LITHOFACIES AND PROVENANCE OF EOCENE REDBEDS ALONG THE NORTH AMERICA-CARIBBEAN PLATE BOUNDARY IN GUATEMALA

Redbeds of the Subinal Formation in the Motagua Valley of Guatemala represent Eocene-Oligocene(?) deposition in a strike-slip basin along the North America-Caribbean plate boundary. The best exposure of the unit (road CA-9 north of Guastatoya) consists of meter-scale lenticular conglomerate bodies and lateral accretion bars that cut into floodplain fines and crevasse-splay deposits. The conglomerates show crude cross-stratification and erosional bases. The sand bodies are composed of coarse- to fine-grained sandstones with lenticular geometry, which accrete laterally and vertically. The floodplain deposits are rich in pedogenetic features such as micritic nodules, bioturbation with skolithes and rhizolites, as well as plant deposits. The overbank fines are interbedded with thin and tabular beds of fine-grained sandstones with ripple cross-laminations. These lithofacies are interpreted as produced by an axial river with moderate sinuosity. Detrital zircon core U-Pb geochronology of five samples show predominance of ~1075–925 Ma ages, a ubiquitous population in the geologic record of Mexico and Central America. The oldest grains are ~1570 ± 50 (n = 4). More indicative of provenance are populations with ages of ~285–235 Ma and ~135–75 Ma. The former is related to the Permian-Triassic East Mexico Arc and possibly gneisses in the highlands of Central Guatemala. We surmise that Cretaceous zircon was sourced from relic arc and forearc deposits of the eastward migrating Great Caribbean Arc that had collided with North America in the Campanian-Maastrichtian. Petrographically, sandstones are angular lithic arenites devoid of matrix mainly bearing quartz, feldspars, serpentinite, volcanic lithics, low-grade metamorphic lithics, white mica, chlorite, and rutile. These features suggest ultramafic, metamorphic, and volcanic sources. Some of the detrital minerals in Subinal are reminiscent of grains in the Chuacús Complex. The presence of abundant antigorite schist and scarce eclogite clasts in conglomerate indicates that northern high-pressure serpentinite mélange had been exhumed at the time of Eocene-Oligocene(?) deposition of Subinal. We propose Eocene depositional conditions in an elongate basin not unlike the current Motagua River.