New Mexico Geological Society Annual Spring Meeting — Abstracts

The upper member of the Horquilla Formation, exposed on the west face of Big Hatchet Peak is predominantly a limestone sequence measuring 230 meters thick.

Biotic constituents and high diversities imply that during deposition, this sequence was open to circulation of normal marine waters. Differences in preservation of biotic constituents in some beds suggest that those units contain fossil assemblages derived, in part, from adjacent sedimentary environments resulting in mixed fossil assemblages. In many cases the derived biota are alga or organisms regarded as contributing to shallow water sedimentation (e.g. Tubiphytes and Komia).

Rock types composing this sequence are principally wackestones with fewer grainstones and packstones. The abundance of wackestones suggests that deposition occurred largely in quiet waters of either a back reef, lagoonal setting or on a fore reef slope below wave base. Contained grainstones and pacKstones indicate greater levels of agitation that may have resulted from occasional lowering of wave base. Either storm events or lowstands of sea level can lower wave base.

Based on the time-distribution of algal constituents, this sequence appears to represent a shallowing-upward sequence. This is supported by the abundance of phylloid algal biostromes and Archaeolithophyllum in the upper units of the sequence. The sequence, with its contained sediments and biota, is interpreted as a transition from a lower-slope to upper-slope environment resulting from basinward progradation of a carbonate shelf. Aggradation brought the substrate into the photic zone allowing the growth of photosynthetic organisms.