ARIMPIA, Formation

See ARIMPIA, Group

ARIMPIA, Group

TERTIARY (Miocene and ? Pliocene)

State of Zulia

Author of name: A. H. Garner (?)

Original reference: A. H. Garner, 1926, p. 682.

Original description: ibid.

The term Arimpia geographically relates to the village of Arimpia, located in the District of Perijá, about 10 kilometers west of La Villa del Rosario. The term has been variously used, both as a group and formation name, since original publication by Garner (1926, p. 682). History of the term prior to Garner's publication seems to be very brief; in available private reports there are no references pre-dating 1925. Evidence on authorship of the name is inconclusive.

It is interesting to note reference to the "Arimpia formation" in a private report dated February, 1925, and a map that attests to use of the term in a form synonymous to the term "Arimpia group" of today (unpublished report by P. W. McFarland, Venezuelan Sun Ltd., currently in Maracaibo files of the Richmond Exploration Company). The outcrop zone of the Arimpia unit shown on McFarland's map extends from the vicinity of La Villa del Rosario to a point about 5 kilometers west of Arimpia, and includes type sections of both the La Villa and Los Ranchos formations.

The original definition by Garner (1926, p. 682) reads as follows: "Arimpia formation.—Type locality: forms small hills in vicinity of and to southwest of town of Arimpia, southwestern Perijá, State of Zulia. Alternating massive, brown and red, coarse, soft sandstones and white to brown clays, ferruginous. Lower part, clays more shaley and sandstones more laminated. Brown, sandy shales." He shows on his correlation chart (ibid., p. 678), an overlying La Villa formation, which he describes as follows (p. 683): "around town of La Villa... Massive, fine-grained, colored sandstones, and mottled white clays." In these data is evidence that Garner's Arimpia formation is the sequence that is commonly and currently called Los Ranchos formation. Soon after this definition was published, Liddle (1928, p. 308) clearly set off the unit in the manner now generally accepted: "Los Ranchos beds, together with the overlying La Villa beds, are often referred to as the Arimpia formation, because they are apparently a depositional unit."

At present, use of the term Arimpia as a group name (accepted by Sutton and Liddle) is preferred; the term "Arimpia formation" (Garner and others, variable definition) is considered to be obsolete. Actually the term "Arimpia group" was used nearly concurrently by Liddle (1946, p. 483) and Sutton (1946, p. 1704). The reference by Liddle repeats his statement made in 1928 (see above), simply substituting the word "group" for "formation". Sutton states that "Arimpia is still occasionally used as a group name to include the Los Ranchos and La Villa formations." He further remarks that "the Los Ranchos forms the lower half of the Arimpia formation of Garner." This interpretation does not agree with Garner's statements cited above, and it is evident that Sutton either misconstructed Garner's definition, or has attributed Liddle's work to Garner. In any event, the "Arimpia group" definition is now generally accepted by the geological profession in Venezuela.

The Arimpia group, in the region between La Villa del Rosario and Machiques, consists almost equally of sandstone and claystone. The lower (Los Ranchos) part is comparatively resistant to erosion, and forms quite rugged topography. Its sandstone members are typified by fair to good stratification. The upper (La Villa) part erodes to relatively low, "sabana" type topography and occasional "cerros", and is distinguished by friable, poorly bedded sandstones generally of light gray color. There is tendency in the La Villa part for mottlings of red and purple in the sandstones, as compared to general rusty-yellow or brown staining in sandstones of the Los Ranchos. Claystones in both units tend to be gray, with red and tan mottling.

Outside the La Villa-Machiques area, the La Villa and Los Ranchos change so that the intervening boundary is indefinite. A common tendency is for proportion of sandstones to diminish and of claystones to increase. Locally the identity of two separate formations disappears; for example, in the subsurface east of La Villa del Rosario all of the sequence is assigned to the La Villa unit. Some geologists may feel that use of the term "Arimpia" will avoid ambiguity.

The Arimpia group is recognized throughout the western part of the State of Zulia between Lake Maracaibo and the Perijá mountain front. The equivalent term El Guayabo group is generally used in the District of Colón, southwestern Zulia. Approximate upper Miocene and Pliocene age is assigned on basis of stratigraphic position.

John B. Miller