New Mexico Geological Society Annual Spring Meeting — Abstracts

On-going STATEMAP geologic mapping of the Black Bluffs 7.5-minute quadrangle provides an opportunity to study the interrelated sedimentary, volcanic, and structural evolution of the southern portion of the east-tilted Engle basin of the southern Rio Grande rift. Mapping efforts to date have focused on the northern portion of the quadrangle around Mitchell Point along Elephant Butte Lake (EBL) eastward toward the Fra Cristobal Mountains (FCM), which lay in the footwall of the Walnut Springs fault (WSf) and bounds the basin on the east. All rift basin-fill sedimentary strata in the studied area belong to the Plio-Pleistocene Palomas Formation (Santa Fe Group), and include axial-fluvial pebbly sands/sandstones interfingering with piedmont sediments derived from the FCM to the east and several highlands to the west. Sedimentary rocks intercalate with locally-erupted basaltic flows and pyroclastic volcanics. The exposed rift basin-fill stratigraphy can be divided into four sequences based on the lateral extent of axial facies sediments and intercalated volcanics. Sequence (1) [oldest] is marked by sand-dominated axial-fluvial sediments occurring notably further east of axial sediments of later sequences. Axial sandstones of sequence (1) are found within 600 m of the FCM, as opposed to the ~2.5 km closest distance of later sequences. Piedmont sediments from the FCM then prograded westward, and sequence (2) axial-fluvial sediments occur only from the eastern margin of EBL on the east to at least 2 km west of EBL on the west. Sequence (3) is marked by intercalated basaltic flows and pyroclastic deposits. At the surface, volcanics are concentrated spatially to the east of EBL, with vents generally along intrabasinal and basin-bounding faults, and concentrated stratigraphically to a discrete interval as opposed to occurring throughout the Palomas Formation section. Toward the end of this volcanic period, piedmont sediments from the west prograded eastward toward EBL, and sequence (4) is marked by axial-fluvial sediments restricted to the area around EBL. The sharp contact between sequence (1) and (2) and basaltic flows in sequence (3) were used to estimate vertical offsets across the intrabasinal Hackberry fault (Hf) and the basin-bounding WSf. The Hf offsets the (1)-(2) contact by 60 m and various basaltic flows of sequence (3) by 10 to 42 m, with offsets decreasing to the north and/or up-section. The WSf offsets various basaltic flows of sequence (3) by 26 to 35 m. Axial-fluvial sandstones of sequence (1) do not cross the WSf, but the significantly greater eastward extent of axial sediments during this time relative to later sequences may be a reflection of significant activity along this basin-bounding structure during or preceding deposition of sequence (1).