Larry J. Doyle

Sediments of the Mississippi, Alabama, and Florida (MAFLA) Continental Shelf

ABSTRACT The eastern Gulf (MAFLA) continental margin may be conveniently divided into two parts of opposing history and character. West of Cape San Blas lies the eastern limb of the Gulf Coast Geosyncline whose surface expression is a clastic sand body, called the MAFLA Sand Sheet, grading westward into the muds of the Mississippi pro-delta. These sediments have a clay mineral suite dominated by smectite. East of Cape San Bias lies the West Florida Margin, a sequence of carbonate and evaporitic rocks which has been cut off from a major clastic source since Jurassic time. The surface expression of this sequence is the West Florida Sand Sheet, predominantly a patchy veneer of shell hash and foraminiferal, algal, and even oolitic sands which is subjected to periodic reworking by frontal system st rms and hurricanes. Kaolinite dominates its clay mineralogy. Seaward of the carbonate sands lies the West Florida Lime Mud facies, slope sediments composed of planktonic foraminifera and coccoliths. Inshore of the carbonate sands and separated from them by a zone of mixed composition lies a mature quartz fine sand, which also makes up the beaches of Southwest Florida. West Florida shelf quartz sands appear to have been deposited at lower sea level stands and to have been transported back and forth with no net drift in a longshore current system which changes seasonally from north to south. Clay mineralogy in portions of the MAFLA region shows distinct changes in composition over a period of a year in the benthos and over periods as short as a few hours in the water column. These changes reflect contribution from two distinct provenances. Benthie variation probably results from occasional intrusion of smectite laden Mississippi River or Loop Current water into the eastern zone. Water column variation may be the result of seiching of the Gulf or the pulsing movement of kaolinite laden eastern shelf water to the west.

Recent Sedimentary Development of Tampa Bay, Florida: A Microtidal Estuary Incised Into Tertiary Platform Carbonates

Tampa Bay, a large, microtidal, clastic-filled estuary incised into Tertiary carbonate strata, is the largest estuary on Florida’s west coast. A total of 250 surface sediment samples and 17 cores were collected in Tampa Bay in order to determine the patterns and controlling factors governing the recent infilling and modern sediment distribution, and to examine the results in terms of current models of estuarine sedimentation and development. Surficial sediments in Tampa Bay consist of three facies types, each occurring in a distinct zone: modern terrigenous clastic muds occurring in the upper bay and around the bay periphery; relict, reworked-fluvial, quartz-rich sands occupying the open portion of the middle bay; and modern carbonate-rich, marine-derived sands and gravels occupying the lower bay. Factors controlling sediment distribution include: sediment source and supply rate; bathymetry, which is a function of the antecedent topography; and the winnowing effect of wind-generated waves that prohibits modern accumulation in the shallow middle bay. These factors also play a major role in the recent infilling history of Tampa Bay, which has progressed in four stages during the Holocene sea-level rise. Recently developed models of estuarine sedimentation are based primarily on mesotidal to macrotidal estuaries in terrigenous clastic settings in which sedimentation patterns and infilling history are a result of the relative contribution of marine and fluvial processes. Tampa Bay differs in that it was originally incised into carbonate strata, and neither fluvial or marine processes are interpreted to be major contributors to modern sediment distribution. Tampa Bay, therefore, provides an example of an unusual estuary type, which should be considered in future modeling efforts. *** DIRECT SUPPORT *** A01BY083 00004

Patterns and controls of surface sediment distribution: West-central Florida inner shelf

Abstract The west-central Florida inner shelf represents a transition between the quartz-dominated barrier-island system and the carbonate-dominated mid-outer shelf. Surface sediments exhibit a complex distribution pattern that can be attributed to multiple sediment sources and the ineffectiveness of physical processes for large-scale sediment redistribution. The west Florida shelf is the submerged extension of the Florida carbonate platform, consisting of a limestone karst surface veneered with a thin unconsolidated sediment cover. A total of 498 surface sediment samples were collected on the inner shelf and analyzed for texture and composition. Results show that sediment consists of a combination of fine quartz sand and coarse, biogenic carbonate sand and gravel, with variable but subordinate amounts of black, phosphorite-rich sand. The carbonate component consists primarily of molluskan fragments. The distribution is patchy and discontinuous with no discernible pattern, and the transition between sediment types is generally abrupt. Quartz-rich sediment dominates the inner 15 km north of the entrance into Tampa Bay, but south of the Bay is common only along the inner 3 km. Elsewhere, carbonate-rich sediment is the predominate sediment type, except where there is little sediment cover, in which cases black, phosphorite-rich sand dominates. Sediment sources are likely within, or around the periphery of the basin. Fine quartz sand is likely reworked from coastal units deposited during Pleistocene sea-level high stands. Carbonate sand and gravel is produced by marine organisms within the depositional basin. The black, phosphorite-rich sand likely originates from the bioerosion and reworking of the underlying strata that irregularly crop out within the study area. The distribution pattern contains elements of both storm- and tide-dominated siliciclastic shelves, but it is dictated primarily by the sediment source, similar to some carbonate systems. Other systems with similar sediment attributes include cool-water carbonate, sediment-starved, and mixed carbonate/siliciclastic systems. This study suggests a possible genetic link among the three systems.

Facies architecture of the mixed carbonate/siliciclastic inner continental shelf of west-central Florida: implications for Holocene barrier development

Abstract Sediment vibracores and surface samples were collected from the mixed carbonate/siliciclastic inner shelf of west–central Florida in an effort to determine the three-dimensional facies architecture and Holocene geologic development of the coastal barrier-island and adjacent shallow marine environments. The unconsolidated sediment veneer is thin (generally

The Hydraulic Equivalence of Mica

ABSTRACT Settling experiments performed on silt- to fine-sand-sized mica flakes with a holographic micro-velocimeter revealed that mica is the hydraulic equivalent of quartz spheres having diameters a factor of 4 to 12 times smaller. Mica in the very fine to fine-sand sizes has been traditionally used by sedimentologists to delineate areas of deposition or nondeposition and potential winnowing of fines, and is here found to be the hydraulic equivalent of silt-sized particles but not of clay. Experiments also showed that mica flakes tend to settle at orientations which are neither perpendicular nor parallel to the gravitational vector and tend to generally maintain their orientation throughout. Equations for the settling of a disc in Lerman and others (1974) and that developed by Komar and Reimers (1978) are shown to be mathematically similar for the coarse-silt to fine-sand ranges of discs and are adequate predictors of settling rates of mica flakes. A comparison of the hydraulic equivalency of quartz spheres to coarse-silt- through fine-sand-sized mica flakes is presented.

Mica: Its use in determining shelf-depositional regimes

Abstract The fine sand-size mica content was determined in over 450 beach, dune, river, estuary, continental shelf and continental slope sediments from the southern United States Atlantic continental margin. Because of hydraulic equivalence considerations, the mica content can be used to delineate depositional and non-depositional regimes of fine-grained sediments. At the present time deposition of fines in the study area is restricted to a narrow nearshore band and the upper continental slope. The central and outer shelf are areas of winnowing.

Shallow Structure, Stratigraphy, and Carbonate Sedimentary Processes of West Florida Upper Continental Slope

An extensive minisparker, 3.5-kHz, piston-coring survey of the continental slope above the West Florida Escarpment has revealed a Pleistocene sequence of up to 160-msec two-way traveltime, overlying a second, probably Miocene, strong reflector. South of 27°20^primeN the contact between the two is clearly an erosional unconformity and includes a band of karstlike features. The Pleistocene drape thins to a minimum, locally even exposing the second layer, at about 500 m (1,650 ft) water depth and thickens dramatically downslope. We attribute this thinning to the north-south flowing Loop current blocking deposition and scouring the bottom. The ancestral Loop may have been responsible for the erosional unconformity, or it could have been due to subaerial erosion. If the l tter is the case, present depth of the erosional surface suggests as much as 400 m (1,320 ft) of subsidence after its formation. From its southern limit to 26°40^primeN, two parallel reefs mark the upper slope. Sediments on the upper slope are a foraminifera-coccolith ooze, the compositional equivalent of a chalk deposit. Radiocarbon dating shows ooze below the erosional minimum accumulating at over 30 cm/1,000 years (11.8 in./1,000 years) for at least 25,000 years--a surprisingly high rate--over an order of magnitude greater than that for a compositionally equivalent deep-sea ooze. High sedimentation rates are also reflected in a variety of mass wasting features from creep to massive slides to gravity-induced folds tens of kilometers long. The upper west Florida slope, therefore, is only a temporary resting place for sediments that are moved downslope by mass wasting processes to the West Florida Escarpme t.

Neogene to recent stratigraphy and depositional regimes of the northwest Florida inner continental shelf

Abstract The late Neogene to Recent depositional history of the inner shelf off northwest Florida was investigated using high-resolution seismic reflection data. Two principal sedimentary provinces, the Apalachicola Embayment and the Alabama-Florida Shelf, are distinguished by different structural trends and sequence stratigraphy. A transition from carbonate to terrigenous clastic deposition occurs vertically and laterally from east to west. The dominant controls on deposition have been sea level history and location of fluvial systems advancing southward and infilling the Apalachicola Embayment. In the Apalachicola Embayment, the upward carbonate-to-terrigenous transition correlates with a change from relatively flat lying reflections, to prograding clinoforms, and then chaotic and reflection-free sequences. Carbonate deposition in the middle and late Miocene is inferred to have occurred during highstands of sea level with minor input of terrigenous material. In the late Miocene, a major erosional unconformity and associated river valley entrenchment cut deeply into the flat-lying carbonate section. Subsequent deposition is distinguished by broad prograding clinoforms and an increase in terrigenous material in an open shelf environment. In the late Pliocene, sea-level fluctuations generated a stratigraphic record estimated by offlapping, seaward-thickening sequences. The interaction between sea level and fluvial supply to the shelf became very important as shifting river inputs resulted in locally thick depocenters bounded by erosional unconformities. Four primary areas of fluvial-deltaic input in the Plio-Pleistocene are identified based on the distribution of channels and prograding clinoforms interpreted to be delta front deposition. The present inner shelf is a sediment-starved clastic depositional regime.

What does percent organic carbon in sediments measure

Weight percent total organic carbon (TOC) is one of the most commonly used descriptors for both Recent and ancient sediments. It is commonly considered to be approximately proportional to organic carbon accumulation rates and, therefore, a gauge of those rates. Weight percent TOC is also used to judge hydrocarbon source rock potential and primary productivity of the overlying water column. On the basis of data from both the Recent and the entire Phanerozoic, we conclude that weight percent TOC is an unreliable measure of the rate of deposition of organic carbon and that arguments and conclusions based on that relationship are suspect.

Stable isotope compositions of sedimentary organic carbon in Tampa Bay, Florida, U.S.A.: implications for evaluating oil contamination

δ13Cpdb values for sedimentary organic carbon in Tampa Bay, Florida and adjacent riverine systems range from −28% up the rivers, to −20% in the middle of the bay, to −10% in several contiguous small bays along the Gulf of mexico coastline. These values reflect the compositions of the plant precursors, that is, C3-land plants, phytoplankton and sea grasses, respectively, which predominate in each of these areas. Organic carbon percentages range from 5.0 to less than 0.1 with a moderately good correlation relative to the<63gm sediment grain size fraction ( = 0.76for67samples). In addition to providing another interesting scenario of δ13C signatures that are useful in understanding the contemporary deposition of organic carbon on the one hand and ancient depositional environments on the other, these data are also shown to be useful in assessing the contribution of oil residues to the sediments should an oil spill occur in Tampa Bay at some time in the future.