Sedimentology of a Carboniferous very fine- to fine-grained sand bed river (Cypress Formation, Illinois): facies, paleohydraulics, and implications for reservoir and aquifer heterogeneity

ABSTRACT Rivers that transport fine-grained sandy sediment, and their associated deposits, remain understudied compared to coarser-grained alluvial channels, particularly in the ancient sedimentary record. This study details the sedimentology of a large (mean and maximum bankfull depth of c. 10 and 20 m, respectively), low-slope, fine-grained sandy river with low variance in grain size from the Carboniferous (Upper Mississippian) Cypress Formation in the Illinois Basin, USA, at the outcrop and regional scale. A combination of newly collected and existing subsurface (geophysical logs and core) and outcrop data was used to detail the subtle variability in the sedimentology of this fine-grained river, which is shown to be significant for interpreting the depositional setting, paleohydraulics, and reservoir heterogeneity of the Cypress Formation. Thick (up to ∼ 50 m) sandstones of the Cypress Formation are very fine- to fine-grained and dominated by six lithofacies associated with fining-upward fluvial channel fills deposited during a significant sea-level lowstand. Up to three stories of stacked channel-fill deposits are present, with the upper-most stories being most complete (∼ 14 to 20 m thick) and capped by a basin-wide paleosol. Small ( 0.27 m thick) and low-angle ( 15°) cross-beds, along with planar bedding and attenuated longitudinal bars (∼ 4 m thick) with low-angle ( 10°) master surfaces, dominate the channel fills and are interpreted to be characteristic of fluvial deposition under conditions of high suspended-load transport. As a result, the scaling of cross-set thicknesses to mean bankfull depths provides shallower depth estimates (∼ 4 m) compared with those derived from measured channel-story thicknesses (7–11 m). The erosional bases of channel stories, although subtle, correspond to abrupt grain-size coarsening and up to fourfold increases in permeability, revealing significant heterogeneity that is otherwise obscured and must be incorporated in reservoir and aquifer models to accurately represent flow behavior. Additionally, the well-sorted and texturally homogeneous sandstones comprising channel fills are interpreted to be the result of high rates of suspended-load transport and subsequent deposition, which can mask preserved stratification, and in some cases renders facies as completely massive. Therefore, seemingly massive, well-sorted, very fine- to fine-grained sandy facies require additional scrutiny, because their stratified sediments can be easily mistaken as structureless.