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Landforms of the Louisiana Coastal Plain

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Landforms of the Louisiana Coastal Plain Landforms of the Louisiana Coastal Plain John Snead, Richard P. McCulloh, and Paul V. Heinrich Louisiana Geological Survey - Maps and Atlas Series No. 15 Landforms of the Louisiana Coastal Plain John Snead, Richard P. McCulloh, and Paul V. Heinrich geographic information systems by R. Hampton Peele and Robert Paulsell cartography by John Snead, Robert Paulsell, and Lisa Pond LOUISIANA STATE UNIVERSITY Louisiana Geological Survey Baton Rouge 2019 Landforms of the Louisiana Coastal Plain 1 ©2019 Louisiana State University mail Louisiana Geological Survey Louisiana State University Baton Rouge, LA 70803 website lgs.lsu.edu office Room 3079 Energy, Coast & Environment Building phone 225/578-5320 2 Landforms of the Louisiana Coastal Plain Introduction The geology of Louisiana is very recent, at least at Conversely, the dynamic processes that shaped recent the surface. While older hard rock can be found in coastal landforms offer insight into the similar processes northwest Louisiana, the geologic units of the Louisiana that formed older, inland deposits where the original coastal plain consist of poorly consolidated sedimentary landforms themselves have long since eroded away or deposits of varying age, compaction, sediment type, and have been consolidated into rock. origin. Landforms exert tremendous influence and control on Most of the surface deposits are alluvial, having been soil development, drainage, natural vegetation patterns, eroded, reworked by water in some fashion and accu- wildlife habitats and the occupation patterns of indige- mulated in distinct sequences by a variety of riverine nous peoples. They affect subsequent human develop- depositional processes, some of which are still ongoing. ments such as agriculture, forest management, highway Other depositional units are coastal deposits of shells patterns, and the suburban sprawl into wetlands. Land- and alluvial sediment reshaped by shallow marine pro- form awareness is essential to the research and engi- cesses into headland beaches, barrier islands, and shoals. neering efforts that help manage Louisiana’s coastline Most sediments in coastal Louisiana are widely varying and rivers. combinations of gravel, sand, silt, and clay with occa- The mapped landforms help define Louisiana’s ex- sional lenses of shell and peat. The composition of these tensive riverine and coastal floodplains, areas that are deposits alone is rarely diagnostic so traditional geologic subject to episodic stream flooding and hurricane storm maps tend to group them together as recent unconsoli- surges. They also offer insight into the increased flood- dated Quaternary deposits. ing potential of very flat upland regions that retain such Fortunately, the processes that deposited these sedi- low gradients that they can still flood during extreme ments, along with the later erosional processes acting rain events. Many of these regions include urban areas. upon them, have produced sometimes subtle, but dis- Although based on geologic map units of the Louisi- tinctive landforms that compose the coastal landscape. ana Geological Survey series of geologic quadrangles This map is an effort to better define these landforms, at 1:100,000 scale, the landforms mapped here have especially for non-geoscientists working in coastal resto- been extensively refined, extended, and reinterpreted ration and environmental disciplines and for the inter- based on high-resolution lidar topographic imagery that ested and informed Louisiana citizen. was unavailable when many of the geologic maps were Louisiana’s coastal plain is the suite of flat and low- developed. One-foot elevation data resolves stunningly lying tablelands, prairies, river valleys, natural levees, and detailed features not identifiable on the 5- and 10-foot coastal marshes that lie between the inland hill country contours of yesterday’s topographic mapping. and the Gulf of Mexico. The inland hill country of north Many landforms have a direct relationship with a tradi- Louisiana is composed of the eroded remnants of much tional geologic formation described by deposition, sed- older coastal plains, now removed by elevation and iment type, and stratigraphy. The landforms described distance from Louisiana’s modern coastal geology issues here are based upon the shape of their surfaces, their that include land loss, hurricane storm surge, subsid- position in the topography, the processes that created ence, delta dynamics, and sea-level rise. and shaped them, and the ecosystems they support. The older coastal plain landforms, now elevated above Some geologic formations have been grouped together the modern floodplains, show distinctive geologic traits into a single landform, while others have had several that are analogous to the newer sediments presently landform elements broken out within them. being deposited and eroded on the coastal plain. The older landforms can provide insight into the evolution of modern coastal systems including the processes of faulting, coastal subsidence, inland uplift, delta develop- ment, and the marine advances and retreats associated with sea level change. Landforms of the Louisiana Coastal Plain 3 Original map scale is 1:380,160. Landforms of the Louisiana Coastal Plain. Image of 4 Landforms of the Louisiana Coastal Plain Geology Geology is an earth science concerned with the plan- Mississippi River Alluvial Valley et, the rocks from which it is made, and the processes The Mississippi River is the continent’s largest river, by which they change over time. Geologically young draining 41% of the United States. Its giant alluvial valley sedimentary deposits that accumulated in river, delta, is the dominant physical feature in Louisiana, with a beach, estuary, lake, swamp, and marsh settings charac- floodplain spanning a width of 50 miles ≈( 80 kilome- terize the Louisiana Coastal Plain. The older, higher land- ters) at the latitude of Baton Rouge. The floodplain scapes consist of Neogene and Pleistocene sediment, sediment consists of diverse types with distinctive land- while the lower areas along the coastline are the sites of forms, including channel, natural-levee, crevasse-splay, Holocene deposition in these same environments that backswamp, distributary, and lake deposits. continues to the present. Mississippi River Delta The Mississippi River Delta is the terminus for the Geography tremendous amount of sediment transported over time Coastal Louisiana is located on the North American by the Mississippi River where it reaches the sea. It continent between 29° and 31° north latitude, a third contains 37 percent of the nation’s estuarine marsh. The of the way from the equator to the North Pole. North delta plain consists of several individual delta complex- of the Tropic of Cancer (23.5° N), it possesses a humid es. Each of the delta complexes went through a natural subtropical climate greatly influenced by its position on life cycle from initiation through growth and aggrada- the Gulf of Mexico, a relatively warm body of ocean. tion eventually leading to overextension, abandonment, and subsidence accompanied by the initiation of a new Physical Provinces course. Louisiana includes portions of three significant North America physical provinces: Gulf of Mexico Coastal Plain The Gulf Coastal Plain in its broadest sense refers to the vast band of young, Philadelphia low-lying landscapes extending from Lou- • 0 100 200 300 400 500 MILES isiana east into the Florida panhandle and west through Texas and Mexico around 0 100 200 300 400 500 KILOMETERS the Gulf to Yucatan. It is continuous with the Mississippi embayment to the north Richmond• and with the Atlantic Coastal Plain to the MISSISSIPPI east and northeast. The coastal plain prov- ALLUVIAL VALLEY Raleigh• ince contains an inner zone of ancient Little coastal deposits and an outer zone of Rock INNER more recent coastal plain deposits. • COASTAL PLAIN Atlanta• •Charleston n a e Montgomery c • O OUTER COASTAL PLAIN Baton Rouge c i • t n a l Houston• t A MISSISSIPPI Tampa DELTA • Mexico of ulf G Physical provinces of the Northern Gulf of Mexico and Lower Atlantic coastal areas. Landforms of the Louisiana Coastal Plain 5 Stratigraphy Pleistocene Stratigraphy is the study of the age and succession The Pleistocene epoch, popularly known as the “Ice of the sedimentary layers of the Earth, including their Age,” spanned Earth’s most recent period of repeated origin, geographic extent, physical form, fossils, and glaciations and ended with the retreat of the last con- other identifying characteristics. On the Louisiana coast- tinental glacier. Glaciers never approached Louisiana al plain, only the most recent of geologic periods are but Pleistocene glacial cycles affected the state by the represented. periodic lowering of global sea levels as growing gla- ciers consumed atmospheric moisture. The state was Neogene also affected by episodic infusions of large amounts of The Neogene geologic period began 23 million years glacial outwash down the Mississippi River Valley as the ago and ended with the beginning of the present glaciers melted. By the late Pleistocene the active deltas Quaternary Period 2.6 million years ago. The Neogene had continued to shift southward and extended to the is subdivided into two epochs, the Miocene and the west. During the periods of glacial advances, sea level Pliocene. During the Miocene, deltaic depocenters of dropped more than 400 feet (≈120 meters) at times. In the Mississippi River system shifted from southwestern response, the Mississippi River cut deep valleys into the to southeastern Louisiana. During the Pliocene, the loci NORTH kilometers SOUTH 0 20 40 60 80 100 120 140 160 90 hills 300 vertical exaggeration = 420 80 250 70 60 tablelands 200 50 150 40 feet meters 30 E 100 N A E R 20 O L Y prairies G P 50 10 E L coastal marshes N E Gulf of Mexico I S E T L 0 O A T E 0 mean sea level C P L H O E E I S L O C E N T O C N E -10 E E N E -50 0 10 20 30 40 50 60 70 80 90 100 miles Diagrammatic cross-section: Stratigraphic units in the Louisiana coastal plain.
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