NPS Paleontology Research Abstract Volume


AMISTAD NATIONAL RECREATION AREA: STRATIGRAPHY AND PALEONTOLOGY

James O. Jones
Geology Department
University of Texas, San Antonio
San Antonio, Texas 78249

Amistad National Recreation Area is located northwest of Del Rio, Texas in Val Verde County. Most of the area is occupied by Amistad Reservoir and except for some isolated acreage, only the shore area up to the high pool level is within the recreation area. The lake was formed by the damming of the Rio Grande River, therefore, the boundary between the United States and Mexico is under water along the former valley of the Rio Grande River. Upper reaches of the lake are over 50 miles from the dam.

All marine fossil bearing rocks in Amistad NRA are Cretaceous in age. Two Lower Cretaceous formations are present and are known as the Devils Ridge and Salmon Peak. Lithology of both units is principally limestone. The Devils River Formation is Albian in age and the Salmon Peak Formation is a middle and late Albian facies. The Devils River was deposited as a basin margin reef around the rim of the Maverick Basin and the Salmon Peak was deposited as reef margin and basin facies. Amistad has both facies present. An open ocean connection to the basin provided the path for many genera of ammonites to move into the area and these fossils are good time indicators. In addition, many varieties of bivalves are present. Of particular interest are the various genera of rudistids that provide information concerning the depositional environment. Gastropods, echinoderms and an occasional scleractinian constitute the major marine fossils. Foraminifera are abundant, probably the most notable is the milliolid variety. Ostracods and various types of algae constitute the other known micro-organisms in the strata.

Lower Cretaceous rocks in the NRA are unconformable with Upper Cretaceous rocks. Upper Cretaceous rocks are composed of the Del Rio Clay, Buda Limestone and the Boquillas Formation. Del Rio and Buda are Cenomanian and the Boquillas is Upper Cenomanian through Turonian. The depositional setting of the Del Rio Formation was a shallow nearshore muddy siliciclastic basal transgressive unit. It is easily recognized by the very abundant bivalve, Ilymatogyra arietina, formerly Exogyra arietina. In addition, numerous other clams and a few gastropods are present. Foraminifera and ostracods are very abundant. Iron stained surfaces with borings and worm tubes mark several beds. Three genera of algae have been reported. The Buda limestone is unconformable with the underlying Del Rio and the Boquillas is unconformable with the underlying Buda. Both the Buda and Boquillas contain ammonites, various bivalves, ophiuroid and echinoid fragments, an abundance of foraminifera and some ostracods, burrows and borings. Several genera of algae are present. Although corals and bryozoans are seldom reported, they probably occur within the formations.



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