Sequence stratigraphy of the Lower Permian Abo member in the Robledo and Dona Ana Mountains near Las Cruces, New Mexico

Abstract

In the Robledo Mountains, approximately 100 m of interbedded red beds, gray shale, and limestone were mapped as the Abo Tongue of the Hueco Formation by Seager et al . (in press), whereas similar strata in the Doña Ana Mountains were assigned to the Abo Formation by Seager et al . (1976) . These strata are considered correlative (Wahlman and King 2002) and are referred to in this study as the Abo member for ease of discussion . Although fusulinids are absent in the Abo member in both mountain ranges (Wahlman and King 2002), a marine invertebrate fauna in the Robledo Mountains suggests a late Wolfcampian (Sakmarian) age (LeMone et al . 1975; Kues 1995; Kietzke and Lucas 1995) . In addition, conodonts collected from the lower part of the Abo member in the Robledo Mountains were assigned to the Sterlitamakian, the upper substage of the Sakmarian Stage valleys separated by interfluves that may be sites of nondeposition and soil formation (Fig . 1B) . The process of fluvial incision may be episodic, creating one or more inset terraces (Fig . 1B; Strong and Paola 2006) . Sediment deposited during the period of sea-level fall and initial rise is referred to as the lowstand systems tract (Fig . 1A, LST) . When the rate of sea-level rise increases, sediments of the transgressive systems tract are deposited (Fig . 1A, TST) . If incised valleys were cut during lowstand, they will be flooded by the rising sea, creating estuaries . The transgressive systems tract may be condensed compared to the other systems tracts, because of erosion beneath one or more marine ravinement surfaces . Near the crest of the sea-level curve, the rate of detrital influx to the shoreline may exceed the space made available by slow sea-level rise, causing the shoreline to prograde seaward, a process that may continue during subsequent slow sea-level fall . Sediment deposited during this interval of progradation and coastal offlap belongs to the highstand systems tract (Fig . 1A, HST) . Deposition of the highstand systems tract proceeds until the rate of sea-level fall accelerates, at which time coastal-plain rivers once again begin to erode, creating the upper sequence boundary . Lower Permian strata along the western margin of the Orogrande Basin of southcentral New Mexico consist of interbedded terrestrial and marine rocks (Fig . 2; Kottlowski 1963), suggesting that this area occupied a position between the limit of coastal incision during sea-level lowstand and maximum onlap during marine transgression (cf . Blum and Tornqvist 2000) . These strata in the Robledo and Doña Ana Mountains near Las Cruces are well suited for sequence stratigraphic analysis, because of the ability to correlate depositional facies and stratigraphic surfaces along canyon walls and between closely spaced outcrops in a direction roughly parallel to the paleoshoreline . This paper builds upon earlier sequence stratigraphic interpretations of the Abo member by Mack et al . (2003a) by presenting more detailed correlations in abstract