PICHAO, Conglomerate

TERTIARY (upper Miocene)

State of Miranda, Venezuela

Author of name: R. J. Smith, 1953.

Original reference: R. J. Smith, 1953, p. 59.

Original description: M. Nicklas, 1953, p. 370-373.

S. E. Aguerrevere and G. Zuloaga (1937, p. 11, 21) were the first to mention the coarse elastics later called the Pichao conglomerate. They wrote (p. 11) that east of Caracas, at Pichao in the Guaire valley, temporary subsidence took place during the Miocene with consequent encroachment of the sea, as indicated by the occurrence of marine Miocene sediments which now have dips up to 42°. They continued (p. 21): "At Pichao, at the crossing of the Caracas-Santa Lucia road on the Guaire River, there is a large outcrop of an unmetamorphosed chocolate-brown conglomerate of some 50 meters thickness with subangular fragments of quartz, quartzite, limestone and schists resting directly on the Zenda and Los Colorados phases of the Caracas Series, the contact being a sharp angular unconformity. The conglomerate is followed by a series of marls, sands, shales and plastic clays which have been classified as Miocene in age from their foraminiferal content".

R. J. Smith (1953, p. 59) named the formation in these words: "The Pichao conglomerate, which crops out 15 km. east of the mapped area, is possibly of early Miocene marine origin (Aguerrevere and Zuloaga, 193T, p. 11)".

M. Nicklas (1953, p. 370-373) repeated the description made by Aguerrevere and Zuloaga in slightly different words and continued: "... it is suggested that the locality described by them, that is, the crossing of the Caracas-Santa Lucia road on the Rio Guaire at Pichao, should be considered the type locality and that the name Pichao conglomerate should be used, ...". He also extended the use of the name Pichao conglomerate from the Santa Lucia basin into the Guatire basin.

P. P. Wolcott (1940, private report) considered the conglomerates at Pichao to be the basal part of his Cumaca formation. Later (1945, private report) he restricted the Cumaca formation to the lower Río Tuy embayment and included the conglomerates at Pichao in his new Tuy formation. It seems elear that if the name Tuy formation is to be retained, the Pichao conglomerate should be ranked as its basal member. According to Wolcott, the conglomerates at Pichao have a thickness of approximately 120 meters (394 feet). He assigned them to the upper Miocene together with the rest of the Tuy formation. So far as known, the conglomerates are unfossiliferous. Woleott was apparently unaware of a basal conglomerate in the Tertiary section of the Guatire basin, all of which he assigned to the "Pliocene" Guatire formation. It thus seems to be a debatable point at present as to whether the Pichao conglomerate is restricted to the Santa Lucia basin of west-central Miranda or ocours also in the Guatire basin of northern Miranda.

A. N. Dusenbury, Jr.