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USGS1980

Neogene diatoms from Petersburg, Virginia

Sixty-two diatom taxa have been identified from a diatomaceous sandy clay that crops out near the outlet of Wilcox Lake, Petersburg. Virginia. These diatoms are mostly cosmopolitan marine forms, but some species suggest deposition in brackish water that had a freshwater inflow. The regional stratigraphy, as well as the pelecypods and foraminifers, indicates that the beds may have been laid down in a lagoonal or estuarine environment. The diatom assemblage suggests that the deposit is of early Pliocene age, although previous biostratigraphic studies have considered it to be latest Miocene.

Virginia↗

USGS1979

Middle Miocene marine diatoms from the Hawthorn Formation within the Ridgeland Trough, South Carolina and Georgia

Diatomaceous sediments in outcrops of the Miocene Hawthorn Formation at Dawsons Landing in South Carolina and at Berrys Landing in Georgia, have yielded a total of 111 marine diatom taxa. Varving of the diatomaceous clay suggests that the diatoms were deposited in a restricted basin or lagoonal area that had oxygen-deficient bottom waters. The Middle Miocene diatom assemblage of the Hawthorn is correlative with that of approximately the upper half of Miocene Lithologic Unit 16 of the Choptank Formation in the Chesapeake Bay region of Maryland and with the Serravallian Stage.

Georgia, South Carolina↗

USGS1978

Marine diatom sequence in Miocene strata of the Chesapeake Bay region, Maryland

The Calvert and Choptank Formations exposed along the west shore of Chesapeake Bay in Maryland have been correlated with strata of early and middle Miocene age. The stratigraphic ranges of marine diatom marker species indicate a distinct diatom sequence in the deposits. A few diatoms that mark deep-sea Miocene deposits are found sparingly in these shallow neritic assemblages. Seven diatom zones based on well-recognized events in the diatom biostratigraphy are proposed for the mid-Atlantic region of the eastern United States. A suggested correlation of these zones is made with some of the deep-sea diatom zonations in the Atlantic and Pacific Ocean basins.

Maryland↗

USGS2008

A 26 million year gap in the central Arctic record at the greenhouse-icehouse transition: Looking for clues

The Cenozoic record of the Lomonosov Ridge (central Arctic Ocean) recovered during Integrated Ocean Drilling Program (IODP) Expedition 302 revealed an unexpected 26 Ma hiatus, separating middle Eocene (∼44.4 Ma) from lower Miocene sediments (∼18.2 Ma). To elucidate the nature of this unconformity, we performed a multiproxy palynological (dinoflagellate cysts, pollen, and spores), micropaleontological (siliceous microfossils), inorganic, and organic (Tetra Ether Index of lipids with 86 carbon atoms (TEX 86 ) and Branched and Isoprenoid Tetraether (BIT)) geochemical analysis of the sediments from ∼5 m below to ∼7 m above the hiatus. Four main paleoenvironmental phases (A–D) are recognized in the sediments encompassing the unconformity, two below (A–B) and two above (C–D): (A) Below the hiatus, proxies show relatively warm temperatures, with Sea Surface Temperatures (TEX 86 ‐derived SSTs) of about 8°C and high fresh to brackish water influence. (B) Approaching the hiatus, proxies indicate a cooling trend (TEX 86 ‐derived SSTs of ∼5°C), increased freshwater influence, and progressive shoaling of the Lomonosov Ridge drilling site, located close to or at sea level. (C) The interval directly above the unconformity contains sparse reworked Cretaceous to Oligocene dinoflagellate cysts. Sediments were deposited in a relatively shallow, restricted marine environment. Proxies show the simultaneous influence of both fresh and marine waters, with alternating oxic and anoxic conditions. Pollen indicates a relatively cold climate. Intriguingly, TEX 86 ‐derived SSTs are unexpectedly high, ∼15–19°C. Such warm surface waters may be partially explained by the ingression of warmer North Atlantic waters after the opening of the Fram Strait during the early Miocene. (D) Sediments of the uppermost interval indicate a phase of extreme oxic conditions, and a well‐ventilated environment, which occurred after the complete opening of the Fram Strait. Importantly, and in contrast with classical postrifting thermal subsidence models for passive margins, our data suggest that sediment erosion and/or nondeposition that generated the hiatus was likely due to a progressive shoaling of the Lomonosov Ridge. A shallow water setting both before and after the hiatus suggests that the Lomonosov Ridge remained at or near sea level for the duration of the gap in the sedimentary record. Interacting sea level changes and/or tectonic activity (possibly uplift) must be invoked as possible causes for such a long hiatus.

Paleoceanography↗

USGS1990

Silicoflagellate and diatom biostratigraphy in successive Burdigalian transgressions, middle Atlantic coastal plain

The earliest Miocene strata on the Middle Atlantic Coastal Plain were deposited in two distinct marine transgressions separated by a hiatus in sedimentation of approximately one million years. The older unit (Bed 3A of the Calvert Formation and its correlatives) was deposited in a relatively restricted basin of middle Burdigalian (middle early Miocene) age. The younger unit (Bed 3B of the Calvert Formation and its correlatives) was deposited in a much more widespread basin of late Burdigalian (late early Miocene) age. The older deposit was completely overlapped by the younger deposit. These two beds of substantially differing geologic age are difficult, if not impossible, to distinguish solely by lithostratigraphy. They are, however, readily differentiated on the basis of their distinctive silicoflagellate and diatom assemblages.

Delaware, Maryland, New Jersey↗

USGS2014

Late 20th Century benthic foraminiferal distribution in Central San Francisco Bay, California: Influence of the Trochammina hadai invasion

The distribution of foraminifera in most of San Francisco Bay is well documented, but this is not the case for the subembayment known as Central Bay. To resolve this, 55 grab samples obtained in 1998 were analyzed to characterize the foraminiferal fauna in the surface sediments of the area. Thirty-five species were identified, including the invasive Japanese species Trochammina hadai that was introduced into the bay in the early 1980s. A cluster analysis of the samples from Central Bay produced three groups (biofacies) and one outlier. The Shallow Subtidal Biofacies is characterized by a marsh to shallow-subtidal agglutinated fauna, dominated by T. hadai but also including T. inflata , T. macrescens , Haplophragmoides subinvolutum , and Miliammina fusca . The Intermediate Subtidal Biofacies, the Intermediate Subtidal Outlier, and the Deep Subtidal Biofacies are dominated by calcareous taxa, most notably Ammonia tepida , Elphidium excavatum , and Elphidiella hannai . Ammonia tepida is most abundant in the warmer, intermediate depths of eastern Central Bay, abundances of E. excavatum peak in the cooler estuarine water near Alcatraz Island, and E. hannai thrives in the cold water west of Angel Island in a transitional setting between the deep subtidal estuarine and the nearshore marine environments. The recovery of oceanic species as far east as Angel Island indicate that western Central Bay is the most marine-influenced region of San Francisco Bay. Samples collected from 1965 onward were also compared with those from 1998 to investigate how the distribution of benthic foraminifera in Central Bay has changed over the latter half of the 20th Century, particularly in response to the invasion by Trochammina hadai . In 1998, T. hadai was recovered at 46 of 55 sites in Central Bay, comprising from 0.3 to 97% (mean = 23%) of the foraminiferal fauna. With the species’ affiliation for shallow environments, it is not unexpected that it dominated the fauna of the Shallow Subtidal Biofacies (68-97%, mean = 77%) and was also a significant component of the Intermediate Subtidal Biofacies (7-51%, averaging 28%). In the deeper waters west of Alcatraz Island, the abundance of T. hadai was significantly less (mean = 8%), most likely reflecting allochthonous specimens that were the result of post-mortem transport. A cluster analysis clearly distinguishes pre- and post-invasion biofacies, illustrating how dominant T. hadai has become in Central Bay.

California↗

USGS2012

Foraminiferal repopulation of the late Eocene Chesapeake Bay impact crater

The Chickahominy Formation is the initial postimpact deposit in the 85km-diameter Chesapeake Bay impact crater, which is centered under the town of Cape Charles, Virginia, USA. The formation comprises dominantly microfossil-rich, silty, marine clay, which accumulated during the final ~1.6myr of late Eocene time. At cored sites, the Chickahominy Formation is 16.8-93.7m thick, and fills a series of small troughs and subbasins, which subdivide the larger Chickahominy basin. Nine coreholes drilled through the Chickahominy Formation (five inside the crater, two near the crater margin, and two ~3km outside the crater) record the stratigraphic and paleoecologic succession of 301 indigenous species of benthic foraminifera, as well as associated planktonic foraminifera and bolboformids. Two hundred twenty of these benthic species are described herein, and illustrated with scanning electron photomicrographs. Absence of key planktonic foraminiferal and Bolboforma species in early Chickahominy sediments indicates that detrimental effects of the impact also disturbed the upper oceanic water column for at least 80-100kyr postimpact. After an average of ~73kyr of stressed, rapidly fluctuating paleoenvironments, which were destabilized by after-effects of the impact, most of the cored Chickahominy subbasins maintained stable, nutrient-rich, low-oxygen bottom waters and interstitial microhabitats for the remaining ~1.3myr of late Eocene time.

Maryl;Virginia↗

USGS2002

Trends in late Maastrichtian calcareous nannofossil distribution patterns, Western North Atlantic margin

First and last occurrences of several Maastrichtian calcareous nannofossil species are shown to be diachronous across paleodepth and paleoenvironment using the graphic correlation method. Calcareous nannofossil assemblages examined from eleven cores from a deep- to shallow-water transect along the eastern United States Atlantic margin document that the first occurrence of Micula murus (Martini 1961) Bukry 1973 is diachronous, appearing 2.0 million years earlier in open ocean sites than in shallow marine sites. The first occurrence (FO) of Lithraphidites kennethii Perch-Nielsen 1984 is also nonsynchronous, appearing in the deep ocean before its FO in neritic waters. The last occurrence (LO) of L. praequadratus Roth 1978 is diachronous across paleodepth, going locally extinct first in deeper water. The LO of Watznaueria bybelliae Self-Trail 1999 is also diachronous, going locally extinct first in shallow-water settings. Ceratolithoides amplector Burnett 1997, C. pricei Burnett 1997, C. self-trailiae Burnett 1997, C. ultimus Burnett 1997, Cribrocorona gallica (Stradner 1963) Perch-Nielsen 1973. Micula praemurus (Bukry 1973) Stradner and Steinmetz 1984, Pseudomicula quadratus Perch-Nielsen et al. 1978, and Semihololithus spp. are present consistently in common to frequent abundances in ODP holes 1050C and 1052E on the Blake Nose, but they are rare or absent from neritic sections in Coastal Plain cores. It is apparent that these species flourished in an open ocean setting, suggesting that differences in assemblage abundance and diversity between deep ocean and nearshore areas were controlled by paleoceanographic factors. These species are not used for biostratigraphy, but may be useful indicators of open ocean conditions. The line of correlation (LOC) for nine Coastal Plain cores clearly defines the Cretaceous-Tertiary (K/T) boundary unconformity at the top of the Maastrichtian section (Peedee Formation) and the Campanian-Maastrichtian (C/M) unconformity at the base of the Maastrichtian section (Peedee/Donoho Creek formational contact). The K/T boundary unconformity is undulatory in nature; updip Maastrichtian sections have been stripped to a greater depth than the downdip sections. The uppermost Campanian, all of the lowermost Maastrichtian, and the basal upper Maastrichtian sediments are missing from the study area.

Micropaleontology↗