Abstract

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 1750 in. - 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 earlier 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 pre-served under distinct environmental conditions in synclinal and downfaulted valleys at elevations exceeding 10,000 ft. The shelfal Lacandon Formation is detrital limestone. Five early Tertiary and 6 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 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 Santa 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 Vista. The thickness of the Peten Group is 1500 m. Mid-Tertiary nondeposition 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 1500 m. 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 1000 in., the Herreria 240 in., and the Armas 2500 m. The seas withdrew at the end of the Pliocene. Quaternary deposits masks older rocks in some areas.

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