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Architectural element analysis and detailed mapping of a 300 m along-strike exposure of the middle member Wood Canyon Formation, southern Marble Mountains, California, USA, provides new evidence for extensive braidedfluvial channel-belt deposits with adjacent overbank environments. Threedimensional models constructed using 'Structure from Motion' techniques, combined with field-based observations, allowed interpretation of outcropscale trends, barforms, channel fills and fine-scale features. The ca 80 m thick member is divisible into five distinct units, including units M1 to M3 that form the bulk of the stratigraphy. Units are defined by stacking patterns of three facies associations (Facies Association 1 to Facies Association 3), each representing the product of a subenvironment within the fluvial system. In Facies Association 1, stacked cosets, interpreted as low-relief fluvial bars and channel fills, preserve vertical-accretion and downstream-accretion elements under unimodal north-north-west palaeoflow, with minor lateral accretion near bar edges. Deposits of Facies Association 2 to Facies Association 3, linked to overbank environments, are found only in unit M2, in the middle 27 m of the middle member. Floodplains, represented by Facies Association 2, include crumbly red-orange intervals of fine to mediumgrained sandstone and thinner sets of cross-bedding than Facies Association 1, interbedded with thicker cross-stratification indicative of overbank splay or overland flow aggradation from adjacent channel belts during flood stage. Possible aeolian beds of Facies Association 3 preserve broad festooned trough cross-strata that average 23 cm in thickness; their small size, medium-grained sandstone and iron oxide cement suggest a high water table. The diverse assemblage of interpreted subenvironments, paired with bedform and facies patterns, implies a perennial fluvial system that gradually built large sand bars as the channel belt migrated and avulsed across an unconfined braided-fluvial reach, leaving the overbank area on its flanks subject to weathering and aeolian transport. Despite the occurrence of strata deposited in low-energy and ponded settings, and a marine influence proposed for nearby sections of middle member, no ichnofossils were encountered.

More than 25 cm of rainfall from Tropical Storm Lee (TS Lee) over 2 days in September 2011 resulted in catastrophic flooding (U.S. Geological Survey estimated recurrence interval >100 yr) on several Susquehanna River tributaries emanating from the Appalachian Plateau in north-central Pennsylvania (USA). Helicopter photography and field work were used to prepare a detailed geographic information system database of geomorphic response to the flood along ~250 km of Loyalsock, Muncy, Lycoming, and Fishing Creeks. Unlike the response of many streams to previously described Appalachian floods, fluvial response to the TS Lee flood was extensive in these gravel bed streams, characterized by (1) large-scale avulsions and chute development on the insides of meanders, (2) erosion of gravel from channel margins and transport downstream in large pulses, (3) headwater landslides and alluvial fan activation, (4) major floodplain erosion and deposition, and (5) breaching of anthropogenic berms and reconnection of the main channel to prehistoric floodplain anabranches. Geomorphic work, expressed both as bedload sediment transport and landform change (geomorphic effectiveness) was significant: as much as 55,000 m 3 /km of gravel was transported within a single watershed. Landform changes included erosion of chutes (to 500 m long), gravel bars (point bars and mid-channel bars), channel widening (in places >100%), and reoccupation of former multithread channels previously cut off from the mainstem by historic channel straightening, berming, and dredging.Streams in this region appear to be in a phase of disequilibrium largely in response to major shifts in sediment delivery from their watersheds caused by historic logging and a series of floods ~100 yr ago. Widespread clearcutting (A.D. 1850-1920) contributed large volumes of sediment to these streams. Dendrogeomorphic data bracket a period of aggradation of these logging legacy sediments between the 1870s and 1930s, creating a significant low terrace inset into Pleistocene outwash and glacial sediments. Recent floods in 1972, 1996, 2004, and especially TS Lee in 2011 initiated an enhanced phase of disequilibrium as a geomorphic threshold was crossed, resulting in widespread erosion of logging legacy sediments deposited nearly 100 yr ago. The change in sediment load (increased coarse bedload) as a result of widespread bed and bank erosion caused a change in channel pattern from single thread to multithread. Pattern change was facilitated by aggradation of gravel bars above floodplain elevations which promoted avulsion and chute formation. Based on preflood and postflood geomorphic mapping, >6,700,000 m 3 of gravel were mobilized during the flood across 4 watersheds. Mobilization of logging legacy sediment is occurring as pulses of gravel move downstream episodically. This paper demonstrates the important influences of drainage basin morphometry (e.g., ruggedness number) and fluvial history (land use and geomorphic) in understanding current channel dynamics and basin response to heav...

Across the Mojave Desert of southeastern California, outcrops of the Cambrian middle member of the Wood Canyon Formation preserve the deposits of pre-vegetation braided-fluvial and braid-delta environments. One 78-meter-thick section in the southern Marble Mountains, documented here through detailed stratigraphic logging, facies analysis, architectural panels based on “Structure-from-Motion” models, and a suite of paleocurrent and accretion-surface measurements, provides insight into the development of a river-dominated, wave- and tide-influenced braid delta at the distal end of a continent-scale braidplain. In contrast to other pre-Devonian braid-delta strata, in which mudrock is largely absent, the greater part of the middle member system contains over 5% mudstone. Four facies associations, FA4–7, constitute the middle member section and represent (in order of stratigraphic height) a braidplain-to-delta transition (FA4), proximal braid delta (FA5), distal braid delta (FA6), and upper braid-delta front (FA7). The 20 meters of braidplain-to-delta transition strata are largely similar to those of fluvial middle member sections, containing approximately 2% mudstone, unimodal north-northwest paleoflow, and vertical, downstream, and downstream-lateral accretion elements representing compound barforms and channel fills. Above, each braid-delta facies association (FA5–7) preserves high-sinuosity paleocurrent indicators, 6–12% mudstone, and symmetrical, wave-formed sand waves. Decimeter-thick fluid-mud deposits found chiefly in FA6 and less commonly in FA7 indicate the presence of a turbidity-maximum zone that records brackish-water conditions in the distal braid delta. Trace fossils concentrated in FA7 suggest that metazoans were confined to the upper braid-delta front and could not tolerate the variable salinity of the braid delta. Increased marine influence with stratigraphic height requires gradual transgression during deposition of the middle member of the Wood Canyon Formation, possibly as part of a lowstand systems tract.

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