Depositional Setting And Sequence Stratigraphic Analysis Of The Lower Permian Lower Hueco Formation On The Western Margin Of The Orogrande Basin, South Central New Mexico

On the western margin of the Orogrande Basin in south-central New Mexico, eight outcrop datasets, in the form of measured stratigraphic sections from the lower interval of the Lower Permian Lower Hueco Formation were analyzed in regard to depositional facies distribution and sequence stratigraphy. Based on petrographic analysis of 247 samples and associated field relationships, 11 depositional facies are recognized: 1) quartz sandstone, 2) quartz siltstone and shale, 3) fenestral dolomudstone, 4) microbial intraclast packstone, 5) dolomudstone, 6) green-algal packstone to grainstone, 7) ostracode foram wackestone, 8) oolitic packstone, 9) fossiliferous packstone and grainstone, 10) fusulinid packstone and grainstone, 11) phylloid algal bafflestone. The spatial and temporal distribution of these lithofacies was used to develop 4 depositional facies associations that describe their relative position on the Wolfcampian depositional profile: 1) shoreface siliciclastics, 2) supratidal carbonate, 3) intertidal carbonate, and 4) subtidal carbonate. Waltherian transitions between these ithofacies and subsequent depositional facies associations suggests this margin of the Orogrande Basin existed as a shallow marine carbonate ramp, deepening to the east during the early Wolfcampian. Phylloid algal bioherms form thin, isolated buildups locally concentrated in the Robledo Mountains while the remaining depositional ramp is dominated by wide facies belts of supratidal carbonate, intertidal carbonate and subtidal carbonate environments. Deposition of these lithofacies was mainly controlled by changes in relative sea level with little effect from local subsidence as compared to the eastern margin of this basin. Sequence stratigraphic analysis of this region shows two distinctive third-order depositional sequences (Sequence 1 and Sequence 2) within the lower member of the Hueco Limestone. Sequence 1 is bounded at its base by a regionally extensive Type 1 sequence boundary that represents a regional unconformity at the Pennsylvanian-Permian boundary across the Orogrande Basin. Highstand systems tract deposition during Sequence 1 are characterized by thick fourth-order highstand systems tracts composed of aggradational and progradational peritidal carbonate cycles with very thin to absent transgressive systems tracts composed of 1 or 2 subtidal carbonate cycles. The upper boundary surface of Sequence 1 is also a Type 1 sequence boundary that forms a distinctive erosive unconformity with an incised valley filled with estuarine marine siliciclastic in the Robledo Mountains. Sequence 2 in the lower Hueco member begins with fluvial incision forming an incised valley on the shelf and early transgressive estuarine shoreface siliciclastics backfilling the incised valley during the late stage of sea-level lowstand. Phylloid algal bioherms are only present in one stratigraphic interval in Sequence 2 and represent transgressive deposits related to increased rates of sea-level rise and initial development on antecedent topographic highs created by differential compaction of lowstand shoreface siliciclastics. Sequence 2 is characterized by transgressive and highstand subtidal carbonate and subtidal carbonate bioherm cycles, suggesting higher average sea level compared to Sequence 1. Peritidal cycles are still present but are confined to exposures in the Robledo Mountains to the west. Sequence 2 is also capped with another Type 1 sequence boundary with fluvial incision from the overlying sequence. The fluvial incision is consistent with another incised valley filled with late lowstand estuarine shoreface siliciclastics. These shoreface siliciclastics represent the transition into the upper interval of the lower Hueco member of the Hueco Limestone in the Robledo Mountains