ABSTRACT

Middle to late Eocene volcanic activity in the Big Bend region of Texas induced dramatic changes in local depositional processes recorded by volcanogenic sequences in the Chisos, Canoe, and Devil's Graveyard formations. Analysis of lithofacies associations, clast lithologies, and paleocurrent trends of tuffaceous strata in these sequences allows recognition of two sedimentary aprons derived from two widely spaced volcanic centers; a southern apron shed from a center in Mexico, and an apron shed from the Christmas Mountains volcanic center. Each volcanogenic apron contains distinctive vertical sequences that reflect in different ways episodic explosive activity at the volcanic source area, and also distinctive records of lateral changes in sedimentation that correspond to distance from the volcanic source area.

The southern apron contains intermediate-source deposits that are the product of alternating modes of deposition that correspond to aggradation during explosive volcanic activity as well as aggradation during volcanic quiescence. Mudflow and hyperconcentrated flood-flow deposition occurred during times of explosive activity, and fluvial deposition and pedogenesis occurred during volcanic quiescence. Stacked beds of lithic-clast deficient mudflow deposits are the result of rapid aggradation of intrabasinal-derived flows on a topography inundated with tephra. Distal-source deposits do not show evidence for alternating modes of deposition but instead were deposited almost entirely by traction and suspension processes in a fluvial system, and subsequently modified by pedogenesis.

Eruptions associated with the Christmas Mountains caldera complex at 42 Ma produced abundant lava and pyroclastic deposits that are interstratified with sedimentary sequences. Proximal-source exposures within 8 km of the calderas consist mostly of ashflow and air-fall tuffs, and lava and debris flow deposits, occurring as five distinctive sequences that reflect the episodic eruptive history at this center. Intermediate-source exposures from 18 to 23 km are predominantly composed of interbedded mudflow and hyperconcentrated flood-flow deposits that are separated by disconformities. Alternating aggradation and degradation apparently resulted from drastically fluctuating sediment loads originating from the episodically active volcanic source. At distal-source exposures, 30 to 35 km from he caldera, there are no apparent disconformities in the apron. Initial aggradation was dominated by mudflows and pumice-fall, followed by traction deposition in sheet-floods and channels.

Evidence independent of the volcanogenic sequences indicates that each apron aggraded under roughly the same tectonic and climatic influences. Hence, this study shows that volcanogenic sequences can record the explosive activity of a source center in 2 different ways in the same climatic and tectonic conditions. In addition, this study shows that this record of explosive activity may not be recorded at distal parts of the aprons.