ABSTRACT The standard subdivision of a distributive fluvial system (DFS) into proximal, medial and distal zones is typically based on the proportion of channel bodies and/or the degree of channel amalgamation, reflecting a downstream fining trend towards a mud‐prone terminal pinch‐out. However, in dryland settings, fluvial–aeolian interactions and limited accommodation maintain sand‐prone deposits basinward, complicating standard classifications. This study builds upon existing dryland DFS models by investigating the Organ Rock Formation and age‐equivalent strata of the Early Permian Cutler Group (Paradox Basin Province, southeastern Utah, USA). Data are presented as three quantified correlation panels comprising 15 outcrop logs and eight subsurface wells, temporally partitioned into intervals defined by climatic cyclicity. Two contrasting depositional transects are analysed: (1) a sand‐rich fluvio‐aeolian profile and (2) a mud‐rich fluvio‐lacustrine profile. An additional along‐strike profile captures lateral, distal transitions from an aeolian‐ to a lacustrine‐fringed fluvial pinch‐out. Preserved thicknesses are significantly greater (180 to 260 m) within the lacustrine pinch‐out than in the aeolian pinch‐out, reflecting enhanced subsidence in the Blanding Sub‐basin. This study proposes subdividing the dryland DFS into three zones based on the proportions of interpreted depositional environments: (1) braided‐dominated, (2) sinuous‐dominated and (3) terminal splay‐dominated zones. These zones reflect variations in discharge and host a predictable spectrum of associated depositional environments, governed primarily by climatic fluctuations and sediment supply rather than proximity to the sedimentary source. Wetting‐upward trends are generally characterised by the progressive emergence of sinuous fluvial systems and an increase in overall succession thicknesses. Drying‐upward intervals typically correspond to the initial expansion of the braided‐dominated zone during wet maxima, followed by aeolian advance and/or lacustrine contraction as aridity increases. This integrated outcrop‐subsurface analysis provides a predictive framework for assessing DFS architecture, reservoir distribution and basin‐scale allocyclicity in dryland systems, with broad implications for subsurface correlation and resource characterisation.
ABSTRACT The transition between the shelf and the upper slope is often considered an area of sediment bypass, where steeper gradients increase flow velocities and maintain particles in suspension. In addition to transferring sediments into deeper basins, upper‐slope areas can also host channelised submarine deposits. Such upper‐slope channel networks are often complicated, showing dendritic patterns of relatively small‐scale channels. Subsurface datasets typically lack the resolution to resolve the complexities of these portions of the slope profile. Outcrop analogues can provide the required resolution and are crucial for understanding the architecture of ancient turbidite systems in upper‐slope settings. During the late Tortonian, the Taza‐Guercif Basin recorded the transition from a mixed carbonate‐siliciclastic shelf to deep‐marine conditions, marked by marlstone deposition. This study documents the lower Melloulou Formation, in which turbidite sandstones fill channels incised into these marls. Mapping at a 1:5000 scale along with a total measured thickness of ~1100 m from 27 stratigraphic logs documented over fifty channel‐fill bodies from well‐exposed outcrops, allowing both the possibility to investigate these features in a downslope perspective and their relationship with the shelf. Key depositional/erosional features, internal architecture, stacking patterns and aspect ratios define four depositional styles for these channel fills: (i) isolated, (ii) laterally migrating, (iii) stepped and (iv) vertically offset. Four preferential channel pathways evolving from shallow‐water areas down the slope profile are recognised, each characterised by variation in channel‐fill style along the stratigraphy. Slope gradient, structural controls on the slope profile, inherited shelf paleotopography and downslope variations in flow parameters governed sedimentation. Reconstruction of the temporal evolution of the upper slope enables the development of a conceptual model for the downslope sediment‐routing framework, linking these pathways to intra‐reef gullies (sharms) on the shelf. This model highlights the possible physical continuity between slope sand bodies and more proximal, permeable units, such as shelf and fluvial deposits, thereby increasing the risk of upslope stratigraphic trap leakage.
In microtidally influenced rivers, tides are generally assumed to leave virtually no signatures in the sedimentary record. However, this hypothesis has been surprisingly poorly tested using modern river analogues, which provide an opportunity to assess whether sedimentary signatures of microtidal regimes can develop in rivers that lack diagnostic evidence of tidal control on large-scale channel morphology. This study investigates the capability of microtidal processes to leave signatures in deposits within the 180 km long backwater, fluvial-marine transition zone of the Po River (Italy). The Po River enters the Adriatic Sea (tidal range ca 1 m) through several deltaic distributary channels that exhibit no evidence of funnelling or cuspate meanders, indicating a clear morphodynamic dominance of fluvial over tidal processes. Hydrological data show that river water levels are modulated by tidal influence up to similar to 90 km and similar to 40 km from the river mouth during low-discharge and -discharge stages, respectively. Approximately 30 km from the mouth, deposits formed by riverine floods exhibit clear tidal sedimentary structures, including sand-mud couplets and bidirectional ripples. Around 40 km from the mouth, cross-stratified sand displays bundles with cyclic organisation, which are absent further upstream and are interpreted here as the effects of semi-diurnal tidal modulation on the migration of fluvial bedforms. Additionally, these bundles are deposited above the average intertidal zone, indicating that flood-enhanced water levels enable tide-modulated sedimentation to extend beyond the typical intertidal range. This study shows that evidence of tidal processes can be common even in deposits of microtidally influenced rivers, highlighting that tidal sedimentary structures may develop where tides exert little to no control on large-scale morphodynamics. Overall, these findings emphasise caution when interpreting dominant morphodynamic processes solely from sedimentary structures.
This study investigated the formation mechanism responsible for anomalously high permeability (AHP) observed in the Eocene sandstones at depths from 3824 to 3832 m in the Huizhou Sag, Pearl River Mouth Basin, China, using helium porosity and permeability measurements, thin section, scanning electron microscopy, electron probe X-ray microanalysis, rare earth element, microthermometry and in situ compositions of individual fluid inclusions, X-ray diffraction, formation water compositions and numerical geochemical simulations. The AHP is similar to 10-fold (average) higher than the baseline permeability at the same depths. Results indicate that authigenic K-feldspar overgrowth is the predominant factor driving AHP formation. The Al3+ and SiO2(aq) required for K-feldspar overgrowth may have been produced by the dissolution of plagioclase. K-feldspar precipitation may be driven by two potential pathways. The first is in an inherently K+-rich connate formation water and driven by the expulsion of organic acids and K+-bearing shale formation water and the intrusion of K+-rich hydrothermal fluids (either before or after the expulsion of shale formation water). The second is driven by the early intrusion of K+-rich hydrothermal fluids, resulting in initial K-feldspar precipitation and K+-rich formation water; subsequent expulsion of organic acids may further promote K-feldspar precipitation. Numerical diagenetic simulations validate the feasibility of these hypotheses and further indicate that the precipitation of K-feldspar overgrowth may inhibit the precipitation of kaolinite and quartz during plagioclase dissolution. This diagenetic mineral assemblage represents a novel coupled mineral dissolution-precipitation process in feldspar-rich sandstones. Therefore, the coupled plagioclase dissolution and K-feldspar overgrowth precipitation is suggested as the predominant mechanism responsible for the preservation of AHP mainly through resisting mechanical compaction and inhibiting secondary kaolinite and subsequent kaolinite-to-illite transformation. This study proposes a novel mechanism for AHP, identifies a novel diagenetic pathway in feldspar-rich sandstones and is, therefore, important for understanding sandstone diagenesis and reservoir quality prediction.
Rhodoliths, formed by crustose coralline algae (CCA), are prominent components in carbonate platform deposits, contributing to the characteristic rhodalgal facies. Their formation is influenced by both biological and physical factors, and under specific conditions, they can accumulate, forming extensive rhodolith beds. This study examines the depositional mechanisms underlying the formation of monospecific (Sporolithon nodosum) shallow-water rhodoliths from the Whangaparaoa Peninsula, North Island, New Zealand. The findings suggest that these rhodoliths originate in subtidal rocky channels at approximately 5 m depth, by the encrustation of nuclei derived from surrounding geological strata. Following their formation, these rhodoliths are transported into intertidal channel systems and eventually deposited in supratidal beach areas. Notably, the structure, morphology and composition of rhodoliths across these three depositional settings exhibit minimal variation, suggesting that the rhodoliths belong to a strongly interconnected coastal system with only brief intervals between translocation events. In this context, the study provides a sedimentological model for the development of rhodoliths in a coastal setting. Furthermore, it challenges the presence of a significant rhodolith bed sensu stricto in deeper offshore waters, as exploratory diving surveys revealed extensive sand deposits that hinder rhodolith development in these areas.
