Bureau of Economic Geology, The University of Texas at Austin (www.beg.utexas.edu).
AAPG Annual Convention, Houston, Texas, April 9–12, 2006
Outer Ramp Carbonate Mudstone Channelization and
Outcrops of the Permian (Guadalupian) San Andres Formation in Last Chance Canyon, NM dissect low-angle, outer ramp mudstones of the Guadalupe-12 high frequency sequence. Measured sections, field mapping and 3-dimensional mapping using ground-based LIDAR document a channelized network of moderately bioturbated, thin bedded mudstones capped by a coral-sponge floatstone. This network displays relief of approximately 25 m and at least 5 channels are observable in the ~10 km2 outcrop area. Mudstone and floatstone facies form levees of the anastamozing channel network and they gradationally change facies into peloidal-skeletal packstones within the channel axes. Observed gradational facies changes suggest that most channels were the sites of continual down-slope currents and low density sediment gravity flows. These processes served to winnow mud from the axes and deposit it as thin-bedded levees in a system analogous to deep water siliciclastic channel-levee systems. Channel depth varies, at times with channel axis packstones overlying previously deposited levee mudstones, while in other cases channel axis deposits are not present and were likely eroded by currents or sediment transport. Overlying facies filling channel axis lows consist of peloidal packstones, bioturbated dolomudstones and silty mudstones. In contrast to siliciclastic systems, channel levees of this network hosted a significant faunal community and display a range of early lithification and compaction features that possibly resulted from silicification or microbial binding. Such features include boudined, nodular, intraclastic and mega-breccia fabrics. Brecciation occurred proximal to some channel axes, suggesting occasional high density flows bypassed the area or that levee slumping occurred.