Coastal Services Center

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Geologic Regimes of the Atlantic and Gulf Coasts

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Introduction

Coastal processes interact with the local coastal geology and sediment supply to form and modify a coastline's physical features – barrier islands, beaches, dunes, bluffs, and upland areas. To understand the nature of barrier islands and their beaches, it is important for the decision-maker to have a general knowledge of the geology of the areas forming these environments. The extensive barrier island systems along the Atlantic and Gulf Coasts of the United States are found primarily adjacent to a coastal plain geologic system. Many of the barrier islands of the north Atlantic have developed along glacial coasts having a wide range of geology found beneath the glacially derived sediments.

Regional differences in coastlines are the product of variations in coastal processes and coastal geology. (Most beaches along the Atlantic and Gulf Coasts are on barrier islands although they may also be on mainland coasts.) This discussion will primarily address the coastal plain system and its influence on the development of barrier islands and their beaches. Several important factors in this system include river drainage across the coastal plain, sea level history of the past 20,000 years, and the more recent (anthropogenic) development of the coastal environmental systems. The configuration of the coastline, the sediments that are delivered there, and the morphology of the barrier islands and beaches are directly or indirectly linked with the geologic regime in the region where they develop.

Major Geologic Provinces

Figure 1. Map of the coastal plain of the Gulf and Atlantic in the United States.

The coastal plain geologic system is the dominant province that is adjacent to the Atlantic and Gulf Coasts. A coastal plain is defined as a low relief region of largely undisturbed sedimentary strata that dip gently in the seaward direction. They commonly have well developed drainage systems, some that are influenced by adjacent major mountain ranges and others that are not. Along the Gulf Coast, this type of coastal plain is generally present throughout, except for the Mississippi Embayment where the Mississippi Delta occurs. In the delta there is a major landward extension of coastal plain strata that has accumulated on a subsiding, gently sloping trough.

On the east coast, the coastal plain is similar in nature to that found in the Gulf provinces. It begins in south Florida and terminates near the mouth of the Hudson River. North of this coastal plain, the continent has been glaciated numerous times over the course of nearly two million years. Glaciers have deposited large quantities of sediment and carved out a varied topography that has had a major impact on the coast, including barrier islands and beaches. Major glacial deposits helped create Long Island in New York and Cape Cod in Massachusetts. Glaciers also scoured bedrock to form the irregular coastlines typified by Maine.

A different aspect of the geology of the coastal regions is found in the Florida peninsula. This peninsula developed essentially as a carbonate platform that supports the existing land mass and is much like that found under the Bahamas. This very thick accumulation of limestone was completely separate from the continental United States as we know it until some 7-10 million years ago.

Gulf Coastal Plain – The coastal plain of Texas is only a few hundred kilometers wide and consists of strata ranging from the Cretaceous (System) to the present. From east Texas, Louisiana and Mississippi this plain extends northward to the southern tip of Illinois in the Mississippi Embayment. The coastal plain of this area is more than twice the width of that found in central Texas. The most northward boundary moves southward across Alabama as it passes below the southern limit of the Appalachians, near Birmingham, Alabama, and across Georgia, near Columbus and Macon. Along the northern Gulf of Mexico, the coastal plain is very complicated, due largely to the presence of the Mississippi River and the wide deltaic plain formed over the last 7,000 years.

Florida Platform – The Florida peninsula developed subsequent to the separation of the continents roughly 190 million years ago. The formation of extensive deposits of calcium carbonate took place in shallow water beginning more than 100 million years ago, causing two major centers of accumulation, the Florida Platform and the Bahama Platform. As thick accumulations of carbonate sediment occurred over time, there was slow subsidence or settling of the land mass. This environment persisted for about 100 million years and, over time, eventually created limestone strata with a total thickness of more than 3,000 meters.

During the time that these carbonate sediments were accumulating, each platform was essentially isolated from influences by continents. The Florida Platform was separated from the adjacent landmass by the Georgia Straits, not unlike the present straits that separate the Florida southeast coast from the Bahamas. Beginning with the Miocene Epoch, the Georgia Straits began to accumulate large deposits of sediments and eventually filled in, making Florida a part of the continent. Since connecting with the continent, the Florida peninsula has received considerable sediment from the southern Appalachians helping to produce the beaches commonly seen today.

Atlantic Coastal Plain – The Atlantic Coastal Plain is similar to that of the Gulf Coast except that it lies seaward of the Piedmont Province consisting of much older rocks. There is a significant change from this province to the coastal plain, with a distinct boundary occurring, commonly called the "fall line." Many important cities are located along this natural boundary because it generally represents the landward limit of shipping along many important east coast rivers. The cities of Augusta, Georgia on the Savannah River, Columbia, South Carolina on the Congaree River, and Washington, D. C. on the Potomac River are each representative of this geologic setting.

New England – The geologic foundation of the northeastern coast of the United States is diverse and complicated. Extensive ice sheets covered this area during the Pleistocene Epoch, resulting in both glacial deposition and erosion. The deposition consists predominantly of elongate terminal moraines typically containing large volumes of sand and gravel. Along Cape Cod and Long Island, reworking of these moraines has produced extensive beaches and barrier islands. In contrast, most of the northern New England coastline has been stripped of sediments and is dominated by irregular, rocky shorelines with only local pocket beaches, many of which are composed of gravel.

Sea Level History

Figure 4. Important shoreline positions along the Gulf and Atlantic Coasts since the Cretaceous Periods.

Much of the coastal plain is comprised of both marine strata (those deposited under water) and continental strata (those deposited on dry land). This indicates that sea level rose and fell many times over the past 100 million years. The modern coast was shaped primarily by sea level changes that have taken place over the past 2 million years as the result of the development and demise of huge ice sheets that covered much of the world's high latitudes. Although these vertical sea level changes had a maximum range of more than 100 meters, the shoreline migrated horizontally up to 200 kilometers in some places. In doing so, important sedimentary deposits accumulated along the shorelines. Along the Atlantic and Gulf Coasts, many were in the form of barrier islands.

The most recent stage of sea level change is also the most important. Up until approximately 20,000 years ago, when glaciers were at their maximum extent, sea level was some 125 meters below its current level, and nearly all of the present-day continental shelf was exposed. As the glaciers melted, all of the water released caused a rapid change in sea level. Between 20,000 and 7,000 years ago, sea level rose at a rate of about one centimeter per year. Although this may seem to be a small rate of increase, it is comparable to the planet's highest modern day rate of sea level rise occurring in the Mississippi Delta area of Louisiana. Beginning about 7,000 years ago, the rate of rise dropped to about two millimeters per year, similar to the present global rate. This permitted barrier islands to form by giving time for waves and currents to accumulate sand along the coastlines.

Also about this time, rising ocean waters began flooding river valleys that were cut through the continental shelf during the low-stand of sea level. These drowned river systems comprise the numerous estuaries along both the Gulf and Atlantic Coasts. The rivers along the coastal plain have carried tremendous volumes of sediment to the coast, much of it sand. The combination of slow sea level rise and wave action with rivers carrying sediment to the coast has produced a huge volume of sediment along the coastal zone. Much of this sand has been molded into the beaches and barrier islands that extend along most of the Atlantic and Gulf Coasts. Dams and river dredging have caused a substantial reduction in the volume of sediment delivered to the coast. The reduction in sediment supply to the coast, along with a rising sea level, has resulted in extensive erosion along the modern beaches.

Influence of Underlying Geology on Coastal Landforms

A variety of geologic conditions and configurations exists along the Gulf and Atlantic Coasts that can influence the growth and morphology of beaches and barrier islands. These geologic units are composed of both unconsolidated sediments and bedrock. They are typically Neogene except in the New England region. Cohesive mud, limestone, and sandstone may be present beneath the modern sediment or even be exposed on the floor of the coastal ocean. Reefs can also be a factor along the Florida coast and in Hawaii and the Caribbean.

Cohesive mud dominates the Mississippi River Delta coast. This mud originated from local rivers and has accumulated at the rivers' mouths and on the inner shelf. Although this area is dominated by mud, there is also some sand. A sand grain is heavier than a mud particle; therefore it is relatively stable and is present along the delta margin. In some places, this sand forms a ribbon of beach along the muddy shoreline; at others it produces barrier islands.

The inner shelf to the east and west of the Mississippi Delta is comprised of a combination of mud and sand, although mud is less prominent than in the Delta region. This material was also reworked by waves and currents as sea level rose to produce the numerous barrier islands that characterize the coasts of Texas and the regions to the east of the Mississippi Delta.

The coastal sand along the Florida peninsula forms a veneer above a limestone, or carbonate, platform. In some places this limestone is exposed or slightly below sea level, especially along the Gulf side. As sea level rose and reworked the surficial sand into barrier islands, this limestone acted as an anchor to fix the position of the barrier islands.

Much of the Atlantic Coast is underlain by the Neogene strata that range from unconsolidated sand and mud to exposed hardgrounds and bedrock. These types of sediments were sorted and reworked by currents during the Holocene Epoch in much the same manner as those found along the northern Gulf Coast. Some of the resistant hardgrounds and bedrock are swept clean and contain only scattered sediments.

The combination of subsurface glacial deposits and irregular bedrock exerts a major control on evolution of the New England coastline. The glacial deposits range from mud to boulders. Typically the mud is carried away by coastal currents. The sand is mobilized into barrier islands, beach ridges, and mainland beaches, while the coarse gravel such as cobbles and boulders tends to stay where the glacier deposited it. The irregular topography produced by the bedrock may also contribute to the formation of tombolos behind bedrock islands. This variety of conditions results in features that comprise the most complex coastline of both the Gulf and Atlantic.

Summary

Sandy shorelines tend to dominate the Gulf and Atlantic Coasts regardless of the underlying geologic materials. Sea level rise over the past several thousand years, along with the waves and currents in the nearshore area, shaped these underlying materials into elongate sand bodies along the coast. Differences in the size, shape, and location of these sand bodies are related to the underlying geology and local variations in coastal processes.