Floodplain Muds Research Articles

Pattern of vertebrate extinctions across an event bed at the Permian-Triassic boundary in the Karoo Basin of South Africa

The extinction of vertebrates around the time of the Permian-Triassic boundary has long been regarded as a gradual event occurring over millions of years. Our new field investigations of fluvial strata in the central and southern Karoo Basin of South Africa have revealed the presence of an event bed coinciding with a mass extinction of terrestrial fauna and flora. The bed is in a sedimentary sequence that is marked by a reddening of flood-plain mud rocks and a change from high- to low-sinuosity river channel systems. Here we show that the pattern of vertebrate taxa disappearing below this boundary and the subsequent appearance of new taxa above the boundary are consistent with a relatively sudden, possibly catastrophic event, perhaps of 50 000 yr duration or less.

Taenidium and Associated Ichnofossils in Fluvial Deposits, Cretaceous Tuscaloosa Formation, Eastern Alabama, Southeastern U.S.A

Exposures of the Tuscaloosa Formation in eastern Alabama (U.S.A.) provided an opportunity to examine ichnofossils and ichnofabrics within Cretaceous fluvial facies. Floodplain mudstones are completely bioturbated and dominated by backfilled burrows most allied with Taenidium serpentinum. These burrows likely reflect the work of insects or oligochaetes that occupied soft, moist proximal floodplain substrates. Although the affinities and ethology of the tracemakers and the precise environmental conditions that prevailed during burrowing cannot be confidently inferred, Tnenidium‐dominated ichnofabrics of the mudstones can be assigned to the Scoyenia ichnofacies in the narrower sense of several recent authors (e.g., Buatois and Mángano, 1985). Ichnofossils are rare to absent in Tuscaloosa sandstones. The tops of some thin crevasse‐splay sands within mudstone‐dominated intervals were intensely disrupted by Taenidium producers. Large, vertically extensive, chambered(?) burrow systems, presumably dug by some unknown vertebrate organisms, locally were excavated in abandoned channel sands. In situ ichnofossils apparently are absent altogether in conglomeratic fades. However, intraformational conglomerates in channel‐fill sequences locally are composed almost entirely of Taenidium backfill segments that survived erosion from semi‐consolidated floodplain muds and subsequent fluvial transport and, thus, represent a form of concentration‐type ichnofossil‐lagerstätte.

Evolution of the Holocene Mississippi River floodplain, Ferriday, Louisiana; insights on the origin of fine-grained floodplains

ABSTRACT The alluvial architecture and soil characteristics of Holo cene Mississippi River floodplain deposits in the southern Lower Mississippi Valley provide evidence for significant changes in floodplain development in response to sea-level rise. Floodplain cores acquired near Ferriday, Louisiana show that Holocene deposits consist of 15-30 m (ave. 20 m) of sands, silts, and clays, which overlie Late Wisconsin sands and gravels. On the basis of differences in sediment grain size, sediment-body geometry, and the abundance of soil features, the Holocene deposits are subdivided into Lower and Upper Holocene units. Lower Holocene deposits (> 5000 yr B.P.) consist of lacustrine and poorly drained backswamp muds that contain authigenic siderite, pyrite, and vivianite and show little evidence of soil formation. Muds encase crevasse-splay and floodplain-channel sand bodies (< 1 km wide), and collectively these deposits represent a mosaic of shallow lakes, poorly drained backswamps, and multichannel streams, similar to modern examples in the Atchafalaya Basin ( 100 km south of Ferriday). Upper Holocene deposits (< 5000 yr B.P.) are represented by large Mississippi River meander-belt sand bodies that are up to 15 km wide and 30 m thick. Natural-levee silts and sands and well drained backswamp muds are present between meander-belt sands. Upper Holocene deposits contain abundant soil features, and sandy and silty soils are Entisols, Inceptisols, and Alfisols whereas clayey soils are Vertisols. The presence of isolated sand bodies surrounded by mud and the scarcity of soil features suggest that Lower Holocene sediments reflect a period of rapid floodplain aggradation during which crevassing, lacustrine sedimentation, and avulsion dominated floodplain construction. No evidence of large meandering Mississippi River channels represented by buried, thick tabular sands exists near Ferriday, and discharge in Lower Mississippi Valley flow was probably conveyed by a network of small, multichannel floodplain streams. Upper Holocene sediments record a dramatic change ca. 5000 yr B.P. from rapid to slower floodplain aggradation, which was accompanied by extensive lateral channel migration, overbank deposition, and soil formation. On the basis of differences in meander belt dimensions and numbers of abandoned channels, Upper Holocene meander belts are subdivided into simple and complex forms. Relative age relationships suggest that the smaller and older simple meander belts represent periods of divided Mississippi River flow and early attempts to establish a large, single-channel meandering regime. This type of meandering regime is represented by the larger and younger complex meander belts and includes the modern meander belt. Similarities in the timing of changes in floodplain processes and fluvial style and decreasing rates of Holocene sediment accumulation in the southern Lower Mississippi Valley strongly suggest that decelerating Holocene sea-level rise in the Gulf of Mexico affected floodplain development at least 300 km inland from the present-day coast. The alluvial architecture of the Lower Holocene deposits and the absence of large meandering Mississippi River channel deposits older than 5000 yr B.P. near Ferriday indicates that most of the floodplain muds were deposited by avulsion-related crevassing and lacustrine sedimentation rather than by overbank flooding of large Mississippi River channels. Similarities between the floodplain history of the Mississippi River and those of modern and ancient rivers elsewhere further suggest that avulsion, rather than simple overbank deposition, contributes to the construction of fine-grained floodplains to a greater degree than generally recognized.

Fire ecology of a Late Carboniferous floodplain, Joggins, Nova Scotia

The facies distribution and palaeobotanical identity of fossil charcoal is described from the Upper Carboniferous (Westphalian A–B) Joggins section, Nova Scotia, in order to examine the fire ecology of early Westphalian floodplains. Three charcoal assemblages are noted. Assemblage (1) consists of channel sandstone bodies, containing beds of calcrete breccia, cordaite logs (up to 3 m long), and coniferopsid wood charcoal. This assemblage records upland coniferopsid forest fires which altered basin hydrology and promoted flooding in downstream floodplain rivers. Assemblage (2) consists of thin (1–8 mm thick) coals containing hollow, sigillarian tree stumps and lepidodendrid and medullosan plant compressions. Fifteen of the 29 stumps observed have a 2–5 cm thick basal layer infilling the stumps, composed entirely of sigillarian charcoal. Sigillarian and medullosan charcoal is also scattered around the outside of these stumps. This assemblage represents fire in lowland peat-forming communities dominated by Sigillaria and medullosan pteridosperms. These forest fire profiles were slowly buried in floodplain muds. Assemblage (3) consists of a heterolithic sheet sandstone body containing a 2.5 m high sigillarian tree trunk with an internal, basal trunk deposit of sigillarian charcoal and a lightly charred bark rind. This also records fire in lowland sigillarian forests, but, in contrast to (2), this forest fire profile was buried rapidly in a distributary lobe.

Anastomosing river sedimentation in the Channel Country of central Australia

Anastomosing river plains of the Channel Country, central Australia, have aggraded slowly over the past 100 ka. Channel sediments accumulate mainly as accretionary benches of mud and sand, sandy channel‐base sheets and vegetation‐shadow deposits. The channels are laterally stable and the sediments have aggraded locally against erosional banks of tough floodplain muds. Channel sediments are profoundly affected by desiccation during dry periods and by bioturbation caused by within‐channel trees and burrowing invertebrates, especially crayfish. Excavations show that mud‐dominated channel bodies of low width:thickness ratio are generated by a combination of vertical and lateral accretion. Levees and braided surfaces, composed mainly of mud aggregates, border the channels and are activated during valley‐wide floods which lay down distal mud sheets. Floodplain muds are converted to vertisols with gilgai, deep desiccation cracks, and impregnations of carbonate and gypsum. A fixed‐channel facies model is applicable to the Channel Country river deposits. Anastomosis apparently results from the need for the system to move large volumes of water and moderate sediment loads across low‐gradient interior basins.Channels distant from upland source areas receive an abundant supply of pedogenic, sand‐sized mud aggregates generated on adjacent floodplains and reworked into braid bars during valley‐wide floods. Some quartz sand is provided from excavation of subsurface Pleistocene sands in deep channels and waterholes and from aeolian dunes on the floodplains. Adjacent gibber plains supply some gravel to the system.

The norian Plateosaurus bonebeds of central Europe and their taphonomy

Plateosaurus is probably the single most common dinosaur in the Late Triassic, known from over 40 localities in the Norian Knollenmergel mudstones of central Europe. In addition, it is also one of the best known Triassic dinosaurs due to the discovery of many complete skeletons. These complete skeletons as well as the majority of all other material are actually derived from only a handful of localities, three of which are compared here to one another in detail. Frick (northern Switzerland), Trossingen (southwestern Germany), and Halberstadt (central Germany) are recognized as the result of the same taphonomic processes and can all be described with the term “ Plateosaurus bonebed”. This type of bonebed is characterized by its large size, measured in thousands of square meters, the large number of incomplete and complete Plateosaurus skeletons, and the lack of other vertebrate fossils, with the exception of the primitive turtle Proganochelys. The uniformly upright carcass position precludes post mortem transport and strongly argues for in situ death. Shallow depressions in the thick floodplain muds of the Knollenmergel must have formed efficient traps for the heaviest animal at that time, Plateosaurus. The animals became mired and were scavenged upon by small theropods whose shed teeth are always present in Plateosaurus bondebeds. Miring explains the deep position of the feet and hindquarters and, in combination with scavenging and weathering, the bias towoards bones of the posterior body region. The lack of juvenile individuals is due to their lower foot pressure, which was below the threshold for becoming mired.

Hierarchical Levels of Heterogeneity in a Mississippi River Meander Belt and Application to Reservoir Systems: Geologic Note (1)

Six hierarchical levels of sand heterogeneity that would be present in analogous reservoirs are recognized in a Mississippi River meander-belt system in southeastern Missouri: (1) meander belt, (2) meander scroll, (3) channel, point bar, and splay, (4) lobe sheet, (5) bedding unit, and (6) laminae. Heterogeneity level 1 is similar in size to an oil field and consists of (1) 9 million ac-ft (11.1 billion m{3}) of high-permeability channel sands in numerous meander scrolls and (2) 4 million ac-ft (4.9 billion m{3}) of low-permeability clay plugs (in numerous abandoned channels) that separate a field into isolated pools and reservoirs. Overbank flood-plain muds capping the meander-belt system are barriers to vertical migration of fluids. Heterogeneity level 2 is the size of pools within a field and has 1.2 million ac-ft (1.5 billion m{3}) of highly permeable, laterally accreted channel and point-bar sands, partly or wholly isolated from other sand bodies by low-permeability, abandoned-channel clay plugs. Abandoned mud-filled chutes impede lateral migration of fluids in the upper pa ts of the meander scroll. The third level of heterogeneity is composed of about 70,000 ac-ft (86.3 million m{3}) of individual, permeable channel, point-bar, and crevasse-splay sand bodies, with numerous thin sheets and lenses of low-permeability muds and silts (derived from chute fill and mud drapes) that impede vertical migration of fluids. Heterogeneity level 4 is the lobe sheet unit (reservoir pay zone about 450 ac-ft or 555,000 m{3} in volume) comprising channel, point-bar, and splay sand bodies. The fifth level of heterogeneity is the single bedding unit (about 10 ac-ft or 12,000 m{3}) at the scale of a reservoir flow unit or perforated interval. Permeable, cross-bedded sand bundles are separated by low-permeability, inclined and horizontal mud-silt layers and lenses present along bed-set boundaries. Heterogeneity level 6 is individual sand laminae (grain-flow lenses or grain-fall sheets) separated by textural variations (and hence permeability-porosity variations) and isolated mud-silt laminae. Permeabilities range from 90-160 d in sand; several millidarcys to 50 d in muddy sand, silt, and sandy silt beds from the levee, crevasse, and abandoned-channel fill; and less than 10 md in muddy silts and muds from abandoned-channel and chute-fill mud sheets and laminae. At all levels within a fluvial meander-belt system, these permeability heterogeneities are the principal controls on productivity throughout the life of a reservoir.

Occurrence of barite-bearing mud balls within the Mio-Pleistocene terrestrial sand (Kuwait Group) in southern Kuwait, Arabian Gulf

Epigenetic barite concretions embedded within reworked mud balls occur in the upper Kuwait Group fluviatile clastic in southern Kuwait. The barite was precipitated displacively within flood plain muds during an epigenetic stage, possibly as a result of mixing of barium-rich brines with sulfate rich water. The mud balls formed during erosion of the barite-bearing flood plain mud layers by flood mass wasting.

Oakville Formation of Texas Coastal Plain--Depositional Systems, Composition, Structure, Geohydrology, and Uranium Mineralization: ABSTRACT

The Oakville Formation consists of deposits of a bed-load fluvial system composed of at least four major and several minor rivers that flowed across the Miocene Texas coastal plain. Rivers of the southwestern part of the system transported polymictic sand and gravel containing abundant volcanic clasts; stream deposits of the northeastern area are uniquely rich in reworked carbonate-rock fragments. Structures suggest highly variable to ephemeral flow and extensive development of crevasse splays. Bounding flood-plain muds consist of kaolinitic calcium to sodium montmorillonite. Illite is present locally. Hydrogeology and uranium mineralization are strongly influenced by a broad belt of subjacent Wilcox (Eocene age) growth faults. Mineralization and alteration patterns reflect the complex flow of groundwater within a stratigraphically and structurally compartmentalized aquifer. With evolution of the Oakville aquifer system, faults have acted both as flow boundaries and as loci for intrusion of deep-seated highly reducing brines and shallow meteoric groundwater, further obscuring primary ore-forming processes. Volcanic glass within and possibly above the Oakville provides a probable source for the uranium. End_of_Article - Last_Page 455------------

Late Cenozoic volcaniclastic deposits, Andean foredeep, Colombia

Volcaniclastic deposits preserved in the upper Magdalena Valley, Colombia, were derived from stratovolcanoes on Cordillera Central during and after the late Cenozoic Andean orogeny. Andesitic lava clasts and suites of mineral grains, commonly mixed with plutonic and metamorphic rock fragments, predominate in proximal debris flows and in torrential channel lenses; reworked dacitic pyroclastic material is more common in distal debris flows, sheeted overbank sands, and flood-plain muds. Earliest Colombian volcanism is recorded by montmorillonite in the mid-Cenozoic La Cira Formation (1,000 m). During early phases of Andean orogeny in middle and late Miocene time (20 to 10 m.y. ago), abundant volcanic clasts and volcanic mineral grains accumulated in coarse channel and flood-plain sediments of the Honda Group (3,000 m), along with a few thin sheets of reworked pyroclastic debris. The overlying conglomeratic Neiva Formation (200 m) records renewed uplift and a fresh supply of lava. In latest Miocene time (8 to 9 m.y. ago) the climax of explosive activity swamped the Andean foredeep with debris that accumulated on coalescing fans of the Gigante Formation (750 m). About 7 to 5 m.y. ago, the final Andean orogenic phase deformed both Cordillera Central and its foredeep. Subsequent excavation of the uplifted eastern lowland was interrupted at least ten times by local incursions of volcanic detritus 10 to 75 m thick. These deposits include polymictic debris flows and torrential sediments that filled valleys and formed large fans, and reworked pumiceous debris concentrated in fluvial valley fill and in debris flows that spread widely across the Magdalena lowland. Older deposits are preserved in high-level mesas, valley terraces, and deeply dissected fans; younger deposits form low, relatively undissected fans and inner terraces along major streams.