carbonate ramp. The north-south trending Sacramento Mountains are oriented slightly oblique to depositional dip on the Lake Valley ramp. East-west trending canyons dissect the Sacramento Mountains and provide outstanding strike-oriented exposures of the Lake Valley slope. Dip-oriented sections are less common, but provide a cross-sectional view of the ramp's slope-to-basin profile. The Lake Valley outcrops allow detailed documentation of slope depositional processes and sequence stratigraphic relationships along a carbonate ramp.

Preliminary field investigations suggest the Lake Valley Formation can be subdivided into three, 2070 m thick, onlap-offlap sequences. Facies within transgressive systems tracts consist of: black phosphatic shale; nodular-to-thin-bedded, locally argillaceous, lime mudstone; and basal parts of downslope Waulsortian mounds. Transgressive facies were deposited in distal ramp, lower slope, toe-of-slope, and basinal settings (Caballero Formation, Alamogordo Member, Arcente Member). Facies within progradational highstand to lowstand systems tracts include coarse-grained, skeletal-lithoclast grainstone/packstone that were deposited in channeled upper slope to lower slope settings (Andrecito Member, Nunn(?)-Tierra Bianca Members, Dona Ana Member). Late highstand to lowstand deposits in the Ti rra Blanca and Dona Ana Members consist of channeled grainstone/packstone units that truncate bedded sediment gravity flow deposits of underlying upper slope facies. Late highstand to lowstand slope deposits also exhibit the following: 1) grain size coarsens and bed thickness increases upward; 2) channel depth/width ratios decrease downslope; and 3) facies change upward from distal to proximal sediment gravity flow deposits. Downslope mud mounds may have continued to aggrade during highstand to lowstand deposition if they were in the proper paleogeographic position. In addition, mound geometries probably were affected by slope erosion. Mud mounds in northern exposures have highly eroded margins (Teepee Mound, Deadman Branch Mound?), but are relatively unaffected to the south (Muleshoe Mo nd, Little Sugarloaf Mound). This probably reflects lower erosive power of sediment gravity flows downslope. Facies characteristics and stratigraphic relationships of late highstand to lowstand deposits in the two upper Lake Valley sequences may be characteristic of systems tracts on other carbonate ramps.

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