Abstract The Indus Delta is constructed of sediment eroded from the western Himalaya and since 20 ka has been subjected to strong variations in monsoon intensity. Provenance changes rapidly at 12–8 ka, although bulk and heavy mineral content remains relatively unchanged. Bulk sediment analyses shows more negative ɛ Nd and higher 87 Sr/ 86 Sr values, peaking around 8–9 ka. Apatite fission track ages and biotite Ar–Ar ages show younger grains ages at 8–9 ka compared to at the Last Glacial Maximum (LGM). At the same time δ 13 C climbs from –23 to –20‰, suggestive of a shift from terrestrial to more marine organic carbon as Early Holocene sea level rose. U–Pb zircon ages suggest enhanced erosion of the Lesser Himalaya and a relative reduction in erosion from the Transhimalaya and Karakoram since the LGM. The shift in erosion to the south correlates with those regions now affected by the heaviest summer monsoon rains. The focused erosion along the southern edge of Tibet required by current tectonic models for the Greater Himalaya would be impossible to achieve without a strong summer monsoon. Our work supports the idea that although long-term monsoon strengthening is caused by uplift of the Tibetan Plateau, monsoon-driven erosion controls Himalayan tectonic evolution. Supplementary material: A table of the population breakdown for zircons in sands and the predicted Nd isotope composition of sediments based on the zircons compared to the measured whole rock value is available at http://www.geolsoc.org.uk/SUP18412

Rotary-drill cores from the Matagorda and Lavaca estuary complex sampled six distinct lithofacies, and high-resolution seismic data reveal four main seismic facies. The facies architecture of the bay indicates a history of Holocene flooding punctuated by major landward shifts in estuarine environments. Radiocarbon ages help to constrain the rate and timing of these flooding events and indicate significant reorganization of the estuarine environments at a decadal to century time scale. The Holocene evolution of the Matagorda and Lavaca estuary complex began ca. 11,600 yr B.P. with initial flooding of the portion of the ancestral Lavaca River incised valley that is now occupied by lower Matagorda Bay. Early flooding of the lowstand valley was punctuated by landward shifts in the river mouth, followed by episodes of fluvial aggradation. The most pronounced phase of aggradation occurred between ca. 11,600 and 9600 yr B.P., when sea level was rising rapidly, and a step in the valley floor slowed the rate of flooding. By ca. 9500 cal yr B.P., the deep, narrow part of the Lavaca incised valley had been flooded and was occupied by a bayhead delta. The river mouth shifted landward within Lavaca Bay between 8500 and 8200 cal yr B.P., followed by another landward shift in the delta between 7900 and 7700 cal yr B.P. These changes were mainly caused by variations in the rate of sea-level rise. The most dramatic change in the bay setting occurred between 7300 and 6700 cal yr B.P., when the bayhead delta stepped landward at least 30 km, establishing most of modern-day Matagorda Bay. This flooding event is attributed to a combination of rapid sea-level rise and to a reduction in sediment supply to the estuary that was associated with a change in the regional climate from cool and moist to warm and dry conditions. During the past 6700 yr, as the rate of sea-level rise decreased from an average rate of 4.2 mm/yr to an average rate of 1.4 mm/yr, the bayhead delta of the Lavaca River retreated slowly up the valley as the present-day outline of Lavaca Bay formed. The most significant change in the estuary setting during the past 6700 yr was the development of the present coastal barrier system, which led to gradual restriction of tidal exchange and associated changes in salinity.

Abstract Deltaic systems are controlled by a complex interaction of sediment supply, accommodation, and coastal energy, each varying in time and space, making three-dimensional study necessary. Numerical simulation models of such complex systems are thus also required to be 3D. This paper evaluates the state of 3D delta modeling and demonstrates that existing forward simulation models are still too limited to be applied to the full range of deltaic environments. Floodplain dynamics, as well as longshore transport, are generally lacking in present models. Many models lack the capacity to deal with multiple grain-size classes. Two advanced 3D models, AquaTellUs and 3D-SedFlux, illustrate how quantitative theoretical experiments provide insight into the dominances of processes acting and the evolution of a fluviodeltaic system. Depositional architecture and coastal progradation patterns depend strongly on the frequency of channel switching. Results from the AquaTellUs model suggest that channel switching may be an even more important control then the rate of change of sea-level fall. To further develop predictive 3D sedimentary models there is a need for field-test cases with quality input data, and with estimates of their associated uncertainties. Furthermore, process descriptions require efficient quantification and appropriate scaling. Only then will numerical models be able to address the responses of complex delta systems to specific forcing variables.

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.

Abstract Analyses of deltaic facies successions highlight recurring ichnological patterns that reflect a variety of physicochemical stresses imposed upon infaunal organisms by the interaction of various delta-front processes. Analysis of numerous ancient deltaic deposits in Canada, the United States, Australia, and offshore Norway persistently show reductions in bioturbation intensity and impoverishment in ichnological diversity, compared to those of nondeltaic shorelines. Some facies locally demonstrate sporadic colonization and recolonization of substrates left denuded by episodic depositional conditions. Deltaic ichnological suites also locally display size reductions of ichnogenera and a paucity of suspension-feeding ethologies. Resulting trace-fossil suites are overwhelmingly dominated by deposit-feeding behaviors, even in sandy facies. Such ichnological characteristics largely reflect increased sedimentation rates and heightened fluvial discharge, which serve to impede infaunal colonization. River-derived stresses are profound: salinity changes, hypopycnal-flow-induced water turbidity, distributary flood discharges with accompanying phytodetrital (comminuted plant debris) pulses, hyperpycnal-flow-induced sediment gravity flows, and fluid-mud deposition all conspire to produce the overall depauperate nature of the ichnological assemblage. Freshet-discharge events during river floods, accompanied by hyperpycnal conditions, may lead to the episodic introduction of reduced-salinity waters immediately above the sediment-water interface in delta-front and prodelta depositional settings. Such conditions may facilitate development of syneresis cracks and promote reductions in infaunal populations. Wave energy generally buffers fluvial effects, by dispersing suspended sediment offshore and encouraging the thorough mixing of waters of contrasting salinity. High mud concentrations near the delta front damp wave energy, however, limiting its effectiveness in remediating the benthic ecosystem, particularly immediately following distributary flood discharges and storm events. In wave-dominated settings, strong alongshore drift also operates to extend river-derived stresses considerable distances downdrift from distributary mouths. Where asymmetric deltas are formed, markedly different ichnological characteristics are expressed on either side of distributary-channel mouths. Updrift settings typically retain classic shoreface assemblages, whereas downdrift environments commonly acquire markedly stressed suites. Storm energy may be effective in dispersing mud and mixing waters, but it also results in erosion and episodic sediment deposition. Concomitant precipitation induces river floods, returning river-derived stresses to the delta front. Tidal energy and its effects on the infaunal communities of deltas are poorly documented. Tidal flux may trap mud plumes against the delta front, elevating water turbidity. Pronounced mud flocculation coupled with increased settling velocity associated with tidal mixing also leads to rapid deposition of thick fluid muds, particularly in low-lying areas, hampering or precluding colonization. Tidal energy also leads to marked changes in energy and salinity near the sediment-water interface at several time scales. Deltaic ichnological suites are characterized by structures of opportunistic trophic generalists, though mainly those of facies-crossing deposit feeders. High water turbidity, particularly near the sea floor, precludes most suspension-feeding behavior, and suppresses the development of the Skolithos ichnofacies, even in many proximal delta-front deposits. Ichnological characteristics record the dynamic interplay and relative importance of these different processes, both temporally and spatially, on delta systems.

Abstract Facies models of tide-influenced river deltas are less developed than models for river- and wave-dominated types because the interaction of river and tidal currents on deltas is complex, flow and sediment transport over bedforms on tide-influenced river deltas has not been extensively studied, and relatively few studies have recognized ancient deposits of tide-influenced deltas. Tide-influenced deltas occur in shallow seas or adjacent to broad shallow shelves, conditions less common in the modern, following early Holocene sea-level rise, than during other times of Earth history. Tides change patterns of deposition on river deltas by increasing rates of river and basin water mixing, elongating sand-body trends perpendicular to the coastline, making deposits more heterolithic by oscillating depositional current speeds and building up tidal flats in interdistributary areas. Mouth bars deposited as flows decelerate out of distributaries are reworked into more elongate sand bars on the subtidal delta top and delta front. Distinguishing facies and the hierarchy of cross strata formed in tide-influenced fluvial and distributary channels, shallow marine subtidal delta-top and delta-front sand bars, and erosion related to fluvial-channel and tidal-current incision are key to interpreting proximal-distal changes in deposition and water-depth variations from deposits of tide-influenced deltas. Facies patterns in tide-influenced deltas are complex, because patterns of erosion and deposition can change significantly from areas actively fed by the river to areas temporarily abandoned where tidal currents continue to rework sediments. Where rivers supply abundant sand, rapidly prograding delta fronts develop steeply inclined beds containing tide-winnowed sands, whereas areas of slower progradation are extensively reworked by migrating tidal bars. Sandy deposits of tide-influenced deltas commonly have basal erosion surfaces that record scouring of prodelta muds, as well as internal erosion surfaces that record local tidal reworking of sediments following distributary-channel switching. Evidence of significant erosion and changes in the basinward extent of sands do notnecessarily reflect changes in water depth. Significant erosion and sediment reworking can also occur during transgression. Distinguishing sequence stratigraphic divisions depends critically on recognizing large-scale shiftsin facies belts that define key allostratigraphic surfaces, but this can be difficult in tide-influenced deltas,where local changes in sediment supply and tidal-current strength can produce pronounced facies variations and significant erosion. Although there has been a tendency for all tide-influenced deposits to be interpreted as transgressive estuarinevalley-fill deposits because they commonly overlie extensive erosion surfaces, it is increasingly recognized that erosion at the base of sand facies is the norm in most strongly tide-influenced depositional settings. Outcropping deposits interpreted to have formed on tide-influenced deltas show a wide variety of facies patterns and stratal architecture. Although there are too few well-documented examples to define general trends, the characterof tide-influenced deltaic systems is interpreted to change with proximal-distal position within deltaic successions, whether deltaic deposition is confined or broadly distributed, and with the sequence stratigraphic setting.Changes in deposits of tide-influenced-deltas associated with expansion of deposition into a basin and varying rates of shoreline progradation may be as pronounced as those that define sequence stratigraphic divisions.

Abstract The exceptional outcrops of the Book Cliffs, Utah, allow detailed reconstruction of 3D shoreline morphology and coeval facies relationships in a series of wave-dominated, shallow-marine parasequences of Late Cretaceous age. Each parasequence records progradation of a wave-dominated delta and/or strandplain system. These systems ar characterized in terms of vertical facies successions, trace-fossil assemblages, shoreline-shelf profiles, paleogeography, and paleo-geomorphic evolution. The ancient shorelines are compared to modern wave-dominated shorelines via three quantitative geomorphic measures: shoreline lobosity, areal proportion of river channels, and spacing of river channels. Our approach is driven by geomorphic insights gained from detailed, intra-parasequence facies architecture, and highlights the limitations of conventional analysis of outcrop and subsurface datasets using 1D logged sections and generalized 2D facies-trend and isopach maps. Many of the wave-dominated shoreline deposits exposed in the Book Cliffs show clear evidence for river processes and river-derived sediment input, implying that fluvial sediment input was required for shoreline progradation. Along the most extensively exposed shoreline successions, broadly coeval rivers and/or valleys constitute 0-68% of the shoreline, and the spacing of rivers and/or valleys varies from 1 km to over 68 km. By comparison, rivers constitute 1-12% of modern wave-dominated deltas and strandplains, and river spacing ranges from 1 to 130 km. Detailed reconstructions of intra-parasequence facies architecture in two parasequences suggest that shoreline morphology, paleogeography, and evolution were controlled by wave-generated longshore drift and river-mouth locations. A combination of river avulsion and efficient alongshore redistribution of sediment by wave processes resulted in relatively linear shoreline paleogeographies for each mapped parasequence, but these trends obscure subtle lateral variations in facies and intra-parasequence stratigraphy. Autocyclic river avulsion and delta-lobeswitching did not generate localized flooding. Instead, parasequence-bounding flooding surfaces are inferred to have been caused by allogenic changes in relative sea level and/or sediment supply.

Abstract The detailed 3-D facies architecture of “terminal” distributary channels in proximal delta-front deposits of the Cretaceous Panther Tongue delta in central Utah is imaged using digital mapping techniques and ground-penetrating radar (GPR). Four lithofacies were identified: massive sandstone, parallel-laminated sandstone, rippled heterolithics, and bioturbated heterolithics. Lithofacies interpretations suggest shallow water in a delta-front environment where river processes dominate deposition, but with seasonal wave and storm influence. “Terminal” distributary channels and upstream-accreting bars were observed on cliffs oriented both perpendicular and parallel to the paleoflow direction. The terminal distributary-channel facies die out over less than 100 m distally into heterolithic deposits representing distal mouth bars of the delta front. GPR and 3-D photorealistic techniques, together with sedimentary section measurements document the 3-D facies architecture. The 3D photorealistic technique consists of draping oblique, close-range photographic images on 3-D terrain models of outcrops to generate a digital three-dimensional model of the outcrop. 2-D GPR profiles, collected parallel to cliff faces, are tied to the 3-D outcrop model using Global Positioning System (GPS). GPR lines are correlated with bedding diagrams of cliff-face exposures to extend mapping of sedimentary features behind the outcrop into three dimensions. Scours elongate downcurrent represent the bases of “terminal” distributary channels and show maximum relief of 5 m.

Abstract Shelf-margin transects across the Eocene Central Basin of Spitsbergen provide seismic-scale (1 km x 15 km) outcrops of shelf- margin clinothem complexes with and without basin-floor fans (Type I and Type II respectively). Type I and Type II shelf margins reflect broadly similar timing of deposition in a sea-level cycle: (1) shelf-margin progradation or basin-floor aggradation during the falling stage, (2) lower-slope or basin-floor aggradation during the early lowstand, (3) intra-lowstand flooding back onto the shelf edge, and (4) shelf- margin progradation during the late lowstand. The Spitsbergen database strongly suggests that the sediment budget was partitioned largely onto the shelf and coastal plain during the development of transgressive and highstand systems tracts, whereas sands were distributed beyond the shelf edge during falling stage and lowstand. The conventional explanation for the differences in architectural and sediment-volume partitioning between Type I and Type II margins is that the magnitude or duration of sea-level fall was greater in the case of Type I. We argue here for a possible alternative explanation, that higher rates of sediment fallout at the shelf edge and upper slope during the falling stage can damp incision and prevent deep channeling at the shelf margin. Type I shelf-margin complexes show severe erosion of the falling-stage shelf-edge deltas by the delta’s own distributary channels. Time- equivalent basin-floor fans can be “walked out” and linked to the eroded falling-stage deltas in these clinothem sets. The late-lowstand part of this type of shelf margin consists of prograding shelf-edge deltas, causing a prominent late episode of shelf-margin accretion. These late- lowstand deltas are generally more muddy than those of the Type II margins. Heterolithic, thin-bedded turbidites dominate the delta-front succession, along with slumped units. In contrast, Type II shelf margins accreted with an amalgamated succession of falling-stage, early lowstand, and late lowstand deltas, and have no basin-floor fans. The falling-stage deltas are highly progradational, fluvially dominated, and have a sandy turbidite-prone delta front that reaches the base of slope. The tops of these deltas are severely eroded by a subaerial to subaqueous unconformity that is overlain by amalgamated channels. This unconformity is the sequence boundary that formed during the entire fall to the lowest sea-level position. Despite the documented sea-level fall below the shelf edge, no sand was partitioned onto the coeval basin floor. All the sandy sediment was trapped in the shelf-margin deltas, causing significant shelf-margin progradation. The falling-stage deltas are succeeded by an aggradational and landward-onlapping succession of turbidites, covered by a widespread intra-lowstand flooding surface. No coeval deltas at the shelf edge, or coeval basin-floor fans, were documented. The flooding surface is overlain by deltas that formed late in the lowstand. These late lowstand deltas prograded onto the slope, but they have a strong aggradational component; their distributaries and mouth bars are wave-reworked, and delta-front turbidites become muddy as high as on the middle slope. These deltas represent an important second episode of shelf-margin progradation. Although the latter deltas were deposited during the rising limb of the sea-level cycle, they are still overlain by a transgressive systems tract and maximum flooding surface.