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The transect of drill sites across the Middle America Trench offshore Guatemala contains portions of the sedimentary record from the Early Cretaceous to the Recent. Hemipelagic muds cover both slopes of the Trench; the thick accumulation on the Cocos Plate indicates transport of these sediments hundreds of kilometers seaward of their landward-slope source area. Beneath those muds, the sedimentary section of the Cocos Plate is a classic, basalt-chalk-abyssal clay sequence that records the passage of Site 495 from the East Pacific Rise to depths greater than the calcite compensation depth. The eight holes drilled at Sites 499 and 500 contain portions of the sequence cored at Site 495, only the abyssal red brown clays of Site 495 are completely missing and a deposit of turbidites caps the hemipelagic muds originally deposited seaward of the Trench. A horst is buried beneath the turbidites ponded in the Trench axis. Facies changes are absent within the turbidites, and biogenic components indicate transport from upslope. The sediments, sedimentary features, and microfossils cored from the landward slope generally indicate deposition beneath nutrient-rich equatorial waters and subsequent downslope displacement. Tranquil accumulation was interrupted by deposition of pebbly conglomerates at Sites 496 and 497. Sigmoidal veinlets are common and probably formed as the slope sediments dewatered. Numerous gas voids and greatly expanded core sections characterize these samples. Gas hydrates occurred in the deeper sandy layers at both sites. Beneath the blanket of hemipelagic mud at Site 494, deposits older than the Miocene have a varied lithology and an age sequence reminiscent of deposits outcropping on the Nicoya Peninsula of Costa Rica. The differences between the lithologic section at Site 498 (drilled just to the east of Site 494) and the section at Site 494 indicate that the structure of the lower slope is more complicated than suggested by interpretive models derived from geophysical data. The lithology at both Sites 494 and 498 puts severe restrictions on the volume that an accretionary prism can occupy offshore Guatemala, the type locality for the Trench-Slope Model.

A Summary of the Sedimentology of Deep Sea Drilling Project Leg 67 Sites: The Middle America Trench and Slope Off GuatemalaAn Active Margin Transect

A Summary of the Sedimentology of Deep Sea Drilling Project Leg 67 Sites: The Middle America Trench and Slope Off GuatemalaAn Active Margin Transect