The Miocene to Pleistocene filling of a mature extensional basin in Trans-Pecos Texas: geomorphic and hydrologic controls on deposition

Abstract

Northwest Eagle Flat Basin, in Trans-Pecos (West) Texas, is a Late Tertiary–Quaternary extensional basin in the Basin and Range Province. The basin has largely filled with sediment, so that bedrock uplands only crop out around its rim. Movement on normal faults has effectively ceased, and the geomorphology and deposition differ from active extensional basins, where tectonism is the primary control on basin evolution. Eagle Flat differs from typical extensional basins because progressive changes in basin morphology caused by the aggrading basin-fill were the principal controls on sedimentation. A low-gradient alluvial basin floor expanded to cover the former, fault-defined basin margins. The playa lies on what was upland through the Miocene and Early Pliocene. Only the upper few meters of basin-fill crop out; however, a suite of 88 cores was drilled in the southern part of the basin. Nine of the cores and one trench were sampled for paleomagnetic reversal dating. Correlating the dated cores with interspersed cores allowed us to piece together the basin filling-history and explain how the mature features developed. The cores record the gradual burial of the southern half of the basin. The oldest basin-fill strata were cored in the deepest part of the basin, 219 m below the surface, at ∼12 Ma, in the Miocene. After deposition of 50 m of alluvial-fan gravel along the trend of an inferred normal fault, the basin floor aggraded and expanded. By the 780-ka Brunhes–Matuyama reversal, basin-floor drainage had reversed and was from the north. Sediment accumulation ended during the mid-Pleistocene. A fine-grained sediment supply that did not decrease, and outpaced subsidence was the primary control on basin deposition. This caused a progressive loss of relief and drainage-basin area as uplands were buried under the aggrading basin sediments. After the basin-margin faults were buried, the shape of the basin and its filling-history were little controlled by the original tectonically formed geomorphic elements.