Descripción litoestratigráfica y aspectos sedimentológicas de las unidades

Abstract

Fhe features of the eight lithostratígraphic Units at Los Santos de Maimona basin are as fóllow: Unit O: Greywackes aud interbedded volcanic beds. Some coal layers occur in the lower part of this unit near Los Santos de Maimona. The estimated thickness of this unit, which usually is not well exposed, is 700 m. It is regarded as shallow marine terrigenous sediments. Unit 1: The so—called Siphonodendron limestone, composed of biostremal mars and limestones containing abundant rugose corals and brachíopods and frequent algae, tubulate corales, foraminifers, bryozoans, echinoderms, ostracods and mollucs. Its thickness altains 35 m at Los Santos hill and decrenses to the north, being only 10 m al El Almendro. It is regarded as the red flat of a fringing red. Unit 2: Composed of shales and sandstones and interbedded volcanic rock. Some calcarcous lenses also occur. It is usually poorly exposed, and contains few fossils. In the calcareous beds, some rugose corals, brachiopods, algae and foraminifers arc identifiable. Its thickness reaches 80 m at Navafría and 100 m at El Portezuelo, although it decreases to the north. It is rcgarded as mixed deposits in shallow sea. Unit 3: Composed of massive limestone and interbedded marl. It occurs only in sections close to the border of the basin (Navafría, Cerro Almeña, La Alameda), and corresponds to the Iowa part of the Unit 4 in sections where it is flor present. It Contains common colonies of Lithostrotion and Siphonodendron, aud foraminifers are quite abundant. Its thickness is very irregular; Ihe maximum, more than 50 m, occuis at Cerro Almeña and Sierra Cabrera. It is regarded as lime sand shoals and calcareous lagoenal deposits in a very shallow platfoxm. Unit 4: Composed of graded crinoidal calearenite and interbedded mart and shale. The best exposures are at El Portezuelo aud Cerro Almeña. The calcareous layas are thicker and more abundant near the border of the basin, but shale is dominant in the míddle. Although macrofossils are scarce in this unit, foraminífers and fragments of crinoids, brachiopods, rugose aud tabulate corals, trilobites, calcareous algae aud bryozoaus are common. The thickness of this unit attains 80 m at El Portezuelo, in the middlec of the basin, and deereases to the borders; at Cerro Almeña it is 42 m, und at La Alameda and El Almendro it is only 25 m. It is regarded as carbonate turbidites. Unit 5: Composed of interbedded shale, marl and some limestone tenses. The contatus sume corals, brachiopods, crinoids, algae and foraminifers. Exposures arcvcry poer. The real thiekness could not be measure, bul it is estimated tobe about 70 m. it in regarded as distal carbonate turbidites. Unit 6: The same lithology as unit 4, but carbonate are less abundant. Some large calcareous blocks up te 20x10x5 m occur in the northern area. The unit cont-ains a rieti fauna of brachiopods and corals. Bryozoans, algae and foraminifers are also comnon. Its thickness attains 40 m at Las Pilitas and decreases toward the north, being only 25 m at El Salamanco Chico. It is regarded as carbonate turbidites. Unit 7: Composed of shale, and interbedded sandstone and conglomerale in Ihe Iowa part. Sorne compressed goniatites, trilobites and plant fragments occur in shale some 50 m aboye the Unit 6. Its estimaled thickness is 250 m, but it is ordinarily poorly exposed. It is regarded as basinal deposits.