- Copyright © 2017, SEPM (Society for Sedimentary Geology)
Trunk rivers transport the bulk of the sediment in a source-to-sink (S2S) system, and total mass passing through any cross section (i.e., fulcrum) of a trunk-river over geologic time should allow matching of source-area sediment delivery budgets to downstream sediment volumes deposited in the basin. We analyze the paleohydrology of ancient trunk channels and link downstream deltaic strata of Allomember E of the Cretaceous Dunvegan Alloformation in the Western Canadian Sedimentary Basin to test the total mass-balance fulcrum approach. Bankfull channel depth and width, grain size, paleoslope, velocity, and discharge are derived from outcrop, core, and well logs. Some parameter estimates use multiple methods, providing a range of values and serve as a cross check of independent methods. Estimates of annual flood frequency and paleodischarge, associated with long-term geologic-time estimates, are derived from chronostratigraphic analysis and allow calculation of cumulative sediment discharge. Isopach maps are used to measure sink-area sediment volumes. The results indicate that the trunk river of Allomember E was 10–15 m deep and 150–250 m wide, carried fine- to medium-grained (< 200 microns) sand, and flowed over a low-gradient paleoslope of 4.1–6.1 × 10–5. Annual total sediment discharge is estimated to have ranged from 5.4 to 12 × 106 m3. Within 25,000 years, the river is estimated to have transported 135–307 km3 of sediment into the basin. This is consistent with the 130 km3 of sediment mapped in the study area. However, the upper-range estimate of sediment delivered into the sink is 2.5 times the measured sediment volume in the map area, which, if accurate, suggests significant sediment escape. This supports the hypothesis that in Dunvegan time, mud was widely dispersed southward, along the Alberta Foreland Basin by geostrophic currents associated with storm processes and counterclockwise oceanic gyres in the Cretaceous Seaway.