Early Diagenesis of Sands and Sandstones from Middle America Trench and Trench Slope, Offshore Mexico and Guatemala: ABSTRACT

Abstract

Siliciclastic sediments from the northern and southern segments of the Middle American Trench are distinctive both petrologically and diagenetically. Samples of terrigenous sands recovered in piston and DSDP cores from the continental margin of southern Mexico are primarily arkose and lithic arkose of plutonic and metamorphic provenance. Accessory constituents and diagenetic features are useful criteria for distinguishing sands from different tectonic provinces along the Mexican (northern) segment of the trench. Basal, early Miocene sandstones from the upper- and middle-slope regions (continental block) contain abundant skeletal grains and are cemented by calcite or gypsum. Early Pliocene to middle Miocene sands from the accretionary wedge are weakly lithified and contain fractured framework grains. Unlithified trench sands of Quaternary age have undergone significant pore-space reduction at very shallow burial depths. Samples of Holocene terrigenous sands recovered in piston cores offshore from central Guatemala (southern segment) are feldspathic litharenite and litharenite of volcanic provenance. Authigenic pyrite is ubiquitous in these sands, and pore-filling phillipsite occurs locally. Partial dissolution of glass fragments, pyroxene, and plagioclase has occurred in sands from every environment sampled. The differences observed in sands from the two segments of the Middle America Trench may also characterize siliciclastic sediments deposited along other segmented convergent margins. Within a trench segment, changes in sand composition with time could indicate intermittent volcanic activity and changes in relative sea level. End_of_Article - Last_Page 505------------