Abstract

Ancient trench-fill deposits within erogenic eugeosynclinal sequences are best recognized by their stratigraphic position above older hemipelagic and pelagic deposits. Such coarsening-upward sequences reflect migration of an abyssal site toward and ultimately into the trench. Axial transport and continuity of turbidites may help to better define trench geometry and sedimentation patterns but alone are not strong evidence for trench sedimentation. Likewise, sandstone petrography, because it is influenced by such variables as tectonic and geologic peculiarities of the source area and transport of detritus from distant terrenes, is not a useful tool for recognition of trench deposits. Melanges and broken formations are best considered as evidence for gravity transport of material onto the trench floor or tectonic deformation of trench fill when they are part of a coarsening-upward sequence.

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