Abstract

The Montoya Group represents a shall owing-upward, carbonate ramp sequence. The five lithofacies within the Group are laterally and vertically gradational. The lithofacies are named after the dominant lithology or lithologies: 1) quartz sandstone, 2) bioclastic grainstone, 3) fossiliferous wacke-mudstone, 4) mudstone, and 5) boundstone. These represent, respectively: 1) clastic tidal flats, 2) shoals, 3) back-shoal lagoon, 4) fore-shoal deeper ramp, and 5) carbonate tidal flats.

A three-dimensional reconstruction demonstrates the evolution and relationship of the ramp lithofacies. The basal ramp deposits are quartz sandstones, crinoidal grainstones, and whole-fossil mudstones, representing deposition as nearshore, clastic tidal flats that graded offshore into middle-ramp carbonate shoals and deeper-ramp muds. Faunal differences indicate the relative depths. These lithologies grade upward into bioclastic and burrowed wackestones, bioclastic grainstones, and whole-fossil mudstones. These indicate deepening conditions on the ramp, with deposition occuring mostly below wave base in middle to outer ramp conditions. These units are overlain by bioturbated wackestones that grade laterally into bioclastic grainstones which grade into fossiliferous mudstones. These represent lagoon, shoal, and outer ramp deposits reflecting a shallowing of the ramp. Creation of the bioclastic shoal allowed for development of carbonate tidal flats in the protected area behind it. The subtidal wackestones are overlain by massive mudstones and wackestones of the intertidal flat and cryptalgal laminites of the high intertidal and supratidal areas. These cap the other ramp deposits. The tidal flats, lagoon, and shoal facies prograded as deposition continued.