ENIGMATIC SILICEOUS LAG GRAVELS AND CONGLOMERATIC SANDSTONE PREVIOUSLY UNDOCUMENTED IN THE SOUTHERN GUADALUPE MOUNTAINS, TEXAS

Formed during Permian time, the Guadalupe Mountains represent an exhumed remnant of a shelf-edge carbonate "reef" uplifted and exposed by block-faulting during Miocene Basin and Range tectonic activity. The Guadalupe Mountains form a part of an extensive carbonate buildup that fringed the Delaware Basin in southeastern New Mexico and west Texas during the Permian. Siliceous pebble "lag" gravels of varicolored quartzite and chert found in association with conglomeratic sandstone paleokarst and/or fissure fills are well documented in the northern Guadalupe Mountains in New Mexico from Jurnigan Draw southwestward as far as Double Canyon, but no further. The occurrence of similar siliceous lag gravels and conglomeratic sandstone have now been documented from a small down-faulted ridge in the southern most end of the mountain range in Guadalupe Mountains National Park, Texas. The discovery of these siliceous gravels and sandstone dispel the notion that these deposits did not occur southwest of Double Canyon because the southwestern section of the mountains had been uplifted above the area of deposition as Hill (1996) implied. Age and origin of these deposits is controversial. However, a paleogeographic model of the Cretaceous Albian time, between 107 (possibly 112) and 95 Ma BP, infers that the area was perhaps a shoreline consisting of non-marine and marginal marine coarse clastic material behind an extensive shallow open-marine carbonate shelf (Goldhammer 1999); both the northern and southern deposits are shelfward of the reef escarpment. Many investigators (e.g., King 1948, Pray and Wilson 1985) also favor the Cretaceous as the age for these deposits. An isolated patch of poorly sorted, subangular, coarse-grained quartz sandstone cobbles and boulders containing quartzite and chert grains were also discovered on this same small ridge. These particular sandstones are not characteristic of the fine-grained, well sorted, well-rounded quartz sandstone lenses interbedded with the Yates Formation in the area. Investigation to compare the lithology and mineralogy of these sandstone boulders and cobbles with that of the Yates sandstone is being conducted.