CARBONERA, Formation

TERTIARY (upper Eocene-Oligocene ?)

Department of Santander del Norte, Colombia

Synonym of OMUQUENA FORMATION

Author of name: F. B. Notestein et al., 1944.

Original reference: F. B. Notestein et al, 1944, p. 1196.

Original description: ibid.

The Carbonera formation is named from Quebrada Carbonera, a tributary of the Río Zulia on the east flank of the Petrolea anticline, Colombia, where the formation is well exposed. The type section is also in this stream, approximately 12 kilometers northwest of Puerto Villamizar on the Río Zulia.

The formation consists of a thick series of claystones and various amounts of associated sandstones; some lignitic coals occur in its upper and lower parts. The claystones, normally the major constituent, are mostly gray with some greenish-gray and brown intervals. Red and yellow mottling is common, and siderite, both in small spherules and irregular masses a few centimeters in size, is fairly abundant. There are some dark-gray, micaceous in part, silty shales, specially at intervals of 50 to 100 meters, as much at the base as at the top.

The sandstones are generally in beds 5 to 10 meters thick but range up to 30 meters in thickness, and beds less than a meter thick are not uncommon. They are gray and greenish gray, and most range from fine to coarse-grained, but there is also a considerable amount of very f ine-grained micaceous sandstone. Well sections show about 25 meters of sandstone 125 to 185 meters below the top of the formation, and sandstones are particularly well developed in the lower 125 to 250 meters of the formation. However, the bottom 10 to 75 meters consist mainly of shales. The sandstones are characteristically more argillaceous than those of the Mirador.

Some thin beds of lignitic and cannel coals and carbonaceous shales occur in the top and bottom 100 meters of the formation; analysis of 10 coal samples show fixed carbon ratios ranging from 30.2 to 46.6 and averaging 39.7. Scarce thin limestones have also been found in the same intervals.

Approximately 100 meters below the top of the formation on the Tibú anticline, remains of a few small mollusks were found in a highly carbonaceous shale about 5 meters above a 2-meter interval of glauconitic very argillaceous siltstone. Cuttings from Socuavó n°. 2 showed some glauconitic sandstone about 60 meters above the base of the formation. On the Tibú and Socuavó anticlines the extreme top of the formation is glauconitic. The glauconite indicates stages of marine to brackish-water invasion.

Surface measurements in the southern part of the Barco concession usually show about 500 meters for the Carbonera formation; 410 meters was measured on the Leoncito anticline, and an abnormal thickness of 720 meters was measured on the Río Nuevo west of Sardinata n° 1. Wells show 461 and 475 meters on the Sardinata anticline, 479 and 500 on the Tibú anticline, and 560 meters on the Socuavó anticline. The wells show a westward and northward thickening, accompanied by an increase in the percentage of sandstones. As previously stated, separation of the Carbonera and Mirador formations is very difficult in the Río de Oro area, where the combined thickness of the two formations is about 1,220 meters.

Much dark-brown staining was found in sandstones in the upper part of the formation in wells on the Tibú anticline and to a lesser extent on the Socuavó anticline. The stained sandstones occur 140 to 183 meters below the top, range from fine to coarse-grained, and are rather clean. Although some residual free oil and a little gas are still present, the sandstones carry fresh water in common with the other sandstones in the formation. The base of the Carbonera formation is drawn where the shales in its base are in contact with the massive clean coarse sandstones of the Mirador. As mentioned, the wide range in thickness of the Mirador formation has suggested the possibility that this contact may be unconformable. In the Río de Oro area the base of the formation is not definitely recognizable. The top is drawn where the thin sandstones and sandy shales of the formation give way to the almost pure shales of the overlying Ledn shale formation. The upper contact is apparently conformable.

The age of the Carbonera formation according to Notestein et al; is upper Eocene to Oligocene. Based on a study of the Hannatoma fauna of the Carbonera formation, Dusenbury (1949, p. 148) believes that this fauna is upper Eocene and Hedberg (1949, p. 150) is inclined to favor an age older than middle Oligocene for the Hannatoma fauna of the Santander del Norte area of Colombia.

According to Sutton (1946, p. 1672) the Carbonera formation apparently includes strata of upper Eocene and middle Oligocene age, whereas the First Coal horizon and Sandy Shale formation of the type section on the Tarra anticline, Colón district seem to be limited to the upper Eocene.

Dusenbury (1949, p. 148) reports that the Omuquena formation of Táchira has been traced across the Venezuelan-Colombian border into the Carbonera formation.

The upper part of the Carbonera formation probably correlates with part of the Peroc formation.

W. A. Mohler