Universality of delta bifurcations

Post by Dr. Robert C. Mahon, Department of Earth and Environmental Science, University of New Orleans

Which morphologic features of sedimentary systems persist into the stratigraphic record? Ancient river deltas preserved as stratigraphic deposits on both Earth and Mars exhibit remarkable morphologic similarities to modern deltas on Earth (Images 1-3). While channel dynamics may be expected to alter the geomorphic expressions of past channel networks, in many cases channel bodies appear to be preserved in their original configurations. Fully understanding the ways in which geomorphic features become preserved as stratigraphy can provide tools for us to both infer past processes from the ancient deposits with greater confidence, as well as to predict the geometries of ancient deposits in the subsurface (i.e. for resource exploration).

Aeolis

Image 1: CTX image mosaic of a delta in the Aeolis region of Mars, showing distributary channel networks. CTX imagery courtesy of NASA-MRO/JPL/UA.

Theory developed in modern river deltas predicts that there should exist a characteristic bifurcation angle at river delta channel mouths of 72°, irrespective of gravitational acceleration or sediment type (Coffey and Shaw, 2017). We sought to test whether modification of this predicted bifurcation angle after initiation occurred in the transfer of geomorphic features to the ancient stratigraphic record on Earth using 3D seismic data. We also sought to test the expected “universality” of this bifurcation geometry by looking at ancient deltaic deposits on Mars, using high resolution orbital imagery and digital terrain models. The initial findings of this analysis of both terrestrial and martian ancient delta stratigraphy indicates that the bifurcation angles are consistent with theoretical predictions, with mean values of approximately 72° (Mahon and Shaw, EarthArXiv pre-print).

Selenga

Image 2: Landsat 8 image of the Selenga River Delta flowing into Lake Baikal, Russian Federation. Landsat 8 image courtesy of the United States Geological Survey.

 

MarshIsland

Image 3: Seismic coherence horizon slice from Pleistocene paleo-Mississippi delta deposits near South Marsh Island, Louisiana (non-copyrighted image, modified from Nissen, 2000).

Further Reading

Coffey, T. and Shaw, J.B., 2017, Congruent bifurcation angles in river delta and tributary channel networks: Geophysical Research Letters, v. 44, no. 22, p. 11,427–11,436, doi: 10.1002/2017GL074873, https://agupubs.onlinelibrary.wiley.com/doi/full/10.1002/2017GL074873

Mahon, R.C. and Shaw, J.B., (in revision), A faithful record of channel mouth bifurcation angles in river delta stratigraphy on Earth and Mars: EarthArXiv pre-print, doi: 10.17605/OSF.IO/9R64A, https://eartharxiv.org/9r64a/

Nissen, S., 2000, Interpretive aspects of seismic coherence and related multi-trace attributes: Kansas Geological Survey, Open-file Report 2000-84, http://www.kgs.ku.edu/PRS/publication/2000/ofr84/index.html

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