Terminal Eocene Events in Venezuela

Terminal Eocene Events in Venezuela

199 TERMINAL EOCENE EVENTS IN VENEZUELA MARIA L. DIAZ DE GAMER0 The Cenozoic geologic history of Venezuela is usually subdivided into two complex se...

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199

TERMINAL EOCENE EVENTS IN VENEZUELA MARIA L. DIAZ DE GAMER0

The Cenozoic geologic history of Venezuela is usually subdivided into two complex sedimentary cycles, separated by an important tectonic event starting in the Late Eocene (Gonzdlez de Juana et. al., 1980). This event is particularly marked in Western Venezuela where,

after a brief but very widespread marine transgression during the late Middle Eocene, corresponding to the Orbulinoides beckmanni and Truncorotaloides rohri zones (Walton, 1966; Furrer, 1971; Hunter, 1974), there is no sedimentation until the Middle Miocene, with a short marine invasion in the Maracaibo Dasin. The terminal Eocene

geological events in this region are dominated by the initial emergence of the Merida Andes, the northeastern tip of the Andes range. Some sedimentary units of fluvio-lacustrine origin might have been deposited during this time in the area south of the present Lake of Maracaibo, which seems to have been a depression all through the Cenozoic, and indeed up until modern times (Gonzdlez de Juana et al., 1980). Outside this region, an important regional unconformity se-

parates the Middle Eocene sequences from the Miocene and younger rocks in all Western Venezuela (see Fig. 1). In a geologically different area, the Falc6n Dasin, situated in Northwestern Venezuela, the youngest Eocene sediments are of Late Eocene age, belonging to the Globigerinatheka semiinvoluta Zone

(Hunter, 1974), and are found in an anticlinal structure in the southeastern part of the basin. Except for a few sandy intervals of Mio-

cene age, the bulk of the sedimentation in the eastern Falc6n Basin is composed of marls and clays deposited mostly at bathyal depths, with no obvious break in sedimentation. There have been no reports

of early Oligocene faunas anywhere in this area, the oldest dated Oligocene being from the Globorotalia opima opima Zone (Diaz de Gamero, 1982). The interval from the latest Eocene to this level is apparently barren of calcareous foraminifera, the only microfaunas studied to date. It is still undecided whether there is a hiatus at the Eocene-Oligocene boundary, as in all the rest of Western Ve-

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nezuela or, perhaps, the marine sedimentation was continuous, with a change in the physicochemical characteristics of the water mass. Within the Eastern Venezuela Basin, the Eocene-Oligocene boundary is, again, essentially marked by a hiatus. The youngest Eocene sedimentation in the basin, dated by planktonic foraminifera, is of latest Middle Eocene age (Hunter, 1 9 7 4 ) , but the Globigerinatheka semiinvoluta Zone of the Late Eocene has been recognized locally (Furrer, personal communication, 1 9 8 4 ) . Unconformably above these sediments a thin clastic sequence, devoid of fossils, is followed by a marine shale with microfaunas belonging to the Globoortalia opima opima and Globigerina ciperoensis zones of the Oligocene (Lamb, 1 9 6 4 ) In short, with the possible exception of the deep marine sequence of the easternmost part of the Falc6n Basin and some non marine sedimentation preserved locally in downfaulted or subsiding blocks, the Eocene-Oligocene boundary event was one of general uplifting and erosion in Venezuela, north of the stable craton of the Guayana Shield. The uplift was relatively short in Eastern Venezuela, but of a much longer duration, and accompanied by structural deformation in Western Venezuela.

REFERENCES Diaz de Gamero, M.L., 1 9 8 2 . Estratigraffa y micropaleontologfa de la parte norte de la subcuenca de Agua Salada, Falc6n oriental. Universidad Central de Venezuela, Unpublished, 158 pp. Furrer, M.A., 1 9 7 1 . La edad de la Formaci6n Paguey. IV Congr. Geol. Venezolano, Mem., I: 4 0 5 - 4 0 9 . Gonzdlez de Juana, C., Iturralde de Arozena, J.M. y Picard Cadillat, X., 1 9 8 0 . Geologla de Venezuela y de sus Cuencas Petrollferas. Ediciones FONINVES, Caracas, 2 vols., 1031 pp. Hunter, V.F., 1 9 7 4 . The mid-Tertiary stratigraphic unit of the southern Caribbean area., In: Jung, P. (Editor). Contributions Dedicated to the Geology and Paleobiology of the Caribbean and Adyacent Areas. The Kugler Volume. Verhandl. Naturf. Ges., Basel, 8 4 (1): 1 7 2 - 1 9 0 . Lamb, J.L., 1 9 6 4 . The geology and paleontology of the rio Aragua surface section, Serranfa del Interior, State of Monagas, Venezuela. Asoc. Venez. Geol. Min. y Petr., Bol. Inf., 7 ( 4 ) : 111-123. Walton, W.M., 1 9 6 6 . The Pauji and Mene Grande formations. Asoc. Venez. Geol. Min. y Petr., Bol. Inf., 9 ( 1 2 ) : 325-337.