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The Orientation of Sandstone-Filled U-Shaped Trace Fossils as Indicators of Deepwater Channel Axis Position, Tres Pasos Formation, Chile

Abstract

Trace fossils are widely established indicators of paleo-environmental conditions, which augment interpretations of various sedimentary successions and processes. In the study of slope channel deposits, accurately constraining channelform body orientations is challenging, traditionally relying upon paleoflow measurements and lateral facies relationships in outcrop belts. We show that trace fossil orientation can be closely tied to the position and trend of paleochannel axes. Well-constrained channelform bodies of the Cretaceous Tres Pasos Formation, Chilean Patagonia, are characterized by thick amalgamated sandstone beds in their axes that transition to finer and thinner bedded units towards their margins. The 3-D nature of the channelforms in the outcrop belt have been constrained through mapping and paleoflow measurements; in the area of interest, the southward flowing channelform under investigation trends 175–190°. A dense population of vertical to sub-horizontal, elongate and u-shaped trace fossils penetrate alternating sandstone and mudstone beds of the channel margin to depths of up to 1.5 meters. The trace fossil shafts are 0.5–1.0 cm in diameter; they are generally sandstone-filled vertical to sub-horizontal paired burrows characterized locally by feint spreite (Diplocraterion-like). The orientation and attitude of 402 burrow pairs were measured in 4 distinct localities. Trace fossils located more proximal to the channel axis demonstrate a strong preferred alignment normal to the channel trend (271°) and are characterized by shallow plunge away from the channel axis (ave = 36°). Trace fossils located further from the channelform axis display more varied trends, with steeper shafts and less sub-horizontal segments (plunge ave = 70°). The preferred alignment of axis-proximal trace fossils is attributed to a suspension feeding strategy adapted to the passing of nutrient- and oxygen- rich flows that flowed along the channel thalweg. This strategy was apparently less advantageous adjacent to the channel edge (i.e., axis-distal), perhaps in response to more varied turbulent flow pathways associated with overspill from the channel axis, as well as reflection off of channel edges. These data suggest that trace fossil orientation can aid paleo-channel reconstruction in less well-constrained outcrops, and perhaps in core-constrained subsurface studies