Whitney J. Autin

A Revised Chronology for Mississippi River Subdeltas

Radiocarbon measurements by accelerator mass spectrometry relating to three of the four late Holocene Mississippi River subdeltas yielded consistent results and were found to differ by up to 2000 carbon-14 years from previously inferred ages. These geological dataare in agreement with archaeological carbon-14 data and stratigraphic ages based on ceramic seriation and were used to develop a revised chronologic framework, which has implications for prehistoric human settlement patterns, coastal evolution and wetland loss, and sequence-stratigraphic interpretations.

Holocene flood-plain soil formation in the southern lower Mississippi Valley: Implications for interpreting alluvial paleosols

Holocene Mississippi River flood-plain soils in central Louisiana provide important insights on alluvial pedogenesis in an aggradational setting, insights that are useful for interpreting alluvial paleosols. The flood-plain soils are younger than ca. 5400 yr B.P. and are grouped into two categories: (1) meander belt and (2) backswamp soils. Meander belt soils are Entisols, Inceptisols, and Alfisols developed on point-bar ridges and natural levee crests; they consist of 3–5-m-thick, brown, silty and sandy, moderately well drained profiles with multiple parent materials, common yellow-brown and gray mottles, and a few iron and calcite nodules. Backswamp soils are Vertisols formed in flood-plain depressions away from active and abandoned Mississippi River channels and consist of up to 10-m-thick, gray, clayey, poorly drained profiles that have abundant yellow-brown mottles, iron and calcite nodules, gypsum accumulations, and slickensides. The thickness of the soils and the presence of multiple parent materials show that as the flood-plain aggrades, concurrent or alternating episodes of sedimentation and pedogenesis produce cumulative profiles. Profile thickness is more closely related to sediment accumulation rates than to the depth of chemical weathering, and the cumulative nature of the soils explains the difficulty of recognizing discrete paleosol profiles in many ancient alluvial deposits. Comparisons of soils formed on the three youngest Holocene Mississippi River meander belts (Saucier and Snead, 1989) show that the soils record episodes of meander belt growth, abandonment, and relocation. Meander belt 1 soils form as the active meander belt develops and consist of either vertically stacked Entisols or a single Inceptisol. The soils contain few calcite nodules and form over intervals of a few hundred years. Meander belt 2 soils are Alfisols, lack free carbonates, and form over intervals of a few thousand years following meander-belt abandonment and soil leaching. Meander belt 3 soils are Alfisols or Vertisols that contain common calcite nodules, and the profiles are developed in 1 to 2 m of backswamp mud that overlies meander-belt silt and sand. The mud inhibits profile leaching and leads to poor drainage and calcite precipitation. This change in soil hydrology complicates interpretations of the effects of time on meander-belt soil development. Backswamp soils are strongly influenced by seasonal episodes of soil wetting and drying, as demonstrated by the gray colors and the abundance of mottles, nodules, and slickensides in the soils. The distribution of calcite nodules in the backswamp and meander-belt soils is more closely related to seasonal water-table position than to mean annual precipitation, which suggests that not all calcic paleosols in ancient alluvial deposits represent semiarid or arid climates. Slickensides attest to seasonal rainfall, but the composition of the backswamp soils primarily reflects source-area lithology and depositional influences, rather than the effects of chemical weathering. In general, these soils provide an incomplete record of the humid subtropical climate of Louisiana. Although short-term sediment-accumulation rates decrease away from Mississippi River channels, the backswamp soils are not better developed (i.e., more mature) than the meander-belt soils, because poor drainage and slow but continuous sedimentation inhibits weathering. Collectively, these observations suggest that flood-plain soils that developed in aggradational settings, such as that of the Mississippi River flood plain, are best suited for interpreting depositional and hydrologic histories of ancient alluvial deposits, rather than paleoclimates.

Use of alloformations for definition of Holocene meander belts in the middle Amite River, southeastern Louisiana

Geomorphological studies in drainage basins of the northern Gulf of Mexico coastal plain have indicated similarities in flood-plain character and comparable influences on basin evolution. Alluvial morphology and stratigraphy can be linked to flood-plain evolution through the use of alloformations, unconformity-bounded stratigraphic units. This procedure extends classical terrace mapping concepts to conform with formally defined stratigraphic procedures. A meander belt segment of the middle Amite River valley in southeastern Louisiana was selected to test the applicability of alloformation mapping in part of a regional-scale (5,000 km 2 ) drainage basin. Three alloformations, identified as Magnolia Bridge (MAG), Denham Springs (DS), and Watson (WAT), can be differentiated by unconformable boundaries, landscape morphology, and relative pedogenic development. The valley fill consists of a lower sandy and upper silty facies. Preserved deposits consist of channel lag, point bar and scroll bar, channel fill, and overbank sequences. Lithofacies and sedimentary environments are dependent on fluvial process, and a suite of meander belt environments can be identified within each alloformation. Surface horizons of alluvial soils are all similar (ochric epipedons), but subsoil horizons are chiefly stratified parent material on the MAG (Entisols), cambic horizons on the DS (Inceptisols), and minimally developed argillic horizons on the WAT (Alfisols). Allostratigraphy provides an objective method for definition of geologic units that contain genetically related, but heterogeneous sedimentary deposits. Alluvial sediments are differentiated in a manner that successfully integrates geomorphic, sedimentologic, and pedologic data into significant stratigraphic models. This approach is applicable to a wide variety of Quaternary settings where prior stratigraphic techniques have resulted in ambiguity.

Rapid and widespread response of the Lower Mississippi River to eustatic forcing during the last glacial-interglacial cycle

The Lower Mississippi Valley provides an exceptional fi eld example for studying the response of a continental-scale alluvial system to upstream and downstream forcing associated with the large, orbitally controlled glacialinter glacial cycles of the late Quaternary. However, the lack of a numerical chronology for the widespread Pleistocene strata assemblage known as the Prairie Complex, which borders the Holocene fl oodplain of the Lower Mississippi River, has so far precluded such an analysis. Here, we apply optically stimulated luminescence (OSL) dating, mainly on silt-sized quartz from Prairie Complex strata. In total, 27 OSL ages indicate that the Prairie Complex consists of multiple allostratigraphic units that formed mainly during marine isotope stages 7, 5e, and 5a. Thus, the aggradation of the Prairie Complex is strongly correlated with the sea-level highstands of the last two glacialinterglacial cycles. Fluvial incision during the sea-level fall associated with the MIS 5a–MIS 4 transition extended as far inland as ~600 km from the present-day shoreline, testifying to the dominant downstream control of fl stratigraphic architecture in the Lower Mississippi Valley. In addition, the short reaction time of the Lower Mississippi River suggests that large fl uvial systems can respond much more rapidly to allogenic forcing than is commonly believed.

Alluvial pedogenesis in Pleistocene and Holocene Mississippi River deposits: Effects of relative sea-level change

Comparison of Pleistocene and Holocene soils developed in Mississippi River meander-belt deposits in Louisiana illustrates the influence of relative sea-level change on pedogenesis. Holocene and Pleistocene meander belts have similar landscape morphologies, lithofacies, and soil characteristics, and probably formed under similar hydrologic regimes. The Pleistocene Avoyelles Prairie meander belt is now terraced, but it was originally graded to a lower relative sea-level position than the Holocene flood plain. Holocene alluvial soils are generally weakly expressed cumulative profiles with abundant redoximorphic features, and formed on an aggrading flood plain with a shallow fluctuating water table. The soil morphology varies with respect to landscape position and overbank lithofacies. Pleistocene alluvial paleosols show evidence for two stages of pedogenesis. During the first stage of pedogenesis, water-table fluctuations and flood-plain aggradation produced profiles with color mottles, iron and carbonate nodules, and weak B and C horizons, similar to Holocene flood-plain soils. The second stage of pedogenesis occurred after the Avoyelles Prairie was terraced and the regional water table fell a minimum of 30 m. Improved soil drainage created oxidizing and acidic conditions, removed free carbonates, dehydrated iron oxides, and transformed clay minerals. This stage produced Pleistocene profiles with brighter matrix and mottle hues, lower pH values, larger and harder iron nodules, and greater kaolinite contents than comparable Holocene flood-plain soils. The primary influence on regional water- table lowering in the southern Lower Mississippi Valley was relative sea-level fall. Mississippi River flood-plain lowering adjacent to the Avoyelles Prairie most likely occurred between the end of oxygen isotope stage 3 and the maximum period of valley entrenchment, during oxygen isotope stage 2. Water-table fall affected pedogenic processes a minimum of 400 km upvalley of the continental shelf margin. Pleistocene Mississippi River paleosols represent excellent analogs for interpreting ancient alluvial paleosols formed during episodes of base-level fall and valley incision. As fluvial entrenchment terraced the meandering Avoyelles Prairie, a pedogenic threshold was crossed that altered the pathway of chemical weathering. The complexity of a multiple-step pedogenic history also highlights the fact that interpretation of relative profile development as a function of time can be problematic.

Complex response of a Midcontinent North America drainage system to late Wisconsinan sedimentation

ABSTRACT The geomorphic evolution of Mud Creek basin in eastern Iowa, U.S.A. serves to illustrate how geomorphic influences such as sediment supply, valley gradient, climate, and vegetation are recorded in the alluvial stratigraphic record. Sediment supply to the fluvial system increased significantly during the late Wisconsinan through a combination of periglacial erosion and loess accumulation. Subsequent evolution of the Holocene alluvial stratigraphic record reflects long-term routing of the late Wisconsinan sediment through the drainage basin in a series of cut-and-fill cycles whose timing was influenced by hydrologic response to change in climate and vegetation. When viewed in a regional context, the alluvial stratigraphic record appears to reflect a long-term complex response of the flu ial system to increased sediment supply during the late Wisconsinan. Hydrologic and sediment-supply changes accompanying the spread of Euroamerican agriculture to the basin in the 1800s dramatically upset trends in sedimentation and channel behavior established during the Holocene.

Landscape evolution of the Five Islands of south Louisiana: scientific policy and salt dome utilization and management

Abstract The Five Islands of south central Louisiana are piercement-type salt domes uplifted from several kilometers depth as the surrounding strata experiences regional subsidence. In general, the domes are nearly circular in plan with maximum land surface elevations 23–52 m asl. Geomorphic evidence of salt-induced uplift includes surface lineations, aligned gullies, excessively steep land surface topography, and shear fractures in surficial sediments. Evidence of subsidence includes sinkhole ponds a few hectares in area, broad topographic saddles over tens of hectares in size, and kilometer-scale collapse structures. On each of the Five Islands, Peoria Loess and silty colluvium bury a paleosol developed in the Late Pleistocene Prairie Complex of the Lower Mississippi Valley. The loess represents a single genetic unit of eolian origin, is typically thickest on lower side slopes, and is thin or absent on ridge crests. The silty colluvium around the perimeter of the islands is a reworked loess derived from higher elevations. Shear fractures with high-angle average dips occur in both Peoria Loess and the Prairie Complex. Conjugate shear pairs probably develop from extensional stress associated with vertical uplift of the underlying salt. Prairie Complex deposition and initial soil development in a low-relief alluvial plain of the Mississippi River predates the latest emergence of the Five Islands. Loess and colluvial stratigraphy indicate that the domes were emergent during loess deposition. Gully incision, shearing of Quaternary sediments, and the distribution of colluvium indicate continued uplift after loess deposition. Sinkholes and collapse structures are influenced by salt dissolution and corrasion, whereas broad subsidence areas and topographic saddles form over areas of structural weakness within the salt. Five Islands landscape evolution is controlled by the interaction of driving and resisting forces that operate over various time scales. Diapiric uplift is a driving force of net upwards motion, and the external and internal salt dome hydrology are driving forces of solution and corrasion. The structure and lithology of the internal salt stocks and the surrounding sediments are heterogeneous and have variable strength. Collectively, this interaction produces both uplift and subsidence features across the salt dome landscape. Resource use at the Five Islands correlate with instability of both underground facilities and the surface landscape. Uplift of the Five Islands has continued since at least the Late Pleistocene, is probably still active at present, and periods of tectonic and geomorphic instability are possible in the future. Sediments overlying salt domes record discrete periods of surface uplift and periods of episodic and incremental subsidence that is common where salt domes pierce surficial sediment. The rate, magnitude, and pattern of landscape modification by salt domes have implications for the safe utilization of mineral extraction and geostorage facilities. Geomorphic evaluation of salt dome landscapes can help to develop policies that ensure safe salt dome utilization. Salt dome resource planning should include detailed characterization of internal and external stratigraphy and structure; modeling of geomorphic, soil and rock mechanic, and hydrologic processes; routine and emergency planning at operating facilities; and closure and post-closure plans.

Depositional and pedogenic influences on the environmental geology of Holocene Mississippi River floodplain deposits near Ferriday, Louisiana

Abstract Core descriptions and the mineralogy of Holocene Mississippi River floodplain deposits in Louisiana provide insights on fluid migration pathways and the origin of iron-rich ground waters in the Mississippi River Alluvial Aquifer (MRAA). Vertical changes in the sedimentologic and pedologic characteristics of floodplain deposits near Ferriday, Louisiana, provide evidence for two stages of floodplain development and suggest that depositional processes and drainage conditions changed substantially during the Holocene. Depositional and pedogenic processes produced complex fluid migration pathways in the MRAA confining unit and also contributed to the formation of iron-rich ground waters. Lower Holocene deposits in the study area are older than ∼5000 yrs BP and show evidence of crevassing, lacustrine delta building, and multi-channel stream deposition. These processes deposited thin and narrow sheet sands, which represent fluid migration pathways in the MRAA confining unit. Poor drainage conditions during this initial stage of floodplain development also favored the precipitation of authigenic siderite and pyrite in poorly-drained swamps and shallow lakes. The pyrite and siderite probably represent the source of iron-rich ground waters in the MRAA. Upper Holocene floodplain deposits are younger than ∼ 5000 yrs BP and represent the transition to the present-day meandering regime of the Mississippi River near Ferriday. This second stage of floodplain development was accompanied by pedogenesis, which produced slickensides in clayey backswamp soils. The abundance of slickensides and the presence of the thin and narrow sheet sands indicates that fluid migration in the MRAA confining unit near Ferriday is greater than generally recognized. Seasonal water table fluctuations and the mixing of oxygenated meteoric and reduced ground waters cause iron oxide reduction and pyrite oxidation in backswamp settings, which releases iron into solution. The presence of high (up to 16 mg/l) dissolved iron concentrations in water wells that are screened beneath pyrite- and siderite-rich, muddy backswamp deposits near Ferriday, suggests that the distribution of fine-grained alluvium is a primary control on the presence of iron-rich waters in the MRAA. In contrast, water wells that are screened in sandy meander belt deposits, which lack abundant iron-bearing minerals, have low (less than 1 mg/l) concentrations of total dissolved iron. Studies of large floodplains such as the Mississippi River, highlight the importance of floodplain histories for evaluating geologic influences on water quality, developing proper floodplain land use strategies, and for improving our understanding of the environmental geology of floodplain systems.

Pleistocene stratigraphy in the southern Lower Mississippi Valley

Abstract Scientific inquiry into Pleistocene stratigraphy of the Lower Mississippi Valley (LMV) dates to early writings of European naturalists in the late 19th century. By the early 20th century, landscape evolution concepts, stratigraphic models, and regional syntheses had developed for most areas. The 1944 monograph of H.N. Fisk marks the advent of a predictive stratigraphic and landscape evolution model that links form and process to a predominantly glacioeustatic mechanism. The Fiskian model gained widespread acceptance, and decades passed before significant alternate models began to emerge. Revised stratigraphic and geomorphic concepts are presently developing from newly acquired environmental and engineering data. Present scenarios classify Pleistocene outcrop areas into erosional and constructional landscapes, and veneers of eolian, colluvial, fluvial, coastal, and marine origin can drape both types of surfaces. The southern LMV and adjacent Gulf Coastal Plain (GCP) experienced significant landscape change during the Pleistocene. Late Tertiary (Pliocene?) to Early Pleistocene deposition of the Upland Complex was by streams with a high sand and gravel load relative to its mud load. The regional drainage network and fluvial system behavior was probably significantly different from the modern. Braided stream alluvial fan complexes received sediment from highland source areas adjacent to the LMV and the glaciated mid-continent. It is plausible that part of Upland Complex deposition predates initial glacial advances. From Early to Middle Pleistocene, an erosional landscape formed during a dissection period that chiefly postdates soil formation on stable landscape positions of the Upland Complex. Slope evolution truncated a regionally extensive geosol in multiple phases, and parts of the erosion surface complex are graded to the oldest preserved constructional alluvial plains in present valleys. Toe and foot slope positions of the erosion surface complex and its correlative alluvial plains are presently delineated as the Intermediate Complex. Constructional landscapes formed at this time are sparsely preserved; Fisks Montgomery Terrace in the Lower Red River Valley (LRRV) is the best preserved example. Influences on the development of erosion surfaces in the LMV are not well understood; however interactions of relative sea level fall, climate change, and epirogenic crustal movement are plausible factors. From the latter part of Middle Pleistocene to the Holocene, there was widespread evolution of modern constructional landscapes. Constructional alluviation preserved lithofacies of mixed load, laterally accreting, meandering streams that developed over large areas of the southern LMV to form parts of the Prairie Complex. Lateral planation in valleys and stable rates of upland sediment generation were dominant processes during Prairie Complex deposition. Pleistocene stratigraphic examples considered important by Fisk are still considered relevant to modern stratigraphic investigators. Presently, Pleistocene units of the southern LMV, the adjacent LRRV, and central GCP can be correlated only by relative stratigraphic relationships. Refined chronostratigraphic and paleoenvironmental models for these areas would help improve the understanding of the geomorphic influences on Quaternary landscape evolution in the region.

Exposure of Late Pleistocene Mississippi River Meander-Belt Facies at Mt. Pleasant, Louisiana

ABSTRACT Exposure of a sedimentary sequence along a Mississippi River bluff at Mt. Pleasant, Louisiana provides insight into the constructional development of the Prairie Terraces. This site serves as a type section for a late Pleistocene meander belt of the Mississippi River, and stratigraphic features have been traced beneath the Prairie Terraces in southeastern Louisiana. A 76.6-ft (23.35-m) measured section reveals upper units of Peoria loess and mixed loess. Loessial sediments overlie proximal flood-plain deposits of a meandering-river system. The upper flood-plain deposits are stratified natural levee facies consisting of coarse silt to fine sand with small-scale trough cross-stratifications, horizontal stratifications, climbing ripples, scour surfaces, and various soft-sediment deformation structures. Within this deposit, a silt loam paleosol was identified with moderately developed blocky structure, clay films, and oxide stains concentrated along root traces. The flood-plain sequence grades downward into abandoned channel-fill facies, with five distinct cycles recognized by textural gradations. Each cycle fines upward from basal loamy or sandy textures to silt loam or silty clay textures. Oxide stains and concretions are common in the upper fine-grained parts of each cycle. The third channel-fill cycle is characterized by a mottled paleosol with blocky structure, clay films, and oxide stains in its upper part. This paleosol represents an unconformable surface upon which the overlying fluvial sequence was deposited. About 1300 ft (400 m) downstream from the measured section is a point bar sequence consisting of fine sand to granule gravel with large- and small-scale trough cross-stratifications, horizontal stratifications, scour surfaces, and clay drapes. The vertical and lateral successions of strata at Mt. Pleasant Bluff indicate deposition by a large-scale, mixed-load, meandering fluvial system. The modern Mississippi River has facies characteristics analogous to those described in the late Pleistocene sequence. Correlation of individual strata away from the Mt. Pleasant Bluff measured section is difficult. However, the upper contact between mixed loess and stratified natural levee facies and the underlying upper paleosol can be traced along the Mississippi River bluffs for considerable distances. Identification of the lower paleosol away from Mt. Pleasant Bluff is more difficult, due to its limited exposure and possible lateral variability. The described meander-belt facies are of a probable Wisconsin age and are considered as a unit, here named the Mt. Pleasant Bluff alloformation. The age designation is based on position in the stratigraphic section, degree of preservation of sedimentary facies, character and degree of development of the upper paleosol, preservation of constructional topography beneath the loess, and correlation of this sequence to nearby sites with Wisconsin-age radiocarbon dates.