The Lower Cenomanian Del Rio Clay Formation is unusual within the Cretaceous succession of West Texas, which is dominated by carbonate sediments. Examination of several stratigraphic sections cropping out in Val Verde and Terrell counties, and in the Big Bend area, indicate that the Del Rio Clay has a variable thickness (0-120 ft) and consists of mostly tan to buff calcareous shale layers interbedded with thin (~0.5-2 ft thick) quartzose sandstones and oyster-rich (Ilymatogyra arietina) calcareous packstones. Sedimentary structures in the sandstones and packstones are consistent with deposition by storms (tempestites). The lithologic and paleontologic data suggest that the Del Rio was deposited in a nearshore, shallow marine to brackish-water environment subjected to periodic storms during a period of relatively low sea level and exposure of a terrigenous source area.

Studies of modern nearshore marine sediments have shown that kaolinite flocculates (settles) at lower salinities than do illite or smectite. X-ray diffraction analyses of bulk and clay-mineral separate (<2 µm) samples from the study area were obtained to determine whether clay mineralogy, in particular illite-to-kaolinite (I/K) ratios, could serve as a proxy for salinity variations during deposition of the Del Rio. In the most complete section (~120 ft) near Terlingua, Texas, I/K ratios increase from between 1.0 and 1.9 in the lower (60 ft) section, to between 2.0 and 2.3 in the middle portion of the section (60-105 ft). These data suggest increasing salinity conditions with time. I/K ratios as low as 1.2 occur in the upper 15 ft of the section below the unconformity with the overlying Buda Formation suggesting a return to lower salinity conditions. The I/K ratios are generally consistent with previous interpretations that the upper contact of the Del Rio Clay Formation represents a subaerial exposure surface (sequence boundary), and that the Del Rio represents a single transgressive-regressive cycle apart from the overlying Buda Formation. An siliciclastic-rich shale layer between about 5 and 8 ft below the Buda contact has an anomalously high I/K ratio of 5.0 and is smectite-rich. This layer may represent landward deposition of marine sediments (storm related?). Alternatively, it may be the result of large flooding event, which carried large amounts of detrital sediment farther offshore.