Blake W. Blackwelder

Stratigraphy, structure, absolute age, and paleontology of the upper Pleistocene deposits at Sankaty Head, Nantucket Island, Massachusetts

The Sankaty Head cliff of Nantucket Island, Massachusetts, exposes drift of at least two glaciations and interglacial marine deposits. Radiocarbon, amino-acid-racemization, and uranium-thorium analyses were used to determine the absolute ages of the beds. The results indicate that (1) the Sankaty Sand correlates with oxygen-isotope stage 5 (Sangamonian), (2) the underlying drift is older than stage 5 (Illinoian or older), and (3) the overlying drift is Wisconsinan in age. Ostracodes and molluscs within the Sankaty Sand indicate that the marine climate during deposition of the lower part was somewhat warmer than the present climate off Sankaty Head and that the marine climate during the deposition of the upper part was as cold as or somewhat colder than the present climate. The paleoenvironmental data support a stage 5 (Sangamonian) age for the marine deposits.

Late Cenozoic marine deposition in the United States Atlantic Coastal Plain related to tectonism and global climate

Abstract Major hiatuses in upper Cenozoic marine deposits in the United States Atlantic Coastal Plain are recognized on the basis of molluscan faunal changes at erosional unconformities. These hiatuses generally coincided with periods of global cooling and ice sheet formation. Such hiatuses provide information to supplement global climatic data. Major hiatuses are recognized within the early Miocene (23-20 m.y. ago), at the end of the middle Miocene (∼ 11-10 m.y. ago), at the end of the late Miocene (∼6.5-5 m.y. ago), at the end of the early Pliocene (∼4.0–2.5 m.y. ago), at the end of the late Pliocene (∼1.9 or 1.8 m.y. ago), within the Pleistocene (∼1.1-0.5 m.y. ago) and several times within the last 0.4 m.y. Estimates of the amount of water contained in ice sheets at different times in the Pliocene and Pleistocene facilitate calculation of probable minimum sea levels on the Coastal Plain during different high stands of the sea. The altitudes of dated shoreline deposits in the Atlantic Coastal Plain show that the amount of uplift in the Cape Fear area has averaged at least 1.3 cm per 1000 years since the beginning of Pliocene time. The Coastal Plain of Georgia has apparently experienced relatively little vertical deformation during this same time.

Late Wisconsinan Sea Levels on the Southeast U.S. Atlantic Shelf Based on In-Place Shoreline Indicators

A new interpretation of late Pleistocene sea levels on the U.S. Atlantic continental shelf is based on in-place lagoonal and salt-marsh sediments obtained from vibra-cores. These data show sea levels during the last Wisconsinan transgression were about 30 meters shallower than is indicated by existing sea-level curves.

The Georgia Embayment continental shelf: Stratigraphy of a submergence

Forty vibracores taken across the continental shelf and in proposed drilling lease areas of the southeast Georgia Embayment are used to document the unconsolidated shelf-sediment cover. The Holocene-Pleistocene sediment veneer is thin, generally less than 4 m thick. Lagoon sediments deposited during the last regression (shelf emergence) or the Holocene transgression (shelf submergence) occur in patches on the inner and central shelf. Because essentially only late Pleistocene and Holocene mollusk shells are present in the shelf-sediment cover, it is believed that most of the carbonate fraction was removed by subaerial leaching during low sea-level stands aided by mechanical abrasion and biological degradation during the regressive-transgressive cycle. During each transgression or submergence, the surficial sand sheet is recharged with a new biogenic carbonate fraction along with the addition of small amounts of clastic sediments derived from “overrun” estuaries and erosion of underlying Tertiary sediments. The stratigraphy based on the vibracores supports the concept of cross-shelf migration of the shore face—barrier island systems in response to rising sea level. Sedimentologic and paleontologic analyses also indicate that extensive in-place mixing of shelf sediments may have occurred, an important factor to consider in evaluation of the fate of particulate pollutants. The establishment of the time frame of such mixing should be given high priority in future studies.

DIAGENETIC CHANGES IN THE ELEMENTAL COMPOSITION OF UNRECRYSTALLIZED MOLLUSK SHELLS

Abstract The Mg, Sr, Mn, Fe, Na and K contents were determined for 230 apparently unrecrystallized mollusk shells (gastropods and bivalves) ranging in age from late Cretaceous to Holocene. Consistent differences between the Holocene and fossil shells with respect to concentrations of all these elements are attributed to postburial diagenetic changes. Fossil-Holocene shell comparisons are made on the intergeneric level, a more severe test of compositional differences than was previous work involved with few species. The observed differences re-emphasize the need for extreme caution in the use of the many geochemical tools which assume that no compositional changes have taken place prior to recrystallization of calcareous materials.

Geology of Continental Shelf, Onslow Bay, North Carolina, as Revealed by Submarine Outcrops

Lithologic and stratigraphic data from rocks dredged from the continental shelf off Onslow Bay, North Carolina, provide surface control for seismic studies of the southeastern United States continental margin and help to explain the distribution of potentially economic phosphate-rich sediments on this shelf. Outcropping Miocene rocks in this area indicate that the region has long been a positive geologic feature and has received relatively little Pliocene and Pleistocene sedimentation. Leached, molluscan-moldic calcareous quartz sandstones of late Oligocene to early Miocene age (Belgrade Formation) and middle to late Miocene age (Pungo River Formation) crop out in southwestern and northeastern Onslow Bay, respectively. These two areas border a band of highly phosphatic su ficial sediments probably derived from unlithified, phosphatic units of the Pungo River Formation. Lower Pleistocene calcarenites that correlate with the Waccamaw Formation crop out on the sea floor near Cape Fear, which bounds Onslow Bay on the south. A core on Frying Pan Shoals off Cape Fear, after passing through Pleistocene coquina, calcareous quartz sandstone, and oolitic sand, penetrated upper Pliocene calcarenites of the Bear Bluff Formation and middle Miocene phosphatic argillaceous sandstones of the Pungo River Formation. Samples from this core hole show that phosphate increases toward the top of the unindurated Pungo River Formation section, indicating that this formation is probably the source of the high phosphate concentrations in the surficial sediments near Cape Fear, wher the Bear Bluff calcarenites also crop out. Upper Pleistocene units include oolitic limestones adjacent to the outer shelf, calcareous quartz sandstones around Cape Fear, and molluscan coquinas near Cape Lookout which bounds Onslow Bay on the north. The outer shelf is blanketed by submarine lithified algal limestones and sandstones of Holocene age.

Late Wisconsin and Holocene tectonic stability of the United States mid-Atlantic coastal region.

Deposits that formed in the intertidal zone during sea-level rise 12,000 to 9,000 yr ago have undergone very little differential vertical deformation in the area between New York City and South Carolina. The lack of north-south vertical deformation contrasts with tide-gauge and with precise leveling measurements that have been used to indicate that considerable differential vertical movement is occurring along the coast. Probably, present rates of deformation cannot be extrapolated to early Holocene. Depths of dated in-place intertidal deposits and estimates that suggest the U.S. mid-Atlantic shelf was downwarped during Holocene glacio-isostatic adjustment are used to indicate that eustatic sea levels were not substantially below 30 m depth about 12,000 yr B.P.

Mineralogy of the sand size carbonate fraction of some Recent marine terrigenous and carbonate sediments

ABSTRACT A wide variety of marine sediments were examined in order to determine the factors controlling modern carbonate mineral distribution. The sand-sized fraction was utilized exclusively in order to make possible identification of components and to minimize masking effects of non-carbonate material during X-ray diffraction analysis. Factors controlling mineral distribution include the original mineralogical components being formed as well as the nature and extent of alteration processes. These factors are directly related to depth, but show little latitudinal correlation.

Environmental Significance of Physical Attributes of Calcareous Sedimentary Particles: ABSTRACT

Physical attributes of carbonate particles closely reflect environmental conditions. Features such as the degree of aging of shells, particle roundness, particle staining, and shell fragmentation are particularly useful in interpretation of shelf sedimentation. Such particle attributes indicate the relict, shallow-water nature of most shelf sediments off the southeastern United States. End_of_Article - Last_Page 2042------------