DIVIDIVE LIMESTONE, Member

TERTIARY (lower Miocene)

State of Falcón, Venezuela

Author of name: H. G. Kugler, 1926 (private report).

Original reference: A. Senn, 1935, p. 81.

See: SOCORRO, Formation and MOSQUITO, Formation

SOCORRO Formation

TERTIARY (upper Oligocene or lower Miocene to lower middle Miocene)

State of Falcón, Venezuela

Author of name: S. H. Williston, 1921 (private report).

Original reference: F. Hodson, 1926, p. 174.

Original description: ibid.

Hodson (1926, p. 174) wrote: "Socorro series: Gypsiferous sandstones and variegated gypsiferous clay-shales.Type locality: Socorro, south of Urumaco, District of Democracia, State of Falcón."

Liddle (1928, p. 286-288) offered a more complete description, from which the following quotations are taken: "Williston, in 1921, first named and described the Socorro formation from Socorro in the District of Democracia, State of Falcón."

"There is no visible break in sedimentation between the Cerro Pelado and Socorro formations, or between the Socorro and overlying Damsite. There appears to be a gradual transition from moderately shallow water to deeper water, probably culminating in Middle Damsite time."

"The Socorro formation is composed of greenish-gray, reddish-brown, arenaceous shales, carrying lignite beds interbedded with grayish-brown, flaggy, ripple-marked sandstone. Throughout, the formation is poorly fossiliferous."

"The area of outcrop of the Socorro formation in the Falcón region follows in general that of the Cerro Pelado formation which directly underlies it, and extends almost entirely across the State of Falcón. Its average thickness is between 2,500 and 3,000 feet."

"A few localities in the Socorro formation have afforded Middle Miocene fossils; and the stratigraphic position of the formation substantiates this fossil evidence."

Liddle then described his type Socorro section along the Urumaco-Pedregal trail (now a road) in some detail and listed some of its molluscan fossils. He included in his Socorro formation the sandstones in the mesas around San Rafael with the underlying intercalated lignitic coal seams and variegated sandy shales, the type Querales formation north of Agua Clara, the El Isiro coal mines (in the Cerro Pelado formation) and clay shales, lignitic coals and sandstone beds at Curamichate (Agua Salada group) in the District of Acosta, eastern Falcón, all of which present usage excludes from the Socorro. It should be noted that Liddle does not even mention the Querales shales of Hodson (1926, p. 174) and that the combined Querales shales and Socorro series of Hodson are equivalent to Liddle's type Socorro section.

Williston and Nichols (1928, p. 447-450) wrote: "Socorro series-Lower Miocene on the basis of fossil evidence. Type locality at Socorro, from the first sandstone bluffs south of Urumaco to the sandstone at San Rafael. Thickness 3,000 feet. This formation may be divided into two units, the lower consisting of sands, shales, marls and limestone with considerable gypsum. Sands are somewhat concretionary, white, yellow, or brown, weathering to light brown. The shales are generally blue, but in places red. The gypsiferous marls are fossiliferous. The upper part of the Socorro consists of 700 to 1,000 feet of nonmarine sandstones, peat, and paper shales. There are no limes, marls or other fossiliferous horizons. Sandstones are crossbedded and give every appearance of continental origin. Below, the Socorro series lies locally unconformably on the Cerro Pelado. Above the Socorro series and below the lower Urumaco member of the Damsite limestone, there is a very slight angular unconformity with fragments of the Socorro series in the basal member of the Urumaco."

This restricted definition of the Socorro and its division into two principal stratigraphic units have won general approval. Again, no mention was made of the Querales shales, but they were apparently assigned in part to the lower Socorro and in part to the Cerro Pelado. The "El Isiro coal series" were considered the equivalent of the "Cerro Pelado-Socorro", and the Curamichate sands were correlated "with the upper part of the Cerro Pelado or possibly lower Socorro".

Senn (1935, p. 80-81, correlation chart) assigned the Socorro formation to the lower Miocene and divided it into a lower part (Querales shales) and an upper part (Socorro sands, Socorro s. str.). In eastern Falcón the formation has a maximum thickness of 2,300 meters. The fauna from Cantaura (misspelled Cantaure) in the Paraguaná  Península was attributed to the Querales shales. The Miogypsina-bearing Dividive limestone was placed at the base of the middle Miocene Damsite formation.

Wiedenmayer (1937, p. 69) omitted the Socorro formation from his stratigraphic chart. He probably included the upper Socorro in the Caujarao formation.

González de Juana (1937, p. 190-195-196) placed the Socorro formation in the lower half of the middle Miocene and relegated to it a thickness of 850 meters of neritic and bathyal deposits beneath the Caujarao formation and above the Cerro Pelado. On page 195, he placed the Dividive limestone at the base of the Caujarao formation, but in his plate 6, opposite p. 202, he depicted it at the top of the Socorro formation. The Cumarebo oil sands above the San Francisco sand were included in the lower part of the Caujarao or Damsite formation. The Socorro formation was described as consisting of gray shale, clayshale and clay for the most part in the La Vela and Cumarebo areas with some fossiliferous marls, a few sands and thin limestones. The upper member of the formation was called the San Francisco sand. It is the thickest and most productive oil sand of the Cumarebo field. The author continued: "In central-western Falcón, the time equivalent of the sediments previously referred to as the Socorro formation, has been subdivided into the Socorro and the Querales formation. There the Querales formation still retains its bathyal character, but the Socorro proper shows already the influence of a neritic environment. In the Cumarebo area the differentiation between the Socorro and the Querales formations is practically impossible."

Liddle (1946, p. 458-462, correlation chart) mentioned evidence for both middle Miocene and lower Miocene age of the Socorro formation in his text bui in his fig. 21 on p. 461 and in his correlation chart he placed the entire formation in the lower Miocene. He further declared that: "The top of the Socorro formation, made up of the San Rafael sandstone, carries oil in the Cumarebo field." This is obviously a confusion of the San Francisco sand of González de Juana with the San Rafael sandstone, a local name for the Las Lomas sandstone member of the Querales formation south of Socorro. The error was corrected on the correlation chart, where the San Francisco sand is locally the top member of the Socorro and is overlain by the Dividive limestone, bottom member of the Caujarao or Damsite. The chart divided the remainder of the Socorro formation into a middle Socorro sands member and a lower Querales shales member. Fig. 21, showing the geological section exposed between Coro and El Isiro in the District of Miranda, Falcón, indicated a "Lomas sandstone horizon" overlying the Querales shales in the Socorro formation. Liddle accepted the restriction of the Socorro made by Williston and Nichols. This placed the El Isiro coal horizon in the upper part of the Cerro Pelado formation. Liddle's "Socorro, Querales series" (p. 454) is a synonym of his Socorro and Querales members.

Payne (1951, p. 1854, 1857-1860) did not use the names Socorro and Querales in his paper on the Cumarebo area because at the time there was much doubt about the correlation of the type sections of these two formations with the Cumarebo section and with each other. Instead, he used local names, among which his Mosquito formation is now known to be the same as the shaly lower part of the type Socorro and his Portachuelo member of the Caujarao formation is now known to be identical with the sandy upper part of the type Socorro. Pertinent quotations include:

"Mosquito formation (new name).-This lower Miocene-upper Oligocene formation, called the Socorro formation in company reports and also by G. de Juana, attains its greatest thickness in the area, about 3,300 feet, in the type section along the Quebrada El Mosquito, about 5 miles southwest of Cumarebo field. In that area, the formation consists of clays and shales with some micro and macrofossiliferous marly layers, and there the Dividive limestone member (new name), a thin (0-30 feet) reef limestone marks the top of the formation. The reef dies out eastward."

"Farther toward the east, near Pueblo San Francisco, 5 kilometers (3 miles) southeast of Cumarebo field (Fig. 2), the uppermost 300 feet of the formation consist of the soft, fine-grained San Francisco sand, which carries plant remains. The sand disappears westward and does not crop out at the same localities as the limestone, ... "

"Sand 15, the thickest and most prolific reservoir in Cumarebo field,.. is, in whole or in part, the subsurface representative of the San Francisco sand. It is typically about 570 feet thick and has a hard, dense calcareous cap with Operculinoides, this cap being regarded as the stratigraphic equivalent of the Dividive limestone and marking the top of the formation."

"Caujarao formation." "The type locality for the oldest or Portachuelo member (new name) is near a place of that name 7 kilometers (4.5 miles) southwest of Cumarebo field." "Both in the type section and in the field area, the Portachuelo member consists of a series of minor sedimentary cycles. In the field area, each cycle starts with a fairly pure shale, which may contain one or more glauconitic, fossiliferous marls 2-6 feet thick."

". . . the shale grades upward through sandy shale into sand. . . ". "The sand almost invariably has a thin cap of calcareous sandstone, sandy limestone, or merely indurated sand, overlain by a few feet of greensand or glauconitic marl. There is then a sharp change to the shale of the succeeding cycle. Sands 1 through 14 are the culminating phases of these cycles, the youngest of which is overlapped unconformably by the Cumarebo limestone."

"In some of the cycles the shale phase contains thin sands. . . " "In addition, a few limestones occur. The thickest of these is the persistent, macrofossiliferous "Algal lime" between Sands 10 and 11, a reliable subsurface marker in the northeast half of the field, with a thickness of 10-12 feet."

"The Portachuelo member is about 2,700 feet thick at the type locality. In the field, the thickness is approximately 2,900 feet on the upthrown side of the Hatillito fault. On the downthrown side of this fault... maximum thickness may be as much as 3,625 feet."

Most geologists now consider the Socorro formation to be upper Oligocene or lower Miocene to lower middle Miocene in age. They subdivide the formation in the Cumarebo area into the uppermost Portachuelo member, the Dividive limestone member, the San Francisco sand member and the lowermost Mosquito member. Further to the west, in the District of Democracia, only the upper or Portachuelo member and the lower or Mosquito member are recognized. In the District of Buchivacoa, even further westward, the formation cannot be subdivided.

A. N. Dusenbury, Jr