Constituent composition of Early Mississippian carbonate buildups and their level-bottom equivalents, Sacramento Mountains, New Mexico

Abstract

Abstract The Alamogordo Member of the Lake Valley Formation (Lower Mississippian-Tournaisian to early Viséan) was deposited on a homoclinal ramp at depths below the fair-weather wave-base and mainly below the photic zone. Depth increased toward the present southwest, and the level-bottom biota of the Alamogordo Member is present in assemblages that parallel the depth gradient (Jeffery & Stanton this volume). Waulsortian-like mounds are abundant in the Alamogordo, Nunn and Tierra Blanca Members of the Lake Valley Formation on the distal portion of the homoclinal ramp. These mounds contain essentially the same biota as the Waulsortian buildups of Europe. Alamogordo mounds are low, tabular features composed mainly of spiculiferous micrite, while Nunn and Tierra Blanca mounds are more domical and are composed largely of fenestrate bryozoan cementstone. Lithology and biota of coeval, level-bottom beds and mounds differ significantly because of habitat differences (cavities and hard substrates in and on mounds), and because of taphonomy, an example of which is the preservation of fenestrate bryozoan sheets in radiaxial fibrous calcite cements on mounds as compared with comminuted and dispersed fenestrate hash in level-bottom muds. Lake Valley mound constituents, although essentially identical to those in the European Waulsortian, do not occur in regular growth ‘phases’ like those described by Lees & Miller (1985). The difference is interpreted to mean that the New Mexican mounds lacked enough vertical relief above the sea floor in deep water to have been affected by any environmental gradients that may have existed across the entire platform.