Evolution of Yucca Mound Complex, Late Pennsylvanian Phylloid-Algal Buildup, Sacramento Mountains, New Mexico

Abstract

The Late Pennsylvanian Yucca Mound complex is a carbonate buildup on the western side of the Sacramento shelf facing the Oro Grande basin, in south-central New Mexico. This complex is an accumulation of algal and foraminiferal boundstones and phylloid-algal packstones and wackestones which have been subjected to several periods of subaerial exposure. The growth history of this organic complex is related closely to late Paleozoic cyclic sedimentation in a tectonically active area, and is thought to be the result of progressive offlapping growth stages down the western flank of the active La Luz anticline. The oldest beds exposed in the complex (plumose masses of algal and foraminiferal boundstone) formed within wave base on the crest of an earlier organic buildup. The crest of the older buildup was subaerially exposed while a thick accumulation of carbonate mud replete with the phylloid algae, Ivanovia, started forming against its western (seaward) flank. As sea level rose and submerged the earlier mound, the phylloid-algal accumulation kept pace with sea-level rise and onlapped the older mound, despite a few brief interruptions. When the phylloid algae built up into wave base, and attained its climax stage of evolution, a typical shallow-water facies developed. This distinctive rock type on the preserved, rounded mound top contains abundant forams, both encrusting and mobile chambered forms, together with fish remains and conodonts. A bedded detrital flank facies, composed of broken phylloid-algal material, and concentrated remains of forams, fish, and conodonts that inhabited the upper mound, is present on the shelfward (lee) side of the complex. Abundant, algally coated skeletal grains characterize the flank facies, which is easily recognizable elsewhere within the area, mainly in mound-flanking beds. When sea level dropped, Yucca Mound growth ceased, and smaller boundstone accumulations were formed down the offshore flank of the main mound complex. The varied growth history of this particular organic buildup demonstrates just how much opportunity existed for subaerial and littoral diagenetic processes to act on and to modify the varied rock facies common in late Paleozoic phylloid-algal buildups.