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This study uses stable isotope variation within individual Mio-Pliocene paleosols to investigate subkilometer-scale phytogeography of late Miocene vegetation change in southeast Asia between ca. 8.1 and 5 Ma, a time interval that coincides with dramatic global vegetation change. We examine trends through time in the distribution of low-latitude grasses (C 4 plants) and forest (C 3 plants) on Indo-Gangetic fl oodplains using carbon (δ 13 C) and oxygen isotopic (δ 18 O) values in buried soil carbonates in Siwalik Series sediments exposed in the Rohtas Anticline, northcentral Pakistan. Revised, high-resolution magnetostratigraphy and a new 40 Ar/ 39 Ar date provide improved age control for the 2020 m Rohtas section. Carbon isotope results capture lateral variability of C 3 versus C 4 plants at fi ve stratigraphic levels, R11 (8.0 Ma), R15 (6.74-6.78 Ma), R23 (5.78 Ma), R29 (4.8-4.9 Ma), and upper boundary tuff (UBT; 2.4 Ma), using detailed sampling of paleosols traceable laterally over hundreds of meters. Paleosols and the contained isotopic results can be assigned to three different depositional contexts within the fl uvial sediments: channel fi ll, crevasse-splay, and fl oodplain environments. δ 13 C results show that near the beginning (8.0 Ma) and after (4.0 Ma) the period of major ecological change, vegetation was homogeneously C 3 or C 4 , respectively, regardless of paleo-landscape position. In the intervening period, there is a wide range of values overall, with C 4 grasses fi rst invading the drier portions of the system (fl oodplain surfaces) and C 3 plants persisting in moister settings, such as topographically lower channel swales. Although abrupt on a geologic timescale, changes in abundance of C 4 plants are modest (~2% per 100,000 yr) compared to rates of vegetation turnover in response to glacial and interglacial climate changes in the Quaternary. Earlier research documented a sharply defi ned C 3 to C 4 transition in Pakistan between 8.1 and 5.0 Ma, based on vertical sampling, but this higher-resolution study reveals a more gradual transition between 8.0 and 4.5 Ma in which C 3 and C 4 plants occupied different subenvironments of the Siwalik alluvial plain.δ 18 O values as well as δ 13 C values of soil carbonate increase up section at Rohtas, similar to isotope trends in other paleosol records from the region. Spatially, however, there is no correlation between δ 13 C and δ 18 O values at most stratigraphic levels. This implies that the changes in soil hydrology brought about by the shift from forest to grassland (i.e., an increase in average soil evaporation) did not produce the shift through time in δ 18 O values. We interpret the trend toward heavier soil carbonate δ 18 O values as a response to changes in external climatic factors such as a net decrease in rainfall over the past 9 Ma.

The lower part of the Cretaceous Sego Sandstone Member of the Mancos Shale in east‐central Utah contains three 10‐ to 20‐m thick layers of tide‐deposited sandstone arranged in a forward‐ and then backward‐stepping stacking pattern. Each layer of tidal sandstone formed during an episode of shoreline regression and transgression, and offshore wave‐influenced marine deposits separating these layers formed after subsequent shoreline transgression and marine ravinement. Detailed facies architecture studies of these deposits suggest sandstone layers formed on broad tide‐influenced river deltas during a time of fluctuating relative sea‐level. Shale‐dominated offshore marine deposits gradually shoal and become more sandstone‐rich upward to the base of a tidal sandstone layer. The tidal sandstones have sharp erosional bases that formed as falling relative sea‐level allowed tides to scour offshore marine deposits. The tidal sandstones were deposited as ebb migrating tidal bars aggraded on delta fronts. Most delta top deposits were stripped during transgression. Where the distal edge of a deltaic sandstone is exposed, a sharp‐based stack of tidal bar deposits successively fines upward recording a landward shift in deposition after maximum lowstand. Where more proximal parts of a deltaic‐sandstone are exposed, a sharp‐based upward‐coarsening succession of late highstand tidal bar deposits is locally cut by fluvial valleys, or tide‐eroded estuaries, formed during relative sea‐level lowstand or early stages of a subsequent transgression. Estuary fills are highly variable, reflecting local depositional processes and variable rates of sediment supply along the coastline. Lateral juxtaposition of regressive deltaic deposits and incised transgressive estuarine fills produced marked facies changes in sandstone layers along strike. Estuarine fills cut into the forward‐stepped deltaic sandstone tend to be more deeply incised and richer in sandstone than those cut into the backward‐stepped deltaic sandstone. Tidal currents strongly influenced deposition during both forced regression and subsequent transgression of shorelines. This contrasts with sandstones in similar basinal settings elsewhere, which have been interpreted as tidally influenced only in transgressive parts of depositional successions.

The Frewens sandstone is composed of two elongate tide‐influenced sandstone bodies that are positioned directly above and slightly landward of a more wave‐influenced lobate sandstone. The 20‐km‐long, 3‐km‐wide Frewens sandstone bodies coarsen upwards and fine away from their axes, have gradational bases and margins and have eroded tops abruptly overlain by marine shales. These sandstones are superbly exposed in large cliffs on the banks of the South Fork of the Powder River in central Wyoming, USA. The deposits change upwards from thinly interbedded sandstones and mudstones to metre‐thick heterolithic cross‐strata and, finally, to metres‐thick sandstone‐dominated cross‐strata. There is abundant evidence for tidal modulation of depositional flows; however, palaeocurrents were strongly ebb‐dominated and nearly parallel the trend of sandstone‐body elongation. Detailed mapping of stratal geometry and facies across these exposures shows a complex internal architecture. Large‐scale bedding units within sandstone bodies are defined by alternations in facies, bed thickness and the abundance of shales. Such bedsets are inclined (5°–15°) in walls oriented parallel to palaeoflow and gradually decrease in dip over hundreds of metres as they extend from the sandstone‐dominated deposits higher in a sandstone body to muddier deposits lower in the body. Where viewed perpendicular to palaeoflow, bedsets are 100‐metre‐wide lenses that shingle off the sandstone‐body axis towards its margins. The sandstone bodies are interpreted as sand ridge deposits formed on the shoreface of a tide‐influenced river delta. Metres‐thick cross‐strata in the upper parts of sandstone bodies resemble deposits of bars (sandwaves) formed where tidal currents moved across shallows and the tops of tidal ridges. Heterolithic deposits lower in sandstone bodies record fluctuating currents caused by ebb and flood tides and varying river discharge. Erosion surfaces capping sandstone bodies record tidal ravinement. The tidal ridges were abandoned following transgression and covered with marine mud as waters deepened.

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Quantitative determination of palaeochannel geometry and hydraulics from point bar deposits requires an understanding of the interaction between channel‐bend migration, temporal and spatial variation of point bar geometry and facies, and outcrop orientation. This interaction is modelled with the aid of a computer program which predicts three‐dimensional (3‐D) geometry and grain size variation of point bars. Synthetic deposits are produced for the cases of down‐valley bend migration and/or increase in channel‐bend sinuosity. Two‐dimensional (2‐D) cross‐sections in varying orientations across these simulated deposits display lateral‐accretion bedset surface geometry, and variation in mean bedset grain size and local palaeocurrent orientation. Most cross‐sections show point bar deposits thickening away from the meander‐belt axis due to a lateral progression from thinner bend‐exit deposits to thicker bend‐apex deposits (caused by down‐valley channel translation), and/or due to a progression from thinner low sinuosity deposits to thicker high sinuosity deposits caused by channel bend expansion. In association with this lateral thickening, bedset surfaces become steeper and more convex upwards while the variation in mean grain size up bedsets commonly increases. Down‐valley point bar translation allows preservation only of deposits formed downstream of the band apex, and produces characteristic fining upwards sequences. Marked lateral and vertical variations in palaeocurrent directions due to varying channel orientation relative to a given cross‐section are also predicted. These results indicate a need in palaeochannel reconstructions, for a more detailed examination of 3‐D variations in bedset surface geometry, palaeocurrent orientation and grain size distribution within and between bedsets of laterally accreted sediment.