HR: 0800h
AN: U51A-06    [Abstracts]
TI: Provenance of the Subinal Formation, Central Guatemala, Based on Point-Counting of Pebbles in Conglomerates
AU: * Gutierrez, A
EM: cobanero_a@yahoo.es
AF: Cunor, Km 210 Carretera a Coban, Coban, Guatemala
AU: Martens, U
EM: umartens@pangea.stanford.edu
AF: Dept. Geological & Env. Sci., Stanford University, 450 Serra Mall, Bldg 320, Stanford, CA 94304, United States
AB: The continental Subinal Formation of Central Guatemala is composed of red conglomerates and sandstones that outcrop in the Motagua valley between the San Agustín and Cabañas fault, and in the southeastern corner of Guatemala. Stratigraphic position implies a post-Cretaceous depositional age, which has not been accurately established. Point counts of pebbles in conglomerates were performed in localities distributed along the Motagua valley from Granados to Los Amates, and in the Jocotán-Esquipulas area near the border with Honduras. Pebble types contained in conglomerates in the Motagua area are very diverse, and include sandstones, limestones, chert, milky quartz, phyllite, marble, chlorite schist, quartz-muscovite schist, amphibolite, eclogite, gneiss, granite, gabbro/diorite, volcanic rocks, and abundant serpentinite. Schist and gneiss resembles rocks of the Chuacús complex. Serpentinite, gabbro, amphibolite, and eclogite clasts are similar to oceanic lithosphere from the Motagua suture. Abundant white mica in interbedded sandstones is ubiquitous, suggesting derivation from the mica-rich Chuacús complex. Unfoliated granites and volcanic clasts were probably derived from the northern edge of the Chortis block or an unknown terrane, as no unmetamorphosed igneous rocks are known from the southern edge of the Maya block. The relative abundance of some clast groups correlates with the rock units exposed immediately adjacent to the north across the San Agustín fault. Serpentinite in conglomerate is most abundant near Juan de Paz; eclogite, amphibolite and gneiss are most abundant in the Granados area; and white mica is most abundant where Chuacús complex schists are most pelitic. This suggests that the Cabañas fault accommodated most of the strike-slip movement of the Motagua fault system. In contrast, conglomerates in southeastern Guatemala are more homogeneous, containing chiefly clasts of volcanic origin, with minor limestone, graphitic phyllite, chert, and milky quartz. Pebbles were probably derived from the underlying Padre Miguel volcanics that occur within the Chortis block, not from rock units in the Motagua suture zone. Pebbles contained in red beds in the Motagua valley were derived from rock units of the Motagua suture and the southern margin of the Maya block, but provenance from rock of the Chortis block is not clear. These findings suggest that red-bed-bearing basins in the Motagua valley and in the southeast of Guatemala were not connected, and may have formed at disparate times.
DE: 1100 GEOCHRONOLOGY
DE: 8106 Continental margins: transform
SC: Union [U]
MN: 2007 Joint Assembly