Sedimentary facies associations within subduction complexes

Abstract

Summary Sedimentation patterns within modern subduction zones are complex and variable, and do not necessarily follow models of submarine fan sedimentation. Environmental reconstructions within ancient subduction complexes should follow modern analogues as closely as possible and consider several criteria, including turbidite facies associations, vertical depositional cycles, regional palaeocurrent patterns, and in many cases, structural style and sandstone petrology. Important variables in trench sedimentation include the volume and texture of sediment entering the trench and the distribution of major sediment sources, especially large submarine canyons. Sediment transport in the trench is commonly longitudinal, although locally, such as at the mouth of a submarine canyon, flow may be at a high angle to the continental margin. Patterns of trench-slope sedimentation depend largely upon the topography of the slope. In general, coarse sediment is either trapped behind tectonic ridges (within slope basins) or bypasses the slope via submarine canyons. Background sedimentation is dominated by hemipelagic settling. Current directions are commonly at a high angle to the margin, but longitudinal flow may occur within large elongate slope basins. Major facies associations include submarine-canyon, slope, mature-slope-basin, and immature-slope-basin.

A High Resolution Geophysical Survey of Deep Sea Drilling Project Leg 84 Site 570

“…The morphology of the middle slope (1000-3000 m) around Site 570 confirms previous observations that smaller canyons and gullies have an important influence on slope sediment distribution (Karig et al, 1978;Underwood and Karig, 1980;Underwood and Bachman, 1982). Many of these canyons and gullies begin or end within the survey area suggesting rapidly varying depositional centers and erosional sites.…”

“…The high relief along the upper slope (200-1000 m) may have developed (1) by erosion of the shelf edge off Guatemala during glacial low sea-level stands (Seely, 1979;Underwood and Karig, 1980), (2) by mass movement processes at the shelf edge without subaerial exposure (Field and Clarke, 1979;Nardin et al, 1979;Underwood and Bachman, 1982), or (3) by some combination of both.…”

The shape of deep-water siliciclastic systems: A discussion

“…The most favorable sites in which non-fan ponded systems develop appear to be active, convergent, margin settings especially in forearc basins as shown by Underwood and Baehman [47]. However, other possible non-fan ponded systems may befound in some of the fault-controlled Atlantic margin basins north of Cape Hatteras: the basins vary from 50 to 350 km wide [48].…”

“…Unfortunatley, the submarine fan facies models and terminology are being over-emphasized in the active margin successions where fan morphology is generally unproven (though channels and sheet sandstones naturally occur together), as for example in the recent paper by Underwood and Bachman [47]. It may prove possible to define non-fan ponded systems on the basis of facies-associations, and this is deafly an area in which future research should develop because we are unable to do this at present.…”

Sedimentology and structure of the trench-slope to forearc basin transition in the Mesozoic of Alexander Island, Antarctica

“…There is no difference in clast compositions between the two types, suggesting similar provenance. It is likely that most of the LeMay Group sedimentary rocks in the area were deposited in a trench, where coarsening P. A. DOUBLEDAY, D. I. M. MACDONALD & P. A. R. NELL upwards sequences are common (Moore et al 1980;Underwood & Bachman, 1982). However, the low degree of deformation and metamorphism in the Light Sandstone Member suggests that these rocks represent either the very top of a frontally accreted slice, or the upper part of the fill of a trench-slope basin.…”