Foster
David S.
Foster
David S.
No Thumbnail Available
6 results
Search Results
Now showing
1 - 6 of 6
-
PreprintCharacter and distribution of exposed glaciodeltaic deposits off outer Cape Cod, Massachusetts, and their effects on hydrogeology and benthic habitats( 2005-05-15) Poppe, Lawrence J. ; Foster, David S. ; Danforth, William W.Sea-bed outcrops of glaciodeltaic sediments were identified in four places east of Cape Cod, Massachusetts during seismic-reflection, multibeam bathymetric and backscatter, bottom photographic, and sediment sampling surveys. These strata record coarser-grained ice-proximal glaciofluvial topset to finer-grained distal glaciolacustrine bottomset deposition within deltaic systems that prograded southwestward into glacial lakes from the South Channel lobe about 18 ka B.P. These beds are important because they (1) influence the outer Cape's hydrogeologic framework, and (2) provide relatively stable, locally rough habitats within an area of seafloor dominated by mobile sand and gravelly sediment, and benefit the benthic fauna by providing shelter and a substrate amenable to burrow construction.
-
ArticleGeologic framework of the northern North Carolina, USA inner continental shelf and its influence on coastal evolution(Elsevier, 2013-11-20) Thieler, E. Robert ; Foster, David S. ; Himmelstoss, Emily A. ; Mallinson, David J.The inner continental shelf off the northern Outer Banks of North Carolina was mapped using sidescan sonar, interferometric swath bathymetry, and high-resolution chirp and boomer subbottom profiling systems. We use this information to describe the shallow stratigraphy, reinterpret formation mechanisms of some shoal features, evaluate local relative sea-levels during the Late Pleistocene, and provide new constraints, via recent bedform evolution, on regional sediment transport patterns. The study area is approximately 290 km long by 11 km wide, extending from False Cape, Virginia to Cape Lookout, North Carolina, in water depths ranging from 6 to 34 m. Late Pleistocene sedimentary units comprise the shallow geologic framework of this region and determine both the morphology of the inner shelf and the distribution of sediment sources and sinks. We identify Pleistocene sedimentary units beneath Diamond Shoals that may have provided a geologic template for the location of modern Cape Hatteras and earlier paleo-capes during the Late Pleistocene. These units indicate shallow marine deposition 15–25 m below present sea-level. The uppermost Pleistocene unit may have been deposited as recently as Marine Isotope Stage 3, although some apparent ages for this timing may be suspect. Paleofluvial valleys incised during the Last Glacial Maximum traverse the inner shelf throughout the study area and dissect the Late Pleistocene units. Sediments deposited in the valleys record the Holocene transgression and provide insight into the evolutionary history of the barrier-estuary system in this region. The relationship between these valleys and adjacent shoal complexes suggests that the paleo-Roanoke River did not form the Albemarle Shelf Valley complex as previously proposed; a major fluvial system is absent and thus makes the formation of this feature enigmatic. Major shoal features in the study area show mobility at decadal to centennial timescales, including nearly a kilometer of shoal migration over the past 134 yr. Sorted bedforms occupy ~ 1000 km2 of seafloor in Raleigh Bay, and indicate regional sediment transport patterns between Capes Hatteras and Lookout that help explain long-term sediment accumulation and morphologic development. Portions of the inner continental shelf with relatively high sediment abundance are characterized by shoals and shoreface-attached ridges, and where sediment is less abundant, the seafloor is dominated by sorted bedforms. These relationships are also observed in other passive margin settings, suggesting a continuum of shelf morphology that may have broad application for interpreting inner shelf sedimentation patterns.
-
ArticleGlaciotectonic deformation associated with the Orient Point–Fishers Island moraine, westernmost Block Island Sound : further evidence of readvance of the Laurentide ice sheet(Springer, 2012-06-29) Poppe, Lawrence J. ; Oldale, Robert N. ; Foster, David S. ; Smith, Shepard M.High-resolution seismic-reflection profiles collected across pro-glacial outwash deposits adjacent to the circa 18 ka b.p. Orient Point–Fishers Island end moraine segment in westernmost Block Island Sound reveal extensive deformation. A rhythmic seismic facies indicates the host outwash deposits are composed of fine-grained glaciolacustrine sediments. The deformation is variably brittle and ductile, but predominantly compressive in nature. Brittle deformation includes reverse faults and thrust faults that strike parallel to the moraine, and thrust sheets that extend from beneath the moraine. Ductile deformation includes folded sediments that overlie undisturbed deposits, showing that they are not drape features. Other seismic evidence for compression along the ice front consists of undisturbed glaciolacustrine strata that dip back toward and underneath the moraine, and angular unconformities on the sea floor where deformed sediments extend above the surrounding undisturbed correlative strata. Together, these ice-marginal glaciotectonic features indicate that the Orient Point–Fishers Island moraine marks a significant readvance of the Laurentide ice sheet, consistent with existing knowledge for neighboring coeval moraines, and not simply a stillstand as previously reported.
-
ArticleMature diffuse tectonic block boundary revealed by the 2020 southwestern Puerto Rico seismic sequence(American Geophysical Union, 2022-02-08) ten Brink, Uri S. ; Vanacore, Elizabeth A. ; Fielding, Eric J. ; Chaytor, Jason D. ; Lopez-Venegas, Alberto M. ; Baldwin, William S. ; Foster, David S. ; Andrews, Brian D.Distributed faulting typically tends to coalesce into one or a few faults with repeated deformation. The progression of clustered medium-sized (≥Mw4.5) earthquakes during the 2020 seismic sequence in southwestern Puerto Rico (SWPR), modeling shoreline subsidence from InSAR, and sub-seafloor mapping by high-resolution seismic reflection profiles, suggest that the 2020 SWPR seismic sequence was distributed across several short intersecting strike-slip and normal faults beneath the insular shelf and upper slope of Guayanilla submarine canyon. Multibeam bathymetry map of the seafloor shows significant erosion and retreat of the shelf edge in the area of seismic activity as well as slope-parallel lineaments and submarine canyon meanders that typically develop over geological time. The T-axis of the moderate earthquakes further matches the extension direction previously measured on post early Pliocene (∼>3 Ma) faults. We conclude that although similar deformation has likely taken place in this area during recent geologic time, it does not appear to have coalesced during this time. The deformation may represent the southernmost part of a diffuse boundary, the Western Puerto Rico Deformation Boundary, which accommodates differential movement between the Puerto Rico and Hispaniola arc blocks. This differential movement is possibly driven by the differential seismic coupling along the Puerto Rico—Hispaniola subduction zone. We propose that the compositional heterogeneity across the island arc retards the process of focusing the deformation into a single fault. Given the evidence presented here, we should not expect a single large event in this area but similar diffuse sequences in the future.
-
ArticleSeismic stratigraphic framework of the continental shelf offshore Delmarva, USA: implications for Mid-Atlantic Bight evolution since the Pliocene(Elsevier, 2020-07-10) Brothers, Laura L. ; Foster, David S. ; Pendleton, Elizabeth A. ; Baldwin, Wayne E.Understanding how past coastal systems have evolved is critical to predicting future coastal change. Using over 12,000 trackline kilometers of recently collected, co-located multi-channel boomer, sparker and chirp seismic reflection profile data integrated with previously collected borehole and vibracore data, we define the upper (< 115 m below mean lower low water) seismic stratigraphic framework offshore of the Delmarva Peninsula, USA. Twelve seismic units and 11 regionally extensive unconformities (U1-U11) were mapped over 5900 km2 of North America's Mid-Atlantic continental shelf. We interpret U3, U7, U9, U11 as transgressive ravinement surfaces, while U1,2,4,5,6,8,10 are subaerial unconformities illustrating distinct periods of lower sea-level. Based on areal distribution, stratigraphic relationships and dating results (Carbon 14 and amino acid racemization estimates) from earlier vibracore and borehole studies, we interpret the infilled channels as late Neogene and Quaternary courses of the Susquehanna, Potomac, Rappahannock, York, James rivers and tributaries, and a broad flood plain. These findings indicate that the region's geologic framework is more complex than previously thought and that Pleistocene paleochannels are abundant in the Mid-Atlantic. This study synthesizes and correlates the findings of other Atlantic Margin studies and establishes a large-scale Quaternary framework that enables more detailed stratigraphic analysis in the future. Such work has implications for inner continental shelf systems tract evolution, the relationship between antecedent geology and modern coastal systems, assessments of eustacy, glacial isostatic adjustment, and other processes and forcings that play a role in passive margin evolution.
-
ArticleMolluscan aminostratigraphy of the US Mid-Atlantic Quaternary coastal system: implications for onshore-offshore correlation, paleochannel and barrier island evolution, and local late Quaternary sea-level history(Elsevier, 2021-05-18) Wehmiller, John F. ; Brothers, Laura L. ; Ramsey, Kelvin W. ; Foster, David S. ; Mattheus, C. R. ; Hein, Christopher J. ; Shawlerd, Justin L.The Quaternary record of the US Mid-Atlantic coastal system includes onshore emergent late Pleistocene shoreline deposits, offshore inner shelf and barrier island units, and paleovalleys formed during multiple glacial stage sea-level lowstands. The geochronology of this coastal system is based on uranium series, radiocarbon, amino acid racemization (AAR), and optically stimulated luminescence (OSL) methods. We report over 600 mollusk AAR results from 93 sites between northeastern North Carolina and the central New Jersey shelf, representing samples from both onshore cores or outcrops, sub-barrier and offshore cores, and transported shells from barrier island beaches. AAR age estimates are constrained by paired 14C analyses on specific shells and associated U-series coral ages from onshore sites. AAR data from offshore cores are interpreted in the context of detailed seismic stratigraphy. The distribution of Pleistocene-age shells on the island beaches is linked to the distribution of inner shelf or sub-barrier source units. Age mixing over a range of time-scales (~1 ka to ~100 ka) is identified by AAR results from onshore, beach, and shelf collections, often contributing insights into the processes forming individual barrier islands. The regional aminostratigraphic framework identifies a widespread late Pleistocene (Marine Isotope Stage 5) aminozone, with isolated records of middle and early Pleistocene deposition. AAR results provide age estimates for the timing of formation of the three major paleochannels that underlie the Delmarva Peninsula: Persimmon Point paleochannel ≥800 ka; Exmore paleochannel ~400–500 ka (MIS 12); and Eastville paleochannel > 125 ka (MIS 6). The results demonstrate the value of synthesizing abundant AAR chronologic data across various coastal environments, integrating multiple distinct geologic studies. The ages and elevations of the Quaternary units are important for current hypotheses about relative sea-level history and crustal dynamics in the region, which was likely influenced by the Laurentide ice sheet, the margin just ~400 km to the north.