Alabama Landscapes
Geology, Geologic Processes and Geologic History
Geology and Geologic History of Alabama
Geologic Time
Geologists use their own time scale to describe history. The history of the Earth is divided into four main times, each representing a time when significantly different organisms lived on earth (Figure G1).
Figure G1. The Geologic Time Scale.
The Precambrian Eon (Pre-Archean,
Archean and Proterozoic of Figure G1) was first described as
the time of the most ancient oldest of rocks that do not contain fossils. However fossils, mainly
of microorganisms, have now been found in rocks of Precambrian age.
Radiometric age dating techniques have placed the Precambrian as ending
about 570 million years ago.
The Paleozoic Era (570-240 million years ago) was the "time of ancient
life," dominated by animals without backbones who lived mainly in the
ocean. During the later part of the Paleozoic, organisms began to live
on land and vegetation began to flourish. At the end of the Paleozoic a
mass extinction occurred that is estimated to have killed over 75% of
all living species.
The Mesozoic (245 - 65 million years) was
"the time of middle life," dominated by vertebrates and land-dwelling
animals such as the dinosaurs. Another extinction occurred at the end of
the Mesozoic, ushering in the Cenozoic (65 million years - to present).
The Cenozoic is "the time of modern life," in which the ancestors of
presently living species can easily be recognized.
Geology and Geologic History of Alabama
Alabama's geology can be divided into three areas (Figure G2): (i) highly deformed Precambrian and Paleozoic metamorphic rocks in the east central part of the state (ii) moderately to gently deformed Paleozoic sedimentary rocks in the central and northeastern part of the state; and (iii) relatively undeformed Mesozoic to Cenozoic sedimentary rocks and sediments in the southern and western parts of the state.
Figure G2. Geologic areas of Alabama.
Precambrian and Paleozoic History
During the Paleozoic, most of present-day
Alabama was at the edge of continental North America, occupying a
similar position to the continental shelf off the east coast of North
America today. Erosion from North America (situated to the north and west)
carried sediment to the ocean, where it was deposited as extensive thick
blankets stretching to the southeast. The ocean was shallow and teemed
with life. The skeletons of the marine life accumulated on the floor of
the ocean. In places reefs grew in the warmer waters. Thick beds of
limestone with lesser
sandstone formed.
As the Paleozoic progressed, deposition took place in progressively shallower waters, until the early Mississippian (about 350 million years ago) when sediments were deposited in barrier bars, beach deposits, deltas and as wave-dominated sands. Sandstones and shales became more abundant, although beds of limestone, teeming with invertebrate fossils, also formed. In the middle Mississippian, the approach of South America from the southwest shed thick piles of sands and muds to deposit on the older rocks in the northwestern part of the State. These sands and muds contained layers of buried decaying vegetation that eventually were transformed into coal seams.
During the Paleozoic a convergent plate boundary (occurs when two tectonic plates collide) was situated to the east. Collisions of pieces of continents and island arcs continued throughout the Paleozoic, forming a jumble of rocks that included rocks of Precambrian age. Most of those rocks were metamorphosed during the collisions. Looking through the veil of metamorphism, it is possible to determine that many were granites, formed by the cooling of magma at great depths beneath the surface, and basalts were common. However, most of the rocks were sandstones and shales, with some limestones. These rocks were finally welded onto the mainland during the Appalachian orogeny at the at the end of the Paleozoic. This orogeny also deformed the rocks of the mainland and caused the Appalachian Mountains to grow. The Paleozoic and older rocks show strong evidence of deformation from compression, with the stress NW-SE and being progressively weaker from SE to NW.
Mesozoic to Recent History
After the events leading to the formation
of the ancestral Appalachian Mountains during the Paleozoic, the history
of Alabama for the last 100 million years has been relatively quiet. Now
away from a plate boundary, Alabama underwent a period of extensive
weathering, erosion and deposition.
The rocks tell a story of deposition along a coastline that slowly
migrated southwards to its present position. The source of most of the
sediment was North America, now situated to the north of Alabama. Some
of the rocks formed in deltas and estuaries, others close off shore and
some in slightly deeper water. Fossils are abundant in many of these
rocks and in places extensive deposits of shark's teeth and oysters can
be seen. These rocks consist of layers of clay, sand, gravel and chalk. Because of their youth, these deposits are not well consolidated. Deposits
of lignite (brown coal) also occur in these rocks.