Jack L. Kindinger

Multiple outer-reef tracts along the south Florida bank margin: Outlier reefs, a new windward-margin model

High-resolution seismic-reflection profiles off the lower Florida Keys reveal a multiple outlier-reef tract system {approximately}0.5 to 1.5 km seaward of the bank margin. The system is characterized by a massive, outer main reef tract of high (28 m) unburied relief that parallels the margin and at least two narrower, discontinuous reef tracts of lower relief between the main tract and the shallow bank-margin reefs. The outer tract is {approximately}0.5 to 1 km wide and extends a distance of {approximately}57 km. A single pass divides the outer tract into two main reefs. The outlier reefs developed an antecedent, low-gradient to horizontal offbank surfaces, interpreted to be Pleistocene beaches that formed terracelike features. Radiocarbon dates of a coral core from the outer tract confirm a pre-Holocene age. These multiple outlier reefs represent a new windward-margin model that presents a significant, unique mechanism for progradation of carbonate platforms during periods of sea-level fluctuation. Infilling of the back-reef terrace basins would create new terraced promontories and would extend or step the platform seaward for hundreds of meters. Subsequent outlier-reef development would produce laterally accumulating sequences.

Quaternary Geomorphology and Modern Coastal Development in Response to an Inherent Geologic Framework: An Example from Charleston, South Carolina

Abstract Coastal landscapes evolve over wide-ranging spatial and temporal scales in response to physical and biological processes that interact with a wide range of variables. To develop better predictive models for these dynamic areas, we must understand the influence of these variables on coastal morphologies and ultimately how they influence coastal processes. This study defines the influence of geologic framework variability on a classic mixed-energy coastline, and establishes four categorical scales of spatial and temporal influence on the coastal system. The near-surface, geologic framework was delineated using high-resolution seismic profiles, shallow vibracores, detailed geomorphic maps, historical shorelines, aerial photographs, and existing studies, and compared to the long- and short-term development of two coastal compartments near Charleston, South Carolina. Although it is clear that the imprint of a mixed-energy tidal and wave signal (basin-scale) dictates formation of drumstick barriers and that immediate responses to wave climate are dramatic, island size, position, and longer-term dynamics are influenced by a series of inherent, complex near-surface stratigraphic geometries. Major near-surface Tertiary geometries influence inlet placement and drainage development (island-scale) through multiple interglacial cycles and overall channel morphology (local-scale). During the modern marine transgression, the halo of ebb-tidal deltas greatly influence inlet region dynamics, while truncated beach ridges and exposed, differentially erodable Cenozoic deposits in the active system influence historical shoreline dynamics and active shoreface morphologies (block-scale). This study concludes that the mixed-energy imprint of wave and tide theories dominates general coastal morphology, but that underlying stratigraphic influences on the coast provide site-specific, long-standing imprints on coastal evolution.

Seismic stratigraphy of the Mississippi-Alabama shelf and upper continental slope

Abstract The Mississippi-Alabama shelf and upper continental slope contain relatively thin Upper Pleistocene and Holocene deposits. Five stages of shelf evolution can be identified from the early Wisconsinan to present. The stages were controlled by glacioeustatic or relative sea-level changes and are defined by the stratigraphic position of depositional and erosional episodes. The stratigraphy was identified on seismic profiles by means of geomorphic pattern, high-angle clinoform progradational deposits, buried stream entrenchments, planar conformities, and erosional unconformities. The oldest stage (stage 1) of evolution occurred during the early Wisconsinan lowstand; the subaerially exposed shelf was eroded to a smooth seaward-sloping surface. This paleosurface is overlain by a thin ( These Upper Quaternary shelf and slope deposits provide models for analogous deposits in the geologic record. Primarily, they are examples of cyclic sedimentation caused by changes in sea level and may be useful in describing short-term, sandy depositional episodes in prograding shelf and slope sequences.

Depositional history of the Lagniappe Delta, northern Gulf of Mexico

The northern Gulf of Mexico continental shelf is characterized by superimposing deltas. One such delta, informally named Lagniappe, extends east of the Mississippi Delta from mid-shelf to the continental slope. This late Wisconsinan delta is adjacent to, but not associated with the Mississippi Delta complex: the fluvial source was probably the ancient Pearl and/or Mobile Rivers. The fluvially dominated Lagniappe Delta is characterized by complex sigmoid-oblique seismic-reflection patterns, indicating delta switching of high-energy sand-prone facies to low-energy facies. The areal distribution and sediment thickness of the delta were partially controlled by two diapirs.

Sediment Characterization and Dynamics in Lake Pontchartrain, Louisiana

Abstract Lake Pontchartrain in southeastern Louisiana is the largest of several shallow estuaries that together cover over 15,000 km2. Wetlands, forests, and large urban areas surround the lake. Primary transport mechanisms of sediments to Lake Pontchartrain include urban runoff, major diversions of the Mississippi River, discharge from streams along the north and west shores, and tidal circulation. Sediments deposited in Lake Pontchartrain are subjected to resuspension and mixing by natural and human activities. Bioturbation and water turbulence throughout the lake are the major mixing agents, and mechanical shell dredging has reworked much of the lake bottom over the last century. Sediment characterization through direct sampling and geophysical surveys indicates that these processes continually rework the top meter of sediment. The lake receives discharge from roadways and industrial and agricultural sources. Contaminants from these sources accumulate in the lake sediments and are an important contributor to the degradation of the estuary. Decline in populations of various benthic organisms, such as shrimp and clams, has been documented in the lake. To characterize the health of this important estuary, the U.S. Geological Survey (USGS) conducted a comprehensive evaluation of the geology, geomorphology, coastal processes, and environmental condition of the Pontchartrain Basin from 1994 to 1997. This report presents an assessment of sediment distribution and quality using a multidisciplinary approach to characterize the influence of various physical and chemical parameters: nearsurface stratigraphy, major trace metal concentrations (Cu, Pb, Zn, and Ni), and short-lived radionuclides (210Pb, 7Be, and 137Cs). The results are compared with water-circulation patterns to determine high-resolution sedimentation patterns in the lake. The data show a significant increase in trace metals in the top 1 m of lake sediments. Above this horizon, pollen analysis indicates a correlation with land clearing in the area, a proxy for increasing human development of the surrounding landscape and an increase in surface run-off. The data also show that the top meter of sediment undergoes frequent resuspension during high-energy circulation events and via circulation gyres in the lake. This regular turnover does not allow stratification of recently deposited sediments, restricting the sequestration of contaminated material that enters the lake.

Abstract: Holocene Geologic Framework of Lake Pontchartrain Basin and Lakes of Southeastern Louisiana

The Lake Pontchartrain Basin is a 12,170-km2 (4,700-mi2) watershed in southeastern Louisiana, stretching from the State of Mississippi on the north and east to the Mississippi River on the west and south, and to Breton Sound at the Gulf of Mexico. The Pontchartrain Basin is about 200 km along strike and 75 km along dip with modern lakes (Maurepas, Pontchartrain, and Borgne) covering the southern portion of the basin. Lake Pontchartrain and its adjacent lakes form one of the largest estuaries in the United States. Nearly 1.5 million people (one-third of the entire population of Louisiana) live in the 14 parishes within the Lake Pontchartrain Basin. The basin is bounded by incised Pleistocene terraces to north, the Mississippi River delta plain to the south/southwest, and the Pine Island barrier shoreline to the south/southeast. Over the last 150 years, urban growth of New Orleans and the north shore communities and associated exploitation of natural resources have severely altered the environmental quality of the basin. In 1994, the USGS began a multidisciplinary evaluation of the geology, geomorphology, coastal processes, and environmental quality of the Pontchartrain Basin for use by Federal, state and local officials in coastal management and restoration planning. Existing geological information has been integrated with newly acquired high-resolution seismic profiles (>700 line km), 76 vibracores, and more than 1000 samples for geochemical data to develop a geologic history and record of sediment distribution of the basin (Fig. 1). The Pontchartrain Basin has a complex depositional history controlled by sea-level change. Lake Pontchartrain originated as a result of sedimentary processes. During the late Wisconsin lowstand, the region was entrenched by rivers. A buried incised channel of the ancestral Mississippi River, identified from seismic profiles (see Fig. 2 A-A’), underlies the southern margin of Lake Pontchartrain. The incision is three to four km across, was cut to a depth of 40 m, and can be traced from west to east into Lake Pontchartrain (Fig. 3). Sea-level rise during deglaciation truncated the filled paleochannel and the surrounding region of the Pontchartrain Basin. The ravinement surface is a sharp contact with siderite nodules and has been identified from vibracores as the Pleistocene-Holocene contact. The late Pleistocene unit is typically described as a stiff, olive-gray to light grayish-yellow clay that is highly bioturbated. The burrows are fill with oxidized organics or sand and silt. The structure contour map (Fig. 3) of the Pleistocene surface shows the contact is shallow (near the sediment surface, 2 m below sea level) in northeast Lake Pontchartrain and deeper (20 m below sea level) to the southwest. Cross section B-B’ (Fig. 3) illustrates how the Pleistocene contact crops out along the northeast shore and dips to the southwest. Sea-level rise flooded the “Pontchartrain Embayment” and

Late Pleistocene—Holocene geology of the central Virgin Island Platform

Abstract Geophysical and sedimentological data collected on the central Virgin Islands insular shelf provide a unique opportunity to investigate carbonate shelf processes in an active tectonic environment. Although complicated by fluctuating sea level during the Quaternary, the sedimentological regimes have been controlled by the tectonic fragmentation of the region. South of St. John, a northeast-southwest intrusive ridge controlled the development of the shallow geology. Profiles across this region, seaward of the ridge, indicate the existence of a buried wave-cut platform. Resting on this eroded surface, at the shelf break, is a reef having a topographic relief of 55 m. This reef, which formed a protective barrier preserving a sequence of lagoonal deposits, has been radiometrically determined to have died ∼7000 yrs B.P., coinciding with the death of similar reefs in Florida and on the Island of St. Croix. South of St. Thomas, the strata are horizontal and are bisected by east-west midshelf faults. Truncation of the shelf edge strata, northern drainage, and unconformities on the northern shelf indicate that this block was tilted to the north during the late Pleistocene. This limited reef development to a narrow zone on the southern shelf edge. Geochemical, biological and sedimentological data indicate that the early to mid-Holocene was a time of vigorous shelf sedimentation. This accumulation peaked and abruptly stopped ∼1200 yrs B.P., due to continued subsidence. Since this time, most sediment accumulation has been limited to the immediate area around the islands.

Sedimentology of Southwestern Roads Region, U.S. Virgin Islands-Origin and Rate of Sediment Accumulation

Sand deposits on southern insular shelf of St. Thomas, U.S. Virgin Islands, were investigated to determine their origin, environmental processes and accumulation rates. Sea-floor samples show that the sand has been derived (in situ) mainly from calcareous algae and molluscs. Zonation of the dominant sand producers is related to the present environmental setting; water depth has the greatest influence. Carbon-14 data (bulk sample) of cores indicate accumulation rates of slightly less than 1 mm/year for the last 5,000 years. Faunal studies show that the climate has remained constant over the past 5,000 years. The only changes in environmental conditions appear to have been an increase in water depth, changes in the patterns of water movement, and an increase in water temperature.--Modified journal abstract.

Abstract: Geologic Framework and Hard Mineral Resources of Petit Bois Pass and Adjacent Inner Shelf, Mississippi-Alabama

ABSTRACT Approximately 200 line-km of high-resolution seismic profile data, 24 vibracores, and several C14 samples form the database to delineate the geologic framework and potential hard mineral resources of the Petit Bois Pass area. Petit Bois Pass is a natural tidal inlet system located between Petit Bois and Dauphin Islands at the Mississippi-Alabama state line. The inlet is 8.2 km wide with the main inlet channel located to the east adjacent to Dauphin Island. The inlet throat depth is 6.7 m below mean low water (MLW). The morphology of the main tidal channel is mixed energy, which is reflected by the absence of a flood-tidal delta and a moderately developed ebb-tidal delta. The largest percentage of sediment occurs directly between Dauphin and Petit Bois Islands. More recently, however, a secondary tidal channel and associated ebb-tidal delta are developing to the west of the main tidal inlet channel. On the adjacent inner continental shelf, several shoreface sand ridges occur and are oriented at about 40° in relation to the barrier shoreline; axes of the sand ridges have azimuths of about 120°. The origin of these ridges may be related to the evolution of Petit Bois Pass and other inlets in the area. Tidal inlets are naturally occurring, concentrated sinks of sediment captured from longshore transport. In transgressive barrier island settings, available mechanisms for concentrating sediment are restricted almost exclusively to tidal inlet systems. However, the preservation potential of the tidal inlet system during transgression is limited except for the lower part of the tidal inlet channel and the ebb-tidal delta. The remainder of the inlet system eventually is removed by shoreface erosion as the coastal system translates landward. Therefore, tidal inlets and the adjacent transgressed shelf areas are primarily locations for potential hard mineral resources. In the study area, primary hard mineral resource targets include tidal inlet channels, flood-tidal deltas, ebb-tidal deltas, shoreface sand ridges, and fluvial channels. Petit Bois Pass appears to be located in an antecedent topographic depression that is cut into underlying pre-Holocene deposits. Pre-Holocene deposits form the eastern core of Dauphin Island where they are subaerially exposed. The pre-Holocene surface dips to the west, providing a platform for the narrow, elongated Holocene split of Dauphin Island, and crops out on the shoreface at the western end of Dauphin Island to -8 to -9 m below MLW. Farther to the west, core data from the main inlet channel of Petit Bois Pass indicate only Holocene sediments occur suggesting that the pre-Holocene surface is > 11 m below MLW. This > 11 m trend continues under western Petit Bois Pass. It appears that Petit Bois Pass may occupy the eastern side of an antecedent Pascagoula River valley. End_of_Record - Last_Page 461-------