Faulty foundations: Early breakup of the southern Utah Cordilleran foreland basin

Anomalous features of Upper Cretaceous strata in southern Utah challenge existing tectonic and depositional models of the Cordilleran foreland basin. Extreme thickness variations, net to gross changes, and facies distributions of nonmarine to marginal marine strata of the Turonian−early Campanian Straight Cliffs Formation are documented across the Southwestern High Plateaus. Contrary to most traditional models of foreland basin architecture, regional correlations demonstrate abrupt stepwise thickening, with a punctuated increase in average grain size of key intervals from west to east, i.e., proximal to distal relative to the fold-thrust belt. Except in the most proximal sections, fluvial drainage systems were oriented predominantly subparallel to the fold-thrust belt. Combined, these results suggest that modern plateau-bounding faults may have had topographic expressions as early as Cenomanian time, and influenced the position of the main axial river system by creating northeast-trending paleotopography and sub-basins. Laramide-style tectonism (e.g., basement-involved faults) is already cited as a driver for sub-basin development in latest Cretaceous−Cenozoic time, but new data presented here suggest that this part of the foredeep was “broken” into distinct sub-basins from its earliest stages. We suggest that flexural foundering of the lithosphere may have caused early stage normal faulting in the foredeep. Regional implications of these new data indicate that both detachment-style and basement-involved structures were simultaneously active in southern Utah earlier than previously recognized. These structures were likely influenced by inherited Proterozoic basement heterogeneities along the edge of the Colorado Plateau. This interpretation suggests that tectonic models for the region should be reevaluated and has broader implications for understanding variability and geodynamics of foreland basin evolution.

When Is a Barrier Island Not an Island? When It Is Preserved in the Rock Record

Existing barrier island facies models are largely based on modern observations. This approach highlights the heterogeneous and dynamic nature of barrier island systems, but it overlooks processes tied to geologic time scales, such as multi-directional motion, erosion, and reworking, and their expressions as preserved strata. Accordingly, this study uses characteristic outcrop expressions from paralic strata of the Upper Cretaceous Straight Cliffs Formation in southern Utah to update models for barrier island motion and preservation to include geologic time-scale processes. Results indicate that the key distinguishing facies and architectural elements of preserved barrier island systems have very little to do with “island” morphology as observed in modern systems. Four facies associations are used to describe and characterize these barrier island architectural elements. Barrier islands occur in association with backbarrier fill (FA1) and internally contain lower and upper shoreface (FA2), proximal upper shoreface (FA3), and tidal channel facies (FA4). Three main architectural elements (barrier island shorefaces, shoreface-dominated inlet fill, and channel-dominated inlet fill) occur independently or in combination to create stacked barrier island deposits. Barrier island shorefaces record progradation, while shoreface-dominated inlet fill records lateral migration, and channel-dominated inlet fill records aggradation within the tidal inlet. Barrier islands are bound by lagoons or estuaries and are distinguished from other shoreface deposits by their internal facies and outcrop geometry, association with backbarrier facies, and position within transgressive successions. Tidal processes, in particular, tidal inlet migration and reworking of the upper shoreface, also distinguish barrier island successions. In sum, this study expands barrier island facies models and provides new recognition criteria to account for the complex geometries of time-transgressive, preserved barrier island deposits.

Valleys, Estuaries, and Lagoons: Paleoenvironments and Regressive–Transgressive Architecture of the Upper Cretaceous Straight Cliffs Formation, Utah, U.S.A

The John Henry Member of the Upper Cretaceous Straight Cliffs Formation preserves deposition of four regressive–transgressive (R-T) cycles in 350 m of strata of the Sevier foredeep in south-central Utah, USA. Each cycle is discussed in detail, with emphasis on the transgressive phases of deposition. Regressive intervals comprise wave-dominated shorefaces and coastal-plain strata, whereas transgressive intervals record tide-influenced coastal-margin and low-energy-bay and lagoonal deposits. One R-T cycle in the lower John Henry Member preserves a compound incised-valley system filled with a complex assemblage of tidal and estuarine facies. In contrast, overlying R-T cycles are not associated with valley formation, but instead preserve sandstone-rich back-barrier platform deposits that transition landward into tidal-creek, tidal-flat, and marsh depositional settings. Excellent outcrop expression permits detailed examination of the complex internal architecture of the compound incised-valley, and demonstrates that: 1) tidal ravinement significantly modified the initial valley shape during transgression, a process not fully recognized in most conceptual models of valley formation and fill; 2) the valley system incised in a basin-axial position (NNE–SSW), subparallel to the thrust front and oblique to the orientation of pre-valley-formation shorefaces, which prograded from west to east. Axial systems are well-known transporters of large volumes of sediment in foreland basins, and yet most incised-valley models imply a direct and oversimplified relationship between up-dip (source area and tectonics) and down-dip (base level) controls; 3) the major subaerial unconformity and bypass surface occurred at a higher (younger) stratigraphic position than previously interpreted, and is herein renamed the lower John Henry Member sequence boundary. The changes in regional correlations necessitated by this discovery have several broader implications for sequence stratigraphic models; 4) finally, correlations down dip along the axial valley system indicate a steep topographic gradient of 0.011, with 47% vertical, compacted expansion of the whole John Henry Member over 14 km from south to the north. This suggests structural control on sediment transport and deposition, with significant lateral variability in accommodation parallel to the fold-thrust belt. This study adds to the growing body of literature documenting the complex nature of transgressive deposits, which will aid in the interpretation, prediction, and management of analogous subsurface reservoirs.