SAN JUAN, Formation

UPPER CRETACEOUS

State of Anzoátegui, Venezuela

Author of name: H. D. Hedberg, 1937.

Original reference: H. D. Hedberg, 1937a, p. 243-244.

Original description: ibid.

The name San Juan sandstone was created by Hedberg (1937a, p. 243-244; 1937b, p. 1994-1998) after Quebrada San Juan, a sidebranch of Río Querecual. The term was originally used as a member of the Santa Anita formation to designate the sediments, exposed on Rio Querecual, between the top of the Cretaceous Guayuta group, 150 meters upstream from the Paso Santa Anita, and the base of a dominantly shale unit (Vidoño shale), at the mouth of the Quebrada San Juan. Here, the sediments consist of about 320 feet of very hard, gray to light gray, fine to medium-grained, well-sorted sandstone in beds usually from 1 to 3 feet thick. The rock is sparingly glauconitic and calcareous in its uppermost part. It is apparently barren of fossils; however, its age is considered to be Upper Cretaceous due to its stratigraphic position above the Guayuta group and on the basis of the foraminiferal fauna of the overlying shaly part (Vidoño shale) of the Caratas member.

The San Juan sandstone is in conformable stratigraphic contact with the underlying San Antonio formation (Guayuta group) and with the shaly part of the overlying Caratas member, which was termed the Vidoño shale by Hedberg and Pyre (1944, p. 12). These authors point out that westward from Río Querecual, the San Juan sandstone grades laterally within a short distance into the Vidoño shale, and that eastward it thickens, at the expense of the Vidoño shale, to about 1,000 feet, constituting one of the most resistant units of the Serranía del Interior.

The Santa Anita formation was later raised to the rank of group (Liddle, 1946, p. 280) and its respective members (San Juan, Vidoño and Caratas) to formation rank (Hedberg, p. 1193).

The San Juan formation is known as far east as Aragua de Maturín, where a large part of the "Aragua formation" of Liddle (1946, p. 347-352) belongs to this formation (Hedberg, p. 1194).

Gustavo Feo-Codecido