skip to main content


This content will become publicly available on October 31, 2024

Title: BRAIDED-RIVER ARCHITECTURE OF THE TRIASSIC SWARTBERG MEMBER, KATBERG FORMATION, SOUTH AFRICA: ASSESSING AGE, FLUVIAL STYLE, AND PALEOCLIMATE AFTER THE END-PERMIAN EXTINCTION
ABSTRACT

The Triassic Katberg Formation has played a central role in interpreting the end-Permian ecosystem crisis, as part of a hypothesis of aridification, vegetation loss, and sediment release in continental settings. We use drone images of an inaccessible cliff near Bethulie to investigate the Swartberg member, a braided-fluvial body 45 m thick, describing remote outcrop facies to identify geomorphic units and using spatial analysis to estimate their proportions in 2-D sections. Here the Swartberg member comprises three channel belts within shallow valleys, the lowermost of which is ∼500 m wide and incised into lacustrine deposits. The component channel bodies consist mainly of trough cross-bedded sand sheets (48%) and channel-scour fills (28%). Recognizable bars (15%) comprise unit bars with high-angle slipfaces and mounded bar cores (components of mid-channel compound bars), bars built around vegetation, and bank-attached bars in discrete, probably low-sinuosity conduits. Abandoned channels constitute 8% and 16% of flow-parallel and -transverse sections, respectively. When corrected for compaction, the average thalweg depth of the larger channels is 3.9 m, with an average bankfull width of 84 m, scaling broadly with the relief of the bars and comparable in scale to the Platte and South Saskatchewan rivers of North America. The fluvial style implies perennial but seasonably variable flow in a vegetated landscape with a humid paleoclimate. The northward paleoflow accords with regional paleoflow patterns and deposition on a megafan sourced in the Cape Fold Belt, where the Swartberg member represents the avulsion of a major transverse-flowing river.

U-Pb dating of in situ and reworked pedogenic carbonate nodules from below the base of the Swartberg member yielded Anisian to Ladinian ages (Middle Triassic), younger than the previously assumed Early Triassic age and implying that considerable gaps in time exist in the succession. An assessment of the interval spanning the lower to mid Katberg Formation is needed to reevaluate the inferred unidirectional trend in fluvial style, aridification, and fossil distributions in this condensed, disjunct succession.

 
more » « less
Award ID(s):
1714759
NSF-PAR ID:
10475560
Author(s) / Creator(s):
; ; ; ;
Publisher / Repository:
SEPM
Date Published:
Journal Name:
Journal of Sedimentary Research
Volume:
93
Issue:
10
ISSN:
1527-1404
Page Range / eLocation ID:
741 to 775
Format(s):
Medium: X
Sponsoring Org:
National Science Foundation
More Like this
  1. The Triassic Katberg Formation has played a central role in interpreting the end-Permian ecosystem crisis, as part of a hypothesis of aridification, vegetation loss, and sediment release in continental settings. We use drone images of an inaccessible cliff near Bethulie to investigate the Swartberg member, a 45 m thick braided-fluvial body, describing remote outcrop facies to identify geomorphic units and using spatial analysis to estimate their proportions in 2D sections. Here the Swartberg member comprises three channel belts within shallow valleys, the lowermost of which is ~500 m wide and incised into lacustrine deposits. The component channel bodies consist mainly of trough cross-bedded sand sheets (48%) and channel-scour fills (28%). Recognizable bars (15%) comprise unit bars with high-angle slipfaces and mounded bar cores (components of mid-channel compound bars), bars built around vegetation, and bank-attached bars in discrete, probably low-sinuosity conduits. Abandoned channels constitute 8% and 16% of flow-parallel and -transverse sections, respectively. When corrected for compaction, the average thalweg depth of the larger channels is 3.9 m, with an average bankfull width of 84 m, scaling broadly with the relief of the bars and comparable in scale to the Platte and South Saskatchewan rivers of North America. The fluvial style implies perennial but seasonably variable flow in a vegetated landscape with a humid paleoclimate. The northward paleoflow accords with regional paleoflow patterns and deposition on a megafan sourced in the Cape Fold Belt, where the Swartberg member represents the avulsion of a major transverse-flowing river. U-Pb dating of in situ and reworked pedogenic carbonate nodules from below the base of the Swartberg member yielded Anisian to Ladinian ages (Middle Triassic), younger than the previously assumed Early Triassic age and implying that considerable gaps in time exist within the succession. An assessment of the interval spanning the lower to mid Katberg Formation is needed to reevaluate the inferred unidirectional trend in fluvial style, aridification, and fossil distributions in this condensed, disjunct succession.

     
    more » « less
  2. Abstract

    Steep channel networks commonly show a transition from constant‐gradient colluvial channels associated with debris flow activity to concave‐up fluvial channels downstream. The trade‐off between debris flow and fluvial erosion in steep channels remains unclear, which obscures connections among topography, tectonics, and climate in steep landscapes. Here, we analyze steep debris‐flow‐prone channels across the western United States and observe: (1) similar maximum channel gradients across a range of catchment erosion rates and geologic settings; and (2) lengthening colluvial channels with coarsening sediment cover. Following this compilation, we hypothesize that steep channel gradients are controlled by two competing thresholds of motion for bed‐sediment cover: bed failure by mass‐wasting and fluvial entrainment. We use downstream patterns in discharge, channel geometry, and sediment size to calculate discharges needed to mobilize sediment cover by both mechanisms across channels in the San Gabriel Mountains (SGM) and northern San Jacinto Mountains (NSJM) in southern California. Across steep colluvial channels in both landscapes, decadal discharges are below fluvial entrainment thresholds but above mass‐wasting entrainment thresholds for (median) sediment sizes, consistent with recent debris flows captured by repeat imagery. Colluvial channel gradient is similar despite > 3× contrasts in surface sediment grain size. In concave‐up fluvial channels downstream, decadal discharges exceed fluvial entrainment thresholds, and mass‐wasting is not predicted on lower gradients. In both landscapes, fluvial channels steepen downstream compared to gradients needed to mobilize sediment cover, which we interpret to reflect downstream increases in sediment flux. Coarser sediment supply in the NSJM than the SGM increases fluvial entrainment thresholds, which increases total channel relief in the NSJM by (1) lengthening colluvial channels shaped by debris flows and (2) increasing fluvial channel gradients. Our compilation and downstream analysis show how differing sensitivity of fluvial and debris flow processes to sediment grain size impacts the relative relief of colluvial and fluvial regimes in headwater channel networks.

     
    more » « less
  3. Abstract Reconstruction of active channel geometry from fluvial strata is critical to constrain the water and sediment fluxes in ancient terrestrial landscapes. Robust methods—grounded in extensive field observations, numerical simulations, and physical experiments—exist for estimating the bankfull flow depth and channel-bed slope from preserved deposits; however, we lack similar tools to quantify bankfull channel widths. We combined high-resolution lidar data from 134 meander bends across 11 rivers that span over two orders of magnitude in size to develop a robust, empirical relation between the bankfull channel width and channel-bar clinoform width (relict stratigraphic surfaces of bank-attached channel bars). We parameterized the bar cross-sectional shape using a two-parameter sigmoid, defining bar width as the cross-stream distance between 95% of the asymptotes of the fit sigmoid. We combined this objective definition of the bar width with Bayesian linear regression analysis to show that the measured bankfull flow width is 2.34 ± 0.13 times the channel-bar width. We validated our model using field measurements of channel-bar and bankfull flow widths of meandering rivers that span all climate zones (R2 = 0.79) and concurrent measurements of channel-bar clinoform width and mud-plug width in fluvial strata (R2 = 0.80). We also show that the transverse bed slopes of bars are inversely correlated with bend curvature, consistent with theory. Results provide a simple, usable metric to derive paleochannel width from preserved bar clinoforms. 
    more » « less
  4. Abstract. We investigate the interaction of fluvial and non-fluvial sedimentation on the channel morphology and kinematics of an experimental river delta. We compare two deltas: one that evolved with a proxy for non-fluvial (“marsh”) sedimentation (treatment experiment) and one that evolved without the proxy (control). We show that the addition of the non-fluvial sediment proxy alters the delta's channel morphology and kinematics. Notably, the flow outside the channels is significantly reduced in the treatment experiment, and the channels are deeper (as a function of radial distance from the source) and longer. We also find that both the control and treatment channels narrow as they approach the shoreline, though the narrowing is more pronounced in the control compared to the treatment. Interestingly, the channel beds in the treatment experiment often exist below sea level in the terrestrial portion of the delta top, creating a ∼ 0.7 m reach of steady, non-uniform backwater flow. However, in the control experiment, the channel beds generally exist at or above relative sea level, creating channel movement resembling morphodynamic backwater kinematics and topographic flow expansions. Differences between channel and far-field aggradation produce a longer channel in-filling timescale for the treatment compared to the control, suggesting that the channel avulsions triggered by a peak in channel sedimentation occur less frequently in the treatment experiment. Despite this difference, the basin-wide timescale of lateral channel mobility remains similar. Ultimately, non-fluvial sedimentation on the delta top plays a key role in the channel morphology and kinematics of an experimental river delta, producing channels which are more analogous to channels in global river deltas and which cannot be produced solely by increasing cohesion in an experimental river delta.

     
    more » « less
  5. ABSTRACT The rarely witnessed process of river avulsion repositions channels across floodplains, which influences floodplain geomorphology and stratigraphic architecture. The way avulsions redirect water and sediment is typically generalized into one of two styles. Avulsions proceeding through rapid channel switching and producing little to no floodplain disturbance are annexational, while those that involve sequential phases of crevassing, flooding, and eventual development of a new channel are progradational. We test the validity of these avulsion style categories by mapping and characterizing 14 avulsion events in Andean, Himalayan, and New Guinean foreland basins. We use Landsat data to identify how avulsions proceed and interpret the possible products of these processes in terms of geomorphic features and stratigraphy. We show that during annexation the avulsion channel widens, changes its meander wavelength and amplitude, or increases channel thread count. During progradation, avulsion channels are constructed from evolving distributary networks. Often beginning as crevasse splays, these networks migrate down the floodplain gradient and frequently create and fill ponds during the process. We also see evidence for a recently defined third avulsion style. Retrogradation involves overbank flow, like progradation, but is marked by an upstream-migrating abandonment and infilling of the parent channel. Avulsion belts in this study range from 5 to 60 km in length, and from 1 to 50 km in width. On average, these events demonstrate annexational style over 22.4% of their length. Eleven of 13 events either begin or end with annexation, and seven both begin and end with annexation. Only one event exhibited progradation over the entire avulsion-belt length. While there are many documented examples of purely annexational avulsions, we see little evidence for completely progradational or retrogradational avulsions, and instead suggest that a given avulsion is better envisioned as a series of spatiotemporal phases of annexation, progradation, and retrogradation. Such hybrid avulsions likely produce significantly greater stratigraphic variability than that predicted by the traditional end-member model. We suggest that a time-averaged, formation-scale consideration of avulsion products will yield more accurate characterizations of avulsion dynamics in ancient fluvial systems. 
    more » « less