Brushy Canyon Formation, Texas: A Deep-Water Density Current Deposit

Abstract

In mid-Permian time, the Delaware basin was a nearly circular deep, about 160 km in diameter. It was ringed by banks or reefs, which were surrounded in turn by very broad shallow shelves, lagoons, sabkhas, and alluvial plains. Broad tectonic down-warping caused deposition of about 1,000 m of sediment in the basin and neighboring shelves in both the Leonardian and Guadalupian Series (an average of 75 m/m.y.), whereas distant areas received only a fraction of those thicknesses. Although the basin and adjacent shelves accumulated nearly equal sediment thicknesses, appreciable slopes existed at the basin margins throughout mid-Permian time. Thus, the basin waters are estimated to have been more than 100 m deep in Leonardian time and as much as 600 m in late Guadalupian time. Simultaneously, bordering banks were very shallow or even emergent, and they prograded basinward several kilometers. The basin margin is spectacularly revealed by a transverse fault in the Guadalupe Mountains. Here the relations of basin, slope, and shelf beds can be clearly seen. The quartzose siltstone and sandstone of the Brushy Canyon Formation (the lower one-third of the Guadalupian Series), where they wedge out at the basin margin, are the topic of this study. These beds have been variously interpreted as shallow marine or deep turbidity current deposits. In my estimation, they are neither. Rather, they have unusual features that suggest deposition in relatively deep water by saline and cold density currents. Surrounding shelves provided these dense water masses. Numerous basinward-trending channels are one product of these density currents. The channels are commonly 20 to 30 m deep, a kilometer or more wide, and extend far into the basin. Channel floors are flat, and the sides commonly slope 20° to 30°. The channels are filled in a special, though unordered, way. Finely laminated coarse-or medium-grained siltstone beds mantle channel floors, walls, and interchannel areas. Fine-grained, locally conglomeratic sandstone beds are confined to channel floors and abut the walls. Chaotic debris beds locally fill channels near the steepest basin slopes. The basin waters were apparently density stratified. Dense shelf water, spilling through channels in surrounding banks, flowed down marginal slopes and along the basin floor. These denser flows cut channels or deposited sandstone beds confined to channels. At other times, less dense shelf water spread over more dense, stagnant basin layers, raining suspended silt over the irregular basin floor. Characteristics that distinguish these density current deposits from the more common turbidity current deposits include (1) less evident proximal-distal changes (the same rock types and channel relations exist near the basin center as in outcrops 50 km away), (2) coarser porous sandstones confined entirely to channels (some channels have much sand and others little), (3) no levees or overbank deposits that would serve as channel proximity indicators, and (4) mostly ungraded sandstone beds that contain little finer matrix. Similar density current deposits have not yet been recognized in other areas or geologic systems. However, they may be anticipated wherever basin waters were restricted sufficiently to become density stratified and where broad evaporitic shelves or lagoons bordered such basins.