MACOA, Formation

TERTIARY (lower Miocene, approximately)

State of Zulia, Venezuela

Authors of name: Geologists of the Richmond Exploration Company.

Original reference: E. Mencher et al. (Correlation chart), 1951.

Original description: none published.

The name Macoa originates with the settlement of Macoa, located 14 kilometers northwest of Machiques in the District of Perijá, and relates the formation to the region of the Macoa anticlinal nose, where it has been widely studied. The formation is the middle formational unit in the El Fausto group. Like other formations of the group (the overlying Cuiba and the underlying Peroc), the Macoa is defined in the subsurface, and is principally recognized only in well sections.

The type section of the Macoa formation is the 1890-2400 foot interval in the Zulia 20D-1 (Z20-1) well, located 24 kilometers northeast of Machiques. A good supplementary section is found in the Macoa-1 well, interval 3900-4550. About 40 percent of the interval in Zulia 20D-1, and 10 percent of the interval in Macoa-1, are cored. A predominance of dark gray-green and olive-gray claystones distinguishes the Macoa from the generally mottled and varicolored claystones of the adjacent Peroc and Cuiba units. Thicknesses recorded in the Zulia 20D-1 and Macoa-1 wells, respectively 500 and 650 feet (152 and 198 meters) are nearly average for the area, although drastic thinning and perhaps complete pinchout occurs over the Totumo structural arch, about 15 kilometers northwest of La Villa del Rosario.

The Macoa formation consists mostly of claystone, some siltstone, and variable but relatively minor amounts of sandstone. Common colors are hues of gray and olive-to brownish-gray, in medium and dark tones. The sandstone colors range from light olive gray and light-olive to medium olive gray, and the grain size from medium to very fine. Thickness of sand beds averages, evidently, less than 5 feet. The siltstones and claystones colors commonly range from medium light gray to medium-gray, light olive gray, greenishgray, and olive-gray. The darker shades within this color span are most common. In addition, the presence of disseminated lignitic material, and streaks or folia of lignite, locally extends the color range to brownish-and olive-black. Minor beds of lignitic coal are present. Pyrite aggregates are frequent and occur in varying amounts. Glauconitic granules are present locally.

The boundaries of the Macoa formation with the Cuiba formation above, and the Peroc formation below, appear to be gradational. It is conceived that the formation represents a transgressive stage in which sub-saline marine waters invaded a generally lagoonal environment. The electric log pattern is not significantly distinguishing and on the basis of well cuttings, the formational boundaries may be arbitrary within and interval of 50 feet or more. The inferred gradation involves a transition from the mottled claystone to olive gray claystone, and tendency toward slightly reduced proportion of sandstone in the middle of the Macoa unit. It is noted that in Zulia 20D-1, the only well where a Macoa formation boundary was continuously cored, the lithic change between the Macoa and Peroc is relatively sharp and can be picked within a few feet.

It is believed that the Macoa formation represents part of the same general transgressive cycle that is recorded by the marine La Rosa formation farther east. Although the time span for the Macoa is presumably shorter, particularly in marginal areas where the formation is thin and evidently incompletely developed, the Macoa is considered to be correlative in age, for all practical purposes, with the lower part of the La Rosa. Lower Miocene age has been assigned on faunal basis, although paleontologists disagree on whether age can be reliably assigned from the observed assemblage.

The recognized distribution of the Macoa formation extends from the Peroc-1 well 14 kilometers northwest of La Villa del Rosario, southward into the vicinity of the Macoa anticlinal nose, and as far southeastward as the well Zulia 36E-1, located 21 kilometers east of Machiques. From preliminary studies, the unit is evidently present also in the Alturitas-1 well somewhat farther south. No effort has been made to trace the formation into surface outcrops south of Machiques. In the Boscán oilfield area, Richmond geologists have elected to use the name Lower La Rosa instead of the term Macoa formation. This practice, believed to be justified by greater marine affinity and larger assemblage of fauna, has restricted use of the term Macoa to the region Iying westward from the Boscán field.

Fatinas in the Macoa formation include some arenaceous and siliceous foraminifera, a few ostracods of thin, smooth-shelled type, and scarce but persistent occurrence of fish teeth, fish bones, and scales; also molluskan shell fragments have been recovered in some localities. Carbonized plant remains are quite common. Foraminifera of the genera Trochammina and Miliammina are specifically mentioned in the faunal assemblage.

John B. Miller