Departmental Research in Earth, Environmental and Planetary Sciences
Graduate Interdisciplinary Earth Science Symposia (GIESS)
“Depositional Environments” by Eric Barefoot and Brandee Carlson
Lunch begins at 11:30 am
|Towards a mechanistic understanding of the linkages between PETM climate modulation and stratigraphy, as discerned from the Piceance Basin, CO, USA
Eric A. Barefoot
The Paleocene–Eocene Thermal Maximum (PETM) was a period of rapid climatic change when global temperatures increased by 5–8˚C in as little as 5 ka. It has been hypothesized that by drastically enhancing the hydrologic cycle, this temperature change significantly perturbed landscape dynamics over the ensuing ~200 ka. Much of the evidence documenting hydrological variability derives from studies of the stratigraphic record, which is interpreted to encode a system-clearing event in fluvial systems worldwide during and after the PETM. For example, in the Piceance Basin of Western Colorado, it is hypothesized that intensification of monsoons due to PETM warming caused an increase in sediment flux to the basin. The resulting stratigraphy records a modulation of the sedimentation rate, where the PETM interval is represented by a laterally extensive sheet sand positioned between units dominated by floodplain muds. The temporal interval, the sediment provenance history, as well as the tectonic history of the PETM in the Piceance Basin are all well-constrained, leaving climate as the most significant allogenic forcing in the Piceance Basin during the PETM. However, the precise nature of landscape change that link climate forcing by the PETM to modulation of the sedimentation rate in this basin remains to be demonstrated. Here, we present a simple stratigraphic numerical model coupled with a conceptual source-to-sink framework to test the impact of a suite of changing upstream boundary conditions on the fluvial system. In the model, climate-related variables force changes in flow characteristics such as sediment transport, slope, and velocity, which determine the resultant floodplain stratigraphy. The model is based on mathematical relations that link bankfull geometry and water discharge, impacting the lateral migration rate of the channel, sediment transport rate, and avulsion frequency, thereby producing a cross-section of basin stratigraphy. In this way, we simulate a raft of plausible, and mutually exclusive, climate-change scenarios for the case study of the Piceance Basin during the PETM, which may be compared to the stratigraphic record through field observation. The method described here represents a step towards connecting the impacts of global climate change to fluvial systems and sedimentation dynamics.
Tie channels on deltas: A case study from the Huanghe (Yellow River) delta, China
On river floodplains, tie channels convey sediment-laden water between the main channel and adjacent floodplain waterbodies. Field-based studies have documented that the bidirectional nature of flow in tie channels evacuates sediment deposited within the channel, thereby preserving an approximately consistent geometry over time. Herein, these findings are tested for an alternative fluvial environment: a deltaic system, where the distributary channel of an abandoned lobe is now occupied by a subordinate tie channel. The study region is the Yellow River delta of China; specifically, a lobe abandoned ten years ago due to a natural avulsion. An active tie channel maintains a connection between the main river and the adjacent Bohai Sea; therefore, water flux arises due to both riverine and tidal inputs. Measurements of the tie channel location, collected using remote sensing data, indicate that it has migrated laterally several hundred meters since inception, while concomitantly width has reduced by a factor of ten. Several field data sets were collected to constrain the morphological evolution of this tie channel, and compare formative processes to those known for floodplain tie channels. These data include sediment cores, measurements of water stage and flow velocity within the tie channel, and detailed elevation surveys of both the tie and abandoned distributary channels. Preliminary analyses show that, under low to moderate riverine discharge, tides are the primary driver for changes in water stage and flow velocity within the tie channel; additionally, sedimentation arises at the main river and tie channel junction, nearly impeding movement of water between the two channels. This observation raises the question: What maintains the tie channel as an open flow conduit? It is likely that during river floods, enhanced water flux removes this sediment, thereby maintaining the tie channel as an open flow path. However, such floods have not occurred in the past two years, and there is notable encroachment of vegetation into the tie channel. Unlike oxbow tie channels, bi-directional flow may not be required to maintain geometry of deltaic tie channels; rather, it is the episodic movement of water during floods that maintains the channel over time, with narrowing due to vegetation growth during prolonged low flow periods.