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Contrasting Pleistocene and Holocene Fluvial Systems of the Lower Pearl River, Louisiana and Mississippi, USA

Paul V. Heinrich, Louisiana Geological Survey, 3079 Energy, Coast, and Environment Bldg.

Louisiana State University, Baton Rouge, LA 70803

Abstract

Pleistocene Fluvial System 1 Hammond Alloformation

delta-like feature Hammond Alloformation

Pleistocene Fluvial System 2 Hammond Alloformation

Pleistocene Fluvial System 3 Gum Hollow and Mitchell Hammock alloformations

Holocene Fluvial System Pearl River Alluvial Valley

Conclusions

The geologic mapping of eastern St. Tammany Parish, Louisiana, and adjacent parts of Mississippi for the preparation of 1:24,000 and 1:100,000 scale maps has revealed the presence of three, possibly four, distinct fluvial systems related to the Pearl River. The oldest fluvial system lies on the surface of the Hammond alloformation of the Prairie Allogroup. It consists of well-defined relict meander belts of the Late Pleistocene Pearl River with relict sinuous courses and oxbows with large meander loops. One relict course ends in a delta-like feature. Another one is associated with a large, well-defined crevasse splay complex. Within the southeastern corner of St. Tammany Parish, an isolated relict channel segment appears to be a second, younger fluvial system characterized by an unusually deeply incised channel. A third, youngest, Pleistocene fluvial system consists of relict courses and oxbows exhibited by the surfaces of the Gum Hollow and Mitchell Hammock alloformations of the Deweyville Allogroup. These relict courses and oxbows, as clearly seen in the eastern half of the Industrial 1:24,000 quadrangle, Pearl River County, Mississippi, are characterized by channel widths and radii that greatly exceed those of the modern Pearl River. The modern Pearl River is characterized by an anastomosing channel system. Individual channels that compose it are sinuous and have actively meandered. These contrasting channel systems preserve a discontinuous record of how the Pearl River has adjusted to changes in climate, discharge, and base-level during the Pleistocene and Holocene epochs.

As defined by Heinrich, McCulloh, and Snead (2004) in the “Gulfport 30 x 60 Minute Geologic Quadrangle”, the Hammond alloformation consists of the deposits of middle to late Wisconsin Coastal Plain streams in the Florida Parishes of southeastern Louisiana. It includes late Pleistocene Mississippi River depoists, exposed in the eastern valley wall of the modern Mississippi River alluvial valley deposits, which Autin et al. (1988) originally defined as the Mt. Pleasant Bluff Alloformation. This name was abandoned because the deposits exposed at this location are atypical of this alloformation. Within the area mapped, the Hammond alloformation consists of alluvial deposits of the Pearl and other coastal-plain rivers, which lacks any loess cover.

Within the Pearl River Valley, the Deweyville Allogroup unit consists of coarse-grained deposits of ancestral Pearl River and its tributaries. The “Gulfport 30 x 60 Minute Geologic Quadrangle” by Heinrich, McCulloh, and Snead (2004) recognized and mapped two unconformity bounded stratigraphic units, the Mitchell Hammock and Gum Bayou alloformations, within the Deweyville Allogroup within the lower Pearl River Valley. The Mitchell Hammock alloformation is the youngest alloformation and the topographically lower surface of the Deweyville Allogroup along the Pearl River. It lies near the surface of the modern Pearl River flood plain and is mostly buried by its Holocene deposits. The Gum Bayou alloformation is oldest alloformation and the topographically highest surface of the Deweyville Allogroup along the Pearl River within the Gulfport 1:100,000 quadrangle. Gum Bayou, for which this alloformation is named, occupies relict meander scars of the Gum Bayou allformation.

Acknowledgements

Geologic map of the Lower Pearl River Valley region showing Pleistocene fluvial features exhibited by the surfaces of the Hammond, Mitchell Hammock, and Gum Bayou alloformations. Fluvial channels and related features were originally mapped using US Department of Agriculture Agricultural Stabilization and Conservation Service (ASCS) aerial photography, 7.5-minute topographic maps, United States Department of Agriculture soil surveys, and USGS digital orthophoto quarter quadrangles (DOQQ). For this map, the geology was modified from the "Gulfport 30 x 60 Minute Geologic Quadrangle" by Heinrich, McCulloh, and Snead (2004). The fluvial features were compiled from 7.5-minute geologic quadrangle maps prepared for USGS STATEMAP projects and revised using images made from LIDAR (LIght Detection And Ranging) digital elevation models.

1940 ASCS aerial photograph (above) and excerpt from St. Tammany 7.5-minute DOQQ (right) showing relict channels of Fluvial System 1. These channels are part of a large alluvial cone created by the Pearl River during late Sangamon sea level highstand. Because of degradation of the surface of the Hammond alloformation, channels (C) and associated swales (S) now consist of shallow swamp. Greater than modern discharge of the Pearl River is indicated by channel widths and meander radii, which greatly exceed those of the modern Pearl River.

LIDAR image of Slidell, Louisiana, area showing large beach ridge (B) along edge of delta-like feature, fluvial channels (C), and fault-line scarps (F). Note lower sets of beach ridges (b), which lie seaward of large beach ridge.

Excerpts from Haaswood 7.5-minute DOQQ and topographic map showing course of Fluvial System 2. Unlike other channels present on the surface of the Hammond alloformation, this channel is deeply incised into its surface. This channel possibly represents a period of initial entrenchment of Pearl River into the surface of the Hammond alloformation as a result of falling sea level at the end of the Sangamon Stage.

1940 ASCS aerial photography (above) and excerpt from Industrial 7.5 minute topographic quadrangle (right) illustrate typical channels exhibited by both the Mitchell Hammock and Gum Bayou alloformations. As argued by Michael Blum for correlative units within the Deweyville Allogroup within the valleys of the Sabine, Trinity, and Colorado rivers of Texas, these channels were graded to Middle Wisconsin sea level highstands. As also discussed by Michael Blum, the lack of thick fines-grained overbank sediments, indicated lack of significant seasonal flooding despite greater than present discharge.

Excerpt (left) from Nicholson 15-minute topographic map illustrating anastomosing channel system which characterizes the modern Pearl River fluvial system. As argued for the Rhine-Meuse delta by T. E. Tornqvist and others, the anastomosing character of the Pearl River within the lower part of its valley is regarded as a response to rapid post-glacial, sea-level rise. LIDAR image (right) of part of the Industrial 7.5-minute quadrangle. Because of the thick vegetation cover, images made from the LIDAR digital elevation model shows channels and ridge and swale topography (RS) associated with them not detectable on aerial imagery, soil surveys, and topographic maps. The ridge and swale topography demonstrates that some of the channels have actively meandered at times.

1. Meandering channels, delta-like feature, and large crevasse system are associated with the Sangamon alluvial cone of Pearl River, which forms part of the surface of the the Hammond alloformation. 2. A late Sangamon (?) meandering course of the Pearl River lies entrenched into the Hammond alloformation. 3. The Deweyville Allogroup exhibits oversize chanels and meander loops. These Pleistocene channel systems, together with the anastomosing channel system of the modern Pearl River fluvial system preserved a discontinuous record of the response of the Pearl River to changes in discharge and sea level.

The mapping of the fluvial features was made possible by and conducted largely under the funding of geologic mapping of the Bush, Haaswood, Hickory, Industrial, Lacombe, Slidell, and St. Tammany 7.5-minute geologic quadrangles and preparation of the Gulfport 30-minute geologic map by the United States Geological Survey’s STATEMAP Program. Without this funding, the mapping of relict fluvial features of the Lower Pearl River Valley would not have been possible.

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