An Unhurried Look at a Lower Ordovician Mound Horizon, Southern Franklin Mountains, West Texas

TLDR: In this paper, the authors examined the evolution of carbonate mounds in the McKelligon Canyon Formation (Cloud and Barnes subunit B1, interval 15) and the largest mound found, Lechuguilla Mound, in detail.

Abstract: Relatively small-scaled carbonate mounds are a conspicuous sedimentary feature within various stratigraphic horizons of lite Lower Ordovician El Paso Group, southern Franklin Mountains, west Texas. Mounds are best developed within the McKelligon Canyon Formation (Cloud and Barnes subunit B1, interval 15) and the largest mound found, Lechuguilla Mound, has been examined in detail in order to shed some light on the factors controlling and determining the morphology, petrology, and biotic sequence of the mound interval. Three distinctive growth stages can be recognized: (1) establishment of a pioneer colony, (2) the development into a mature biotic entity, and (3) development of a climax community. This sequential growth series may reflect a simple response to organism require ents with increasingly shallower depositional environments. Many of the McKelligon Canyon mounds are cut by channels filled with rather coarse calcarenites. It is believed that the channels are erosional features formed during repetitive intervals when mounds were subaerially exposed, eroded, and subsequently filled in with available skeletal debris. These erosional intervals are probably related to shifts in sea level stages occurring within a shallow intertidal to subtidal depositional environment. The Lower Ordovician mounds present in the southern Franklin Mountains of west Texas closely resemble the Middle Ordovician (Chazyan) mounds of northeastern New York and western Vermont in gross morphology, petrology, and basic biotic composition and sequence, although there are important temporal biotic differences when evolutionarily more advanced organisms occupied similar mound niches.