GEOLOGY AND METAMORPHISM OF CRETACEOUS ROCKS IN EASTERN CARABOBO STATE, VENEZUELAN COAST RANGES

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Abstract
Geological mapping was done in the eastern part of Carabobo State, Venezuela, and a map prepared at a scale of 1 / 75,000. The Caracas Group of earlier workers forms a southward-dipping homocline in the northern part of the area. The Las Brisas Formation is not exposed, and in this area the Caracas Group comprises the Las Mercedes Formation (Barremian?) below, consisting of garnet-quartz-mica schists and quartzites and graphitic schists, and the Tucutunemo Formation (Aptian-Albian?) above, consisting of graphitic phyllites, impure quartzites, and recrystallized limestones. The garnet isograd falls within the Las Mercedes Formation and runs parallel to the Las Mercedes-Tucutunemo boundary. The southern part of the area is composed of green, metavolcanic rocks of the Villa de Cura Group (green-schist grade). The Caracas and Villa de Cura Groups are separated in the eastern three quarters of the area by a narrow (½ to 5 km) fault-bounded strip of slightly metamorphosed phyllites, graywackes, conglomerates, and limestones of the Paracotos Formation (Santonian?-Maestrichtian). In the southwestern part of the area, the Tucutunemo Formation is conformably overlain by a sequence of black phyllites (60 per cent), metalavas (30 per cent), and black chert (10 per cent), here named the Araguita Formation. The formation is tentatively correlated with the Querecual Formation (Cenomanian?-Turonian) of the southern Serrania del Interior. The Araguita Formation is overlain by the Paracotos Formation both conformably (observed) and elsewhere uncomformably (inferred from the presence of Araguita pebbles within Paracotos conglomerates). On its southern side the Araguita formation is faulted both against the Villa de Cura Group and against hornblende gneisses of the Tinaco Complex (pre-Cretaceous). This complex is unconformably overlain by patches of calcareous, recrystallized sedimentary rocks for which an Albian age is suggested by comparison with nearby, unnamed fossiliferous beds of similar lithology. These Albian rocks probably underlie the Araguita Formation conformably on its southern side and correlate in part with the Tucutunemo Formation, upon which the Araguita Formation rests conformably farther north. No direct evidence is available on the age of the Villa de Cura Group, but it is unconformably overlain by slightly altered lavas of the Tiara Volcanic Formation (pre-Turonian-Coniacian) which somewhat resemble the Araguita lavas. Villa de Cura rock types are absent from the Barremian?-Maestrichtian metamorphic sequence previously mentioned and from adjacent Cretaceous sedimentary sequences. Paleogeographic considerations make it unlikely that the Villa de Cura Group was deposited in situ during earliest Cretaceous times. It is suggested that if the Villa de Cura Group is Cretaceous in age it must be an allochthonous block (280 km long and 28 km wide) which slid to its present position at the end of the Cretaceous; alternatively it may be part of the pre-Cretaceous metamorphic basement. The latter alternative is preferred. The principal faults in the area border the central strip of Paracotos rocks and are repeatedly offset by minor northwest-trending cross faults. In the Coast Ranges sedimentary basin there is locally a continuous sequence from the Las Mercedes (Barremian?) to the Paracotos Formation (Maestrichtian); this sequence shows a decrease in grade of metamorphism southward and upsection from albite-epidote-amphibolite facies rocks to beds almost unmetamorphosed. Paracotos conglomerates, however, contain metamorphic fragments of the Las Mercedes Formation which were metamorphosed before erosion. This, in addition to evidence of continuous Cretaceous sedimentation on the northern sides of the eastern and western Venezuelan basins adjacent to the metamorphic belt, suggests that the mid-Cretaceous metamorphism was a gradual process. It seems that all Cretaceous deposits in the Coast Ranges basin underwent a degree of metamorphism related to their depth of burial and their situation in a zone where abnormal thermal conditions prevailed during the Cretaceous, rather than to discontinuous and diastrophic episodes of metamorphism separated by periods of uplift and erosion.

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