ABSTRACT

Sedimentary rocks underlie all parts of Mississippi. They range in age from Recent to early Paleozoic, and all geologic period except Permian and Triassic are known to be represented. The combined thickness of all the formations is approximately 50,000 feet, but all of the formations are not present under one area. Many of the formation extend to the surface, but many others are completely covered by younger sediments and they are known to be present because wells drilled in seach for petroleum have penetrated them.

The state has had a complex and interesting geological history. Its location on the eastern side of the Mississippi embayment and at the junction of the plunging Appalachian and Ouachita folded mountains makes it a key area in the understanding of the geological history of the southeastern United States. Also in Mississippi are the transition zones between thick clastic Tertiary sediments to the west and thin Tertiary carbonates to the east. There is thinning seaward of several Upper Cretaceous and Tertiary formations in the southern part of the state, and this is the only area known at present in the Gulf Coast where such thinning occurs on a subregional scale. The southern margins of some of the Paleozoic epicontinental seas were in central and northern Mississippi.

There was no Gulf of Mexico during the Paleozoic Era, but instead the area now occupied by the northern Gulf and adjacent coastal plain was land which provided sediments to northward-flowing streams. The land was probably low-lying and had little topographic relief during the early and middle Paleozoic, permitting marine carbonates to form in the shallow seas almost to their southern margins. In late Paleozoic time, however, the old land areas of Appalachia-Llanoria were uplifted while the region to the west and north, including northern Mississippi, was tilted down. It was during this time that great thicknesses of sand and shale accumulated in the Appalachian folded belt and plateau region, the Ouachita Mountain area, and in the Warrior basin of northeastern Mississippi. Coal beds also formed during this period in some areas of the coastal regions which were then in existence.

After the coal-forming period (lower Pennsylvanian) the region which had been receiving sediments was uplifted and subjected to erosion, and there is no record in Mississippi, so far as now known, of any sedimentation in this area for a period of about 140 million years. This gap in the record embraces the middle and upper Pennsylvanian, the Permian and Triassic, and the lower and middle Jurassic. There must have been erosion of the lands during this long interval of time but it is not known where the sediments came to rest. There are some down-faulted blocks with continental Triassic sediments under parts of the eastern Gulf coastal plain, and it is possible that similar troughs are present under the central and western coastal plain, but if so they have not been penetrated by wells.

The Gulf of Mexico basin formed during upper Jurassic time and Llanorialand sank below the sea, never to be uplifted again. This is known because marine sediments of upper Jurassic and all younger periods are present on all sides of the basin. But already a sea was in southern Mexico and probably also in the southern Gulf area, in lower and middle Jurassic.

The northern Gulf area was down-warped during upper Jurassic to permit the sea to spread northward far beyond the present northern limit of the Gulf. At first there were restricted seas where thick sections of salt and anhydrite formed. One of these evaporite basins is in southern Mississippi where many salt domes occur. Open sea conditions followed, when the Smackover limestone was deposited. In this shallow sea there were many environments which permitted several kinds of limestone (reefoid, pelletoid, etc.) to form. Conditions changed so that evaporites were deposited over the limestone in some areas. Then came the first large mass of land-derived sediment, from the north, to this newly formed mediterranean sea. These shales and sandstones, deposited mostly in coastal and nearshore environments, are the Cotton Valley formation. All of these upper Jurassic formations lie deeply buried under southern Mississippi.

The Jurassic period ended with further withdrawal of the sea due to a faster rate of sedimentation. The lower Cretaceous Hosston formation, which attains a maximum thickness of about 2000 feet in Mississippi, was deposited mainly in alluvial environments as far down dip as wells have penetrated the formation. Gravels are abundant in the lower half of the Hosston, and they were brought to the coastal area by steep-gradient streams. These coarse grained nonmarine sediments must grade seaward, in the Mississippi coastal area, into finer grained shallow marine sediments. The sea

End_Page 43-------------------------

moved into southern Mississippi at the end of Hosston time, and the shoreline fluctuated considerably in that region while the Glen Rose shale and sand were being deposited in coastal and shallow near-shore marine environments, and impure limestone was forming in very shallow seas with poor circulations. It was during this period that evaporites (anhydrite) formed when the sea was restricted by a barrier, possibly a large reef, south of the present shoreline. The sea retreated southward during Paluxy time when sand and shale were deposited mainly in alluvial and deltaic plains. Then followed a rather widespread transgression of the sea when the Fredericksburg limestone was formed in shallow open sea conditions in southern Mississippi while sand and shale were deposited in coastal areas farther north. The Lower Cretaceous period ended with a withdrawal of the sea almost to the present coast and with deposition of sand and varicolored clay in continental environments.

More than 9,000 feet of Lower Cretaceous sediments were deposited in the Mississippi salt basin where they now lie at depths of about 8,000 to 16,000 feet. This section thins to zero in northern Mississippi, where it is overlapped by Upper Cretaceous strata. Southward the section thins to about 6,000 feet at the present shoreline. Most of the Lower Cretaceous was deposited slightly above sea level and none was laid down in water with depth greater than about 100 feet. Therefore it is obvious that southern Mississippi was sinking during deposition of the Lower Cretaceous, and it continued to subside in later periods.

In early Upper Cretaceous (Tuscaloosa) time the sea began to move inland, though there were large streams building deltas in the advancing sea in southwestern Mississippi, and thick barrier island were forming farther east. Then followed a period when the streams had reduced gradients and were able to transport only clay to a shallow, restricted arm of the sea which covered the southern one-third of the state. This middle Tuscaloosa clay underlies most of southern Mississippi. The surrounding lands were then uplifted, the streams were rejuvenated, the shallow sea was soon filled, and a coastal plain was formed where sand and shale were deposited in alluvial and transitional environments. In northern Mississippi large quantities of chert gravel from the Paleozoic limestone of the Appalachian region were deposited in alluvial valleys during this closing phase of Tuscaloosa time. Then began a very widespread transgression of the sea in the central Gulf coast. At first much clay was still brought down the ancestral Mississippi River and deposited in the advancing sea. This shale is called Eagle Ford. Less and less terrigenous clastic material was brought to the Gulf Coast; the sea moved far up the Mississippi embayment and covered almost all of Mississippi; and in this shallow epicontinental sea about 1000-1500 feet of chalk, marl and some shallow marine near-shore sand accumulated. These deposits are called Selma-Ripley. Volcanoes formed in the shallow sea in west-central Mississippi during this period.

The Cenozoic Era began with a retreat of the sea throughout the Gulf Coast, including the northern part of Mississippi. However, at the beginning of Tertiary deposition a shallow sea was still over most of the state, and glauconitic marl (Clayton formation) was deposited. The sea was soon restricted from the open Gulf; streams were rejuvenated sufficiently to transport great quantities of clay to the shoreline, and it was spread widely in that large stagnant body of water. The clay, which reaches a macimum thickness in Mississippi of about 900 feet, is called Porters Creek. At the end of clay deposition open sea conditions obtained briefly in the southern half of Mississippi when the Matthews Landing marl was laid down in shallow water. Then followed one of the most important regressions of Tertiary time, which began in the middle Paleocene when coarser sediments reached the Gulf Coast from the recently uplifted Rocky Mountains and plains area. The sands, shales and lignites which accumulated during this period mostly in coastal and alluvial environments with a thickness of about 1500 feet in southwestern Mississippi, belong to the Naheola formation. Near the end of this period the shoreline advanced into southern Mississippi and the Coal Bluff marl was deposited in very shallow water. The Paleocene epoch ended with a retreat southward of the shoreline.

An important advance of the sea took place in early lower Eocene time, when the Nanafalia shale was deposited in a shallow sea which covered the southern one-third of Mississippi. These marine conditions lasted only a short time before streams loaded with sand, silt and shale pushed the shoreline southward again. During most of this regressive period coastal environments prevailed over all of southern Mississippi, though the largest deltas were built in the southwestern part of the state. The sea advanced only a few times and for very brief periods during deposition of this section which is called the Tuscahoma formation. A widespread advance of the sea followed, and the thin shallow marine Bashi marl was deposited. The lower Eocene epoch ended with a retreat of the shoreline when the Hatchetigbee sand, shale, and lignite were deposited in broad alluvial valleys and in coastal lakes and marshes.

During middle Eocene time the sea advanced twice over the southern half of Mississippi and it also retreated far to the south two times. Thus there are two cycles of deposition represented. The Tallahatta siliceous claystone and Winona glauconitic sand and clay were deposited in the shallow sea which moved in during early middle Eocene time. The Sparta-Kosciusko sand was deposited mainly in alluvial environments during the regressive period which followed. Then there was a major northward advance of the shoreline and the Cook Mountain-Wautubbee marl was deposited in the shallow sea. The middle Eocene epoch ended with a major regression when the Cockfield sand, shale, and lignite were deposited in coastal and lower alluvial plain environments. The middle Eocene formations have a maximum total thickness of about 2400 feet in west-central Mississippi where they are composed of sand, silt, shale, marl, and some lignite. In coastal Mississippi the section is only about 200 feet thick and is made up of limestone which formed in a shallow, open sea. Thus the area of the depocenter subsided twelve times as fast as the present coastal area during the middle Eocene epoch.

End_Page 44-------------------------

The upper Eocene was a transgressive period in Mississippi when shallow marine marl (Moodys Branch) and moderately deep marine clay (Yazoo) were deposited in central and western Mississippi, and limestone (Ocala) formed in the shallow clear sea which covered coastal Mississippi. At the beginning of this period the shoreline moved rapidly over the coastal plain, and there was almost no retreat of the sea throughout the epoch.

Oligocene time in Mississippi was also a period of marine deposition except for an initial slight regression when the Forest Hill sand was deposited in coastal environments. The Vicksburg limestone and marl were deposited in a shallow sea which covered southern Mississippi. Limestone was deposited farther north and west during this period than in any other part of the Tertiary. The Oligocene ended with a general uplift of the eastern Gulf coastal plain which pushed out the sea and permitted the exposed rocks to be weathered and eroded before they were covered with younger sediments.

A thin section of early Miocene (?) shallow marine sediments (Bucatunna, Chickasawhay, Paynes Hammock) was deposited when subsidence brought the sea over southern Mississippi again, following the development of the unconformity. But during this period of uplift and marine transgression in the eastern coastal plain, the western region was warped down to recieve more than 5,000 feet of alternating shallow marine and nonmarine sediments (Frio formation). Subsidence continued throughout the Miocene epoch and about 4,000 feet of mostly nonmarine sand, silt, and shale were deposited in coastal Mississippi. More than 20,000 feet of similar sediments accumulated during this period in the coastal and off-shore region of Louisiana. In the latter area there were many brief transgressions of the sea whereas in southern Mississippi the sea advanced inland from the present coast only a few times. Thus the Miocene time was mainly a regressive period due to uplift of the area drained by streams flowing to the Gulf, although the coastal and shallow sea areas continued to subside rapidly.

It appears probable that the Pliocene epoch is not represented by sediments in Mississippi, for the strata which overlie the Miocene formations are believed to be of Pleistocene age. It was during the Pliocene that land areas in the northern hemisphere were greatly uplifted, and this was no doubt a major cause of the Ice Age which followed. There was erosion of most of North America, including Mississippi, and the sediments were deposited seaward of the present shoreline.

Sediments of Pleistocene age are prerent in all parts of Mississippi. They include the loess and gravel of the Bluff Hills; the river terrace deposits along all the major streams; the gravels, orange-colored sand and varicolored silt and clay in many of the stream divide areas, especially in the southern part of the state; and the coastal terrace deposits. The gravels include the coarsest sediments ever brought to the Gulf coast from distant sources; therefore the large flooded streams had steep gradients. The shoreline was far south of the present coast during the glacial stages of this period because sea level was lowered as ice was drawn from the ocean to build the ice sheets. Several thousand feet of Pleistocene sediments were deposited in what is now the continental shelf.

The Pleistocene epoch came to an end when sea level reached its present stand, 3,000 to 5,000 years ago, and sediments deposited since then are referred to the Recent or Holocene. In Mississippi a thin section of such deposits is under the flood plains of the major streams, especially the Yazoo-Mississippi alluvial plain; they are under Mississippi Sound and the coastal lakes and lagoons; and a thin venner of Recent sediments is under most parts of the Gulf of Mexico. The off-shore island have also formed since the end of Pleistocene. The coastal area is very likely still subsiding, as it has been during the past 100 million years, but there is sufficient sediment brought to this area by streams to keep the sea from moving inland. In fact, the shoreline is building seaward, and at the present rate of sedimentation the Mississippi Sound will be filled in about 5,000 years.

End_of_Record - Last_Page 45--------