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Upper Cretaceous and Tertiary sediments cover more than half of Guatemala and were deposited in the Chapayal basin, an extension of the Gulf Coast embayment. The basin's foredeep lies between a southern borderland and the Yucatan foreland. Deposition in the Caribbean was connected during late Mesozoic and early Cenozoic time with the deposition in the Chapayal basin by the Sarstun portal. The first age-identifiable sediments are the Carboniferous and Permian. The earliest Gulf Coast embayment deposition in the Chapayal basin was probably that of the late Jurassic Todos Santos Formation, followed by the thick Lower Cretaceous carbonates and evaporites of the Coban Formation. Overlying the Coban, the Upper Cretaceous units--which have an aggregate thickness of approximately 1,750 meters--include Senonian limestones of the Campur Formation and younger clastics of the Verapaz group. The latter consists of the Chemal, Sepur, and Lacandon Formations deposited in early episodic Laramide stages, with the Chemal and Sepur in the foredeep region and the Lacandon on the foreland. The Chemal and Sepur are an argillaceous facies, but the Chemal was deposited only in the region of the present-day Cuchumatanes Mountains and is preserved under distinct environmental conditions in synclinal and downfaulted valleys at elevations exceeding 10,000 feet. The shelfal Lacandon Formation is detrital limestone. Five early Tertiary and six late Tertiary formations occur in the Chapayal basin and Amatique embayment, and cover more than one-third of Guatemala. Previously, only the Rio Dulce and Toledo have been mentioned formally in the literature. The Sepur, thought by Sapper (1937) to be Eocene, is now known to be Cretaceous at the type locality and in other areas. The early Tertiary formations are assigned to the lower Eocene Peten Group. They were deposited contemporaneously and are differentiated on lithology and basin position. The Reforma, Cambio, and Toledo are the shaly formations in the foredeep region. The Toledo occurs only in the Amatique embayment. Shales with conglomeratic lenses characterize the Cambio, and softer clay shales identify the Reforma. The shelfal formations are the anta Amelia and Buena Vista, defined separately by an extensive basal gypsum bed in the overlying Buena Vista. Carbonates and evaporites occur in both formations, but evaporites predominate in the Buena End_Page 425------------------------------ Vista. The thickness of the Peten Group is 1,500 meters. Mid-Tertiary non-deposition divided Tertiary sedimentation into early and late periods. Late Tertiary sedimentation is represented by Oligocene to Pliocene deposits in the Chapayal basin and by Miocene to Pliocene(?) deposits in the Amatique embayment. The Chapayal basin formations are the Desempeno conglomerate, the Lacantun, and the Caribe which have an aggregate thickness of 1,500 meters of terrestrial and deltaic claystones, sandstones, and conglomerates. The Amatique embayment is a graben continuation of the Bartlett trough. The Miocene Rio Dulce Limestone is its oldest late Tertiary formation. Following late Miocene uplift, clastics of the Pliocene(?) Herreria and Armas Formations were laid down. The Rio Dulce has a thickness in excess of 1,000 meters, the Herreria 240 meters, and the Armas 2,500 meters. The seas withdrew at the end of the Pliocene. Quaternary depo its mask older rocks in some areas.

Nuclear Central America comprises the eastern part of the Sierra Madre del Sur geanticline and its flanking geosynclinal portion of the Gulf Coast and Caribbean embayments. Southeastern Mexico, Guatemala, British Honduras, Honduras, and Nicaragua make up the principal land area. Nuclear Central America disappears toward the east into the Caribbean Sea in easterly trending tectonic lineaments. The north flank of this geanticline is the crucial area for regional geologic interpretation. The Mesozoic-Cenozoic Chapayal basin, or the eastern extension of the Chiapas foredeep, and the southern part of the Yucatan platform are the prime sedimentary areas involved. Chapayal basin, one of the local deep basins that ring and nearly surround the Gulf Embayment, is sharply asymmetric, having a steep and highly folded and faulted south limb and a gentle opposing limb which shelves northward over the Yucatan platform. The eastern part of the shelf area is interrupted by the Maya Mountain uplift in British Honduras which developed during the Paleozoic and was rejuvenated at the end of the Paleozoic. The Maya Mountains represent a remnant of an older Paleozoic hinterland that provided a source for later Paleozoic and Mesozoic sedimentation. It was a stable or slightly positive are during much of Mesozoic and Cenozoic time. The Late Paleozoic-Mesozoic mobile belt, which sets the pattern for the geology of the nuclear Central America and the Antillean region, extends eastward End_Page 411------------------------------ from the mainland through the Greater Antilles. This mobile belt separates the Gulf and Caribbean regions, and the forelands to the bordering basins were in the Gulf of Mexico and Atlantic Ocean. The initial and main period of geosynclinal subsidence and sedimentation is Mesozoic in age; however, thick deposits were laid down during the Tertiary in the Chapayal basin and eastward in various island areas. In nuclear Central America the sediments were derived, primarily, from the south and from an important older Paleozoic mobile belt and its subsequent ancient Maya Mountains-Cayman Basin landmass lying on the northwest flank of the Caribbean. The partly foundered northwest Caribbean hinterland, and the arcing of the Late Paleozoic-Mesozoic and Late Mesozoic mobile belt (which incorrectly suggest a foundered craton and surrounding rim syncline in the Antillean-Caribbean region) provide the basic framework for the tectonic relationships between North and South America. It is highly questionable that the Lesser Antilles is related in time with the earlier Late Paleozoic-Mesozoic mobile belt, but is more likely late Cretaceous and early Tertiary. The foundering of the Gulf craton occurred ith the beginning of the Mesozoic, and the initial deposits of the Gulf Embayment are the red-bed clastics and associated sediments of Triassic-Jurassic age. The older and partly metamorphosed sediments in nuclear Central America and its environs include undifferentiated Carboniferous and older Paleozoic, and possible Precambrian. Younger unmetamorphosed sedimentary rocks are Permian, Triassic (?), Jurassic, Cretaceous, lower Eocene, Oligocene, and Late Tertiary in age and include one of the thickest Mesozoic evaporite sequences known in the world. Important orogenies are reflected in the sediments during all the major diastrophic events and during the early Oligocene. Intended as a principal contribution to the geology of the region is the introduction here of a supporting stratigraphic chart showing those formation names and ages which are accepted in Guatemala by the local Stratigraphic Nomenclature Committee. A late Tertiary and Quaternary volcanic belt follows the Sierra Madre axis for a short distance in eastern Mexico and Guatemala and diverges southeastward through the remainder of Central America, forming the physiographic Rocky Mountain backbone. No more than two dozen wells have been drilled for oil or gas in all northern Central America. Although the results have been negative, numerous encouraging shows indicate a future petroleum province. Evaporitic deposits in the Cretaceous and Jurassic limit the potential section of reservoir porosity. This poses no insurmountable problem, however, for explorationists utilizing thorough regional stratigraphic and tectonic studies. End_of_Article - Last_Page 412