60th Annual Report on Research 2015

Numerical Modeling: In this this study, we developed a new method to track landscape change in numerical models using a vector-based tracking method, rather than the more typical method of a grid which biases simulation results.  In Limaye and Lamb [2013], we demonstrate the utility of the new framework for three topics of relevance to reservoir exploration: 1) floodplain evolution with sand, mud and variable bank heights; stratigraphic architecture of stacked channel bodies; and the evolution of bedrock river valleys.  In Limaye and Lamb [2014], we explore the key parameters that control whether incising meandering rivers bevel wide valleys with terraces or become entrenched into canyons. Finally in Limaye and Lamb [in review, Journal of Geophysical Research], we show that terrace formation regularly occurs by meandering rivers in the absence of external changes in climate or tectonics (Fig. 1).  We explore the frequency and geometry of autogenic terraces and discuss how they may overlap and be distinguished from climate-change driven terraces (Fig 2). 

Fig. 1.  Example numerical simulation where river terraces are highlighted in color in planview (left) and appear as benches stranded above the valley floor in cross section (right).  These terraces formed in the absence of external perturbations in climate and tectonics.

Field Application: We have developed an automated, objective computer algorithm for detecting river terraces that can be applied to digital elevation data from natural landscapes and data generated in our simulated landscapes. The algorithm allows quantitative characterization of terrace geometries (e.g., heights, lateral extents, and slopes), which can be used to attempt to decipher intrinsically generated terraces from those caused by climate change. In Limaye and Lamb [2014; in review], we applied the terrace detector to six rivers across the US and Canada.  The Colorado River, TX, in particular has been a key field site where river terraces are interpreted to record pulses of incision driven by climate and sea-level change, which in turn is the basis for widely used stratigraphic models to characterize reservoirs (i.e., sequence stratigraphy).  In contrast to prevailing conceptual models, our analysis indicates that many of the terraces in these rivers, including the Colorado, have the fingerprint of intrinsically generated terraces rather than those forced from climate change (Fig. 2).  In ongoing work in the San Gabriel River, CA, we collected and have analyzed samples for cosmogenic exposure age dating to help determine the whether terraces there are forced by climate change [Scherler et al., in review, GSA Bulletin] 

Fig. 2.  Phase space where meandering rivers are expected to make autogenic terraces, and whether those terraces are expected to be short and unpaired versus long and paired.  Autogenic terrace predictions overlap with many field cases.  

Implications for the Rock Record.  In addition to field studies of modern rivers, our work through this grant on the effect of climate change on river systems has led to two additional scientific contributions with implications for deeper geologic time.  In Ganti et al. [2014] we describe a new method to separate external forcing on landscapes, like climate change, from the internal dynamics of landscapes across a broad suite of environments including river terraces, deltas, and wind dunes.  In Johnson et al. [2014], we apply some of these concepts to determine the likely oxygen content of the atmosphere during the Paleoproterozoic time by analyzing detrital pyrite and uraninite in river deposits.

Publications: This work to date has resulted in four published journal papers, with two more in the final stages of review, and one in preparation. We have also presented this work at a number of conferences (American Geophysical Union Fall Meeting, American Association of Petroleum Geologists Meeting, and Geological Society of America Meeting).

Overall Scientific Impact:  Sediment transfer from rivers to oceans is the foundation of sedimentary basin filling models used to evaluate reservoir potential and correlate between boreholes.  The research from this grant is impacting these models by providing new expectations for the mechanisms that create river terraces, the key landforms used to understand river response to perturbations in climate, tectonics in sea level, which in turn ultimately drives the architecture of sedimentary basins. Our work on landscape evolution by meandering rivers also applies directly to the stratigraphy of fluvial reservoirs including the distribution of sand and mud, and reservoir connectivity through channel-body stacking. 

Research Team and Impact: This grant supported one summer undergraduate fellow, Michael Jensen, who assisted with field data compilation in the summer of 2013.  Most of the funding of this project has gone to support PhD student Ajay Limaye, who graduated in 2015, and Luca Malatesta, who is a third year PhD student. Ajay is now a postdoctoral scholar at the University of Minnesota.  This project also provided partial support for postdoctoral scholar Vamsi Ganti, who is now a lecturer at Imperial College, London.  Finally, this grant allowed the PI, Michael Lamb, to open up a new direction of research into meandering rivers, terraces and climate change, topics he had not investigated previously. The work performed under this award is expected to be used as a springboard for a larger research effort to be submitted to the National Science Foundation.