Does fluvial channel-belt clustering predict net sand to gross rock volume? Architectural metrics and point-pattern analysis of a digital outcrop model

ABSTRACT Spatial point-pattern analyses (PPAs) are used to quantify clustering, randomness, and uniformity of the distribution of channel belts in fluvial strata. Point patterns may reflect end-member fluvial architecture, e.g., uniform compensational stacking and avulsion-generated clustering, which may change laterally, especially at greater scales. To investigate spatial and temporal changes in fluvial systems, we performed PPA and architectural analyses on extensive outcrops of the Cretaceous John Henry Member of the Straight Cliffs Formation in southern Utah, USA. Digital outcrop models (DOMs) produced using unmanned aircraft system-based stereophotogrammetry form the basis of detailed interpretations of a 250-m-thick fluvial succession over a total outcrop length of 4.5 km. The outcrops are oriented roughly perpendicular to fluvial transport direction. This transverse cross-sectional exposure of the fluvial system allows a study of the system's variation along depositional strike. We developed a workflow that examines spatial point patterns using the quadrat method, and architectural metrics such as net sand to gross rock volume (NTG), amalgamation index, and channel-belt width and thickness within moving windows. Quadrat cell sizes that are ∼ 50% of the average channel-belt width-to-thickness ratio (16:1 aspect ratio) provide an optimized scale to investigate laterally elongate distributions of fluvial-channel-belt centroids. Large-scale quadrat point patterns were recognized using an array of four quadrat cells, each with 237× greater area than the median channel belt. Large-scale point patterns and NTG correlate negatively, which is a result of using centroid-based PPA on a dataset with disparately sized channel belts. Small-scale quadrat point patterns were recognized using an array of 16 quadrat cells, each with 21× greater area than the median channel belt. Small-scale point patterns and NTG correlate positively, and match previously observed stratigraphic trends in the fluvial John Henry Member, suggesting that these are regional trends. There are deviations from these trends in architectural statistics over small distances (hundreds of meters) which are interpreted to reflect autogenic avulsion processes. Small-scale autogenic processes result in architecture that is difficult to correlate between 1D datasets, for example when characterizing a reservoir using well logs. We show that 1D NTG provides the most accurate prediction for surrounding 2D architecture.

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.