Probabilistic source-to-sink analysis of the provenance of the California paleoriver: implications for the early Eocene paleogeography of western North America
The Latest Paleocene to Early Eocene Colton and Wasatch Formations exposed in the Roan Cliffs on the southern margin of the Uinta Basin, UT make up a genetically related lobate wedge of dominantly fluvial deposits. Estimates of the size of the river that deposited this wedge of sediment vary by more than an order of magnitude. Some authors suggest the sediments are locally derived from Laramide Uplifts that define the southern margin of the Uinta Basin, the local recycling hypotheses. Other authors suggest the sediments were transported by a river system with headwaters 750 km south of the Uinta Basin, the California paleoriver hypothesis. This study uses source-to-sink analysis to constrain the size of the river system that deposited the Colton-Wasatch Fm. We pay particular attention to the what magnitude and recurrence interval of riverine discharge is preserved in the Colton-Wasatch Fm. stratigraphy, and consider the effects this has on scaling discharge to the paleo-catchment area of the system. We develop new scaling relationships between discharge and catchment area using daily gauging data from 415 rivers worldwide. Previous studies using a global database of rivers use average discharge in scaling relationships to catchment area. We demonstrate that average discharge is a flow of variable magnitude and recurrence interval in rivers with different hydrology, and that at higher magnitudes of discharge, especially 99th percentile discharge and higher, the discharge yields and recurrence intervals of flow events of given magnitude are similar across climates and hydrological regimes. These scaling relationships between 99th percentile and higher magnitudes of flow and catchment area have greatly improved predictive precision compared to existing relationships using average discharge. The discharge events our new scaling relationships are based on are also of similar magnitude and recurrence interval as bankfull flow in modern rivers. Bankfull flow is typically considered the flow state that is in geomorphic equilibrium with deposits of ancient river systems. We test our new scaling relationships between 99th percentile and larger discharge and catchment area in modern rivers on a dataset of six ancient rivers of variable size, climate, hydrology where the catchment size is constrained by other methods. We demonstrate that our new scaling relationships introduce less selection bias and upscaling issues than regional curves that are used in other source-to-sink studies. We also demonstrate the utility of a probabilistic approach to source-to-sink analysis of the catchment area of ancient river systems using a Monte Carlo simulator. Using this method we quantitatively evaluate the likelihood of multiple hypotheses of catchment area for the river systems of interest. We show that discharge events 99th percentile and higher have somewhat characteristic recurrence intervals across climatic and hydrological regimes, and that the higher the magnitude of the recurrence interval that characterizes it will be longer. Given that in modern systems rivers with highly variable hydrology have longer recurrence intervals of bankfull, or the channel-forming discharge, than in rivers with persistent hydrology, it then follows that these systems are in geomorphic equilibrium with higher magnitude discharge events. This has an important impact on our source-to-sink analysis as it predicts that given two river systems with the same catchment area, the trunk channel in the river with more variable hydrology will have larger equilibrium channel dimensions. Of our six ancient river systems studied using our probabilistic source-to-sink method, the Colton-Wasatch Fm. is the endmember of a flood-dominated fluvial style. The Colton-Wasatch Fm. was deposited concomitantly with the Paleocene-Eocene Thermal Maximum, a punctuated greenhouse climate change event associated with seasonal to flood and drought prone hydrology in mid-latitude continental interiors. The sedimentology of the Colton-Wasatch Fm. is dominated by Froude-supercritical and high-deposition rate sedimentary structures, downstream accreting barforms, in-channel pedogenically modified fine-grained deposits, and abundant in-channel continental bioturbation that suggest deposition during the waning stage of floods and long-term channel abandonment between depositional events. The river was highly avulsion prone due to the high variability in discharge and built a large fluvial fan system in its depositional terminus in the Uinta Basin. We statistically project the apex position of the fan in the lower part of the Colton-Wasatch Fm. based on paleocurrent measurements across the basin to a location ~250 km south of the Uinta Basin. The maximum progradation of the fan system is recorded in the middle part of the Colton-Wasatch Fm., and we interpret the fan apex was near Sunnyside, UT at this time. This suggests the fluvial fan prograded more than 100 km in response to punctuated climatic forcing from the lower to the middle part, and later back-stepped in the upper part of the Colton-Wasatch Fm. in what was likely a climatic recovery phase. We apply a scaling relationship to the ~25 m deep and ~750 m wide channel deposits in the Colton-Wasatch Fm. that implies the channels are in geomorphic equilibrium with a ~50-100 year flood event. This scaling relationship is based on the maximum recorded discharge in our dataset of gauging data for 415 modern rivers that has a median duration of 66 years. Of our scaling relationships based on 99th percentile discharge and higher, this relationship produces the most conservative estimates of catchment area. Our probabilistic evaluation of the two hypotheses as to the provenance of the Colton-Wasatch Fm. suggests the California paleoriver hypothesis is more than ten times as likely as the local recycling hypothesis. This is supported by our interpretation that the size of the fluvial fan that is the terminus of this river is larger than the catchment area predicted by the local recycling hypothesis. In this way, our source-to-sink and sedimentological/stratigraphic analyses are complimentary, and strongly support a far-traveled provenance for the Colton-Wasatch Fms. transported by an arid and flood-prone California paleoriver that was the dominant axial drainage system in the southwestern US during the Early Eocene.
discharge, Monte Carlo, scaling relationship, fluvial fan, California paleoriver, provenance
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