Abstract The stratigraphic fill of incised valleys has traditionally been interpreted to be primarily modulated by allogenic controls. The use of this concept has been so dominant that the possibility of autogenic mechanisms controlling fluvial organization of incised valley fills (IVFs) is largely overlooked, particularly in rock-record interpretations. This has been mainly due to the fact that deconvolving autogenic from allogenic signals remains challenging, especially for IVF deposits. Using integrated light detection and ranging (LiDAR), outcrop, and core data, we investigated the fluvial architecture of two IVFs in the lower Blackhawk Formation (Upper Cretaceous) of the Western Interior Seaway, Utah. Contrary to conventional interpretation, our analyses demonstrate that an autogenic signal linked to differential compaction of coal-precursor peats underlying IVFs likely exerted substantial control in both the vertical and lateral organization of sand-body architecture in these two IVFs, which are up to ~15 to 20 m thick individually. Trends in vertical-amalgamation thickness, number of channel-story sand bodies stacked vertically, and width constraints of multilateral sand bodies (lateral amalgamation) of these two IVFs are correlated with thickness variation of underlying coal seams. Decompaction analysis of coal seams indicates that the magnitude of accommodation-creation by coal-precursor peat compaction was potentially much higher to overcome allogenic modulation. This is invoked as the principal reason for broad correlation between fluvial architecture of the IVFs and coal thickness in our data set. These findings contribute to isolating autogenic from allogenic signals in complex systems such as IVFs. They further provide insights on signal-shredding mechanisms in the depositional architecture of the Cretaceous Western Interior Seaway, and they supply evidence that paleovalley fluvial architecture should not be automatically attributed to allogenic processes.

Abstract Detailed logging of ichnological variations within parasequences of several Cretaceous (Upper Turonian) delta complexes from Wyoming and Utah are correlated with inferred short and long-term changes in depositional processes. These changes reflect various proportions of river, flood, wave, storm, and tide influences. Event beds, such as storm and river-flood deposits, tend to show low BI (Bioturbation Index) values of 0-2, owing to high accumulation rates, although this also depends on event frequencies. Upper surfaces of individual storm/river-flood beds may show BI values of 4-5, reflecting the transition to longer-lived fair-weather conditions. Fair-weather waves facilitate persistent agitation near the bed, buffering environmental stresses. Therefore, wave-dominated deposits that are not affected by storms yield climax communities with robust and diverse ichnofacies signatures reflecting “uniform and high” BI trend with values that average 4. River-dominated intervals show the least uniform trends of BI, because of the highly variable conditions related to river jet and plume behavior. BI values vary from 0 to 4, with generally low ichnogenera diversities. These alternations likely record seasonal to centennial fluctuations in sedimentation rate (river discharge) and water turbidity, which influences substrate conditions near distributary mouths. Tide-dominated intervals tend to show the most ‘stressed’ conditions, reflecting “non-uniform and low” trend of BI, with values of 0-2. These reflect salinity fluctuations, heightened water turbidity, rapidly shifting substrates, and narrow colonization windows associated with daily and monthly changes in tidal periodicity. Individual parasequences are characterized by either upward-increasing and upward-decreasing trends of BI, indicating protection from storm erosion and proximity to river input, respectively. Ichnological signatures change significantly across initial flooding surface, principally showing a marked increase in BI as delta lobes are quickly abandoned and transgressed. In contrast, across the maximum flooding surface, changes in ichnological signatures are subtle and rather uniform. We suggest that different parts of a single delta may experience marked differences in river, wave, and tide influence over time, reflecting the enormous complexity of operative processes at various temporal and spatial scales. Detailed intra-parasequence, bed-scale analyses of trace fossils help to reveal this complex evolutionary history of a single delta.

Abstract Outcrop and high-resolution seismic studies show that prograding delta deposits consist of seaward-dipping, offlapping clinoform strata. Despite this, many studies of Quaternary deltas, particularly those based on correlation of sediment cores, commonly depict sharp to gently undulating facies boundaries, similar to those originally shown by Scruton in 1960. The Scruton model emphasizes “layer-cake” lithostratigraphy that correlates similar-appearing but highly diachronous environmental facies, bounded by solid lines that cut across time lines. In contrast, facies architectural and sequence stratigraphic studies of ancient subsurface deltas have largely abandoned this lithostratigraphic approach. The alternate “chronostratigraphic” approach uses outcrop and seismic examples as training images that are used to derive conceptual models that drive the correlation of the internal facies architecture of subsurface strata. These outcrop and seismic examples suggest that there is no observable physical boundary between Scruton’s diachronous facies units. The conceptual “norm” depicts prograding deltas as seaward-dipping clinoform strata. Dipping delta-front sandstone beds roughly parallel time lines and interfinger with muddy prodelta bottomsets. If individual beds cannot be resolved, then diachronous, transitional facies boundaries are routinely drawn in a way that indicates that boundaries of this type are gradational rather than sharp, specifically by using lightning-stroke-type “shazam” lines. We use the method of bedding correlation (i. e., correlation of beds and bedsets) derived from geometries observed in outcrops and seismic analogs as a conceptual guide to recorrelate beds and facies for several recently published modern examples, where data are limited to a few, widely spaced cores. The new correlations, although imprecise because of long correlation distances, are potentially more accurate depictions of the bed-scale facies architecture, and may be more useful in applications that involve modeling bed-scale growth of deltas or that require prediction of 3-D fluid-flow behavior of deltaic reservoirs and aquifers.