Coarse fluvial deposits of the Soledad Member of the Gueydan Formation were derived from the Trans-Pecos volcanic field (TPVF), but the proportions of clast types in the deposits do not match present outcrop abundances in the TPVF. Basalt and mafic trachyte are most abundant in Soledad conglomerate and sandstone, durable trachyte forming the largest clasts. In contrast, the surviving remnant of the TPVF is dominated by silicic lava and tuff.

The basaltic and trachytic clasts most closely resemble rocks within some of the younger (∼32-27 Ma) units in the Davis, Chinati, and Bofecillos mountains of the TPVF. Our data suggest that these and similar units were much more widespread at the time of Gueydan deposition and were largely stripped from the TPVF during erosion (1-2 km).

Gueydan deposition may have been associated with regional uplift of the TPVF accompanying the onset of Basin and Range extension. This uplift would have provided the steep gradient necessary to transport coarse detritus from the TPVF to the Gulf Coast. Basin and Range extension eventually disrupted the course of the ancestral Rio Grande-Rio Conchos, thereby cutting off the supply of volcanic detritus and ending Gueydan deposition.

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