- Abstract
- Affiliation
- All
- Authors
- Book Series
- DOI
- EISBN
- EISSN
- Full Text
- GeoRef ID
- ISBN
- ISSN
- Issue
- Keyword (GeoRef Descriptor)
- Meeting Information
- Report #
- Title
- Volume
- Abstract
- Affiliation
- All
- Authors
- Book Series
- DOI
- EISBN
- EISSN
- Full Text
- GeoRef ID
- ISBN
- ISSN
- Issue
- Keyword (GeoRef Descriptor)
- Meeting Information
- Report #
- Title
- Volume
- Abstract
- Affiliation
- All
- Authors
- Book Series
- DOI
- EISBN
- EISSN
- Full Text
- GeoRef ID
- ISBN
- ISSN
- Issue
- Keyword (GeoRef Descriptor)
- Meeting Information
- Report #
- Title
- Volume
- Abstract
- Affiliation
- All
- Authors
- Book Series
- DOI
- EISBN
- EISSN
- Full Text
- GeoRef ID
- ISBN
- ISSN
- Issue
- Keyword (GeoRef Descriptor)
- Meeting Information
- Report #
- Title
- Volume
- Abstract
- Affiliation
- All
- Authors
- Book Series
- DOI
- EISBN
- EISSN
- Full Text
- GeoRef ID
- ISBN
- ISSN
- Issue
- Keyword (GeoRef Descriptor)
- Meeting Information
- Report #
- Title
- Volume
- Abstract
- Affiliation
- All
- Authors
- Book Series
- DOI
- EISBN
- EISSN
- Full Text
- GeoRef ID
- ISBN
- ISSN
- Issue
- Keyword (GeoRef Descriptor)
- Meeting Information
- Report #
- Title
- Volume
NARROW
GeoRef Subject
-
all geography including DSDP/ODP Sites and Legs
-
Arctic region (1)
-
Asia
-
Middle East
-
Turkey
-
Anatolia (1)
-
-
-
-
Atlantic Ocean
-
North Atlantic (1)
-
-
Chesapeake Bay impact structure (11)
-
James River (1)
-
North America
-
Gulf Coastal Plain (1)
-
-
North Slope (1)
-
United States
-
Alabama (1)
-
Alaska (2)
-
Atlantic Coastal Plain
-
Central Atlantic Coastal Plain (1)
-
-
Chesapeake Bay (7)
-
Delaware
-
Sussex County Delaware
-
Bethany Beach Delaware (1)
-
-
-
Delmarva Peninsula (1)
-
Maryland
-
Calvert County Maryland (1)
-
Dorchester County Maryland (1)
-
Talbot County Maryland (1)
-
-
New Jersey (1)
-
Potomac River (2)
-
Salisbury Embayment (1)
-
South Carolina
-
Dorchester County South Carolina (1)
-
-
Virginia
-
Mathews County Virginia (1)
-
New Kent County Virginia (1)
-
Northampton County Virginia (9)
-
Pamunkey River (1)
-
Westmoreland County Virginia (1)
-
-
-
-
elements, isotopes
-
isotope ratios (1)
-
isotopes
-
stable isotopes
-
Sr-87/Sr-86 (1)
-
-
-
metals
-
alkaline earth metals
-
strontium
-
Sr-87/Sr-86 (1)
-
-
-
-
-
fossils
-
borings (1)
-
Chordata
-
Vertebrata
-
Tetrapoda
-
Mammalia
-
Theria
-
Eutheria
-
Cetacea
-
Archaeoceti (1)
-
Odontoceti (1)
-
-
-
-
-
-
-
-
ichnofossils (2)
-
Invertebrata
-
Arthropoda
-
Mandibulata
-
Crustacea
-
Ostracoda (1)
-
-
-
-
Mollusca (2)
-
Protista
-
Foraminifera (2)
-
-
-
microfossils (13)
-
palynomorphs
-
acritarchs (1)
-
Dinoflagellata (9)
-
miospores
-
pollen (2)
-
-
-
Plantae
-
algae
-
nannofossils (3)
-
-
-
-
geologic age
-
Cenozoic
-
Quaternary
-
Pleistocene (4)
-
-
Tertiary
-
Neogene
-
Miocene
-
Calvert Formation (4)
-
lower Miocene (1)
-
middle Miocene
-
Choptank Formation (3)
-
-
Saint Marys Formation (3)
-
upper Miocene
-
Eastover Formation (2)
-
-
-
Pliocene
-
Yorktown Formation (1)
-
-
-
Paleogene
-
Eocene
-
lower Eocene
-
Aquia Formation (2)
-
-
middle Eocene
-
Lutetian (1)
-
-
upper Eocene
-
Piney Point Formation (1)
-
-
-
Oligocene
-
lower Oligocene (1)
-
-
Paleocene (3)
-
Paleocene-Eocene Thermal Maximum (2)
-
-
-
-
Mesozoic
-
Cretaceous
-
Potomac Group (2)
-
Upper Cretaceous
-
Maestrichtian (1)
-
Senonian (1)
-
-
-
-
Paleozoic (3)
-
Precambrian
-
upper Precambrian
-
Proterozoic (3)
-
-
-
-
metamorphic rocks
-
metamorphic rocks
-
impactites
-
impact breccia
-
suevite (2)
-
-
-
-
-
Primary terms
-
Arctic region (1)
-
Asia
-
Middle East
-
Turkey
-
Anatolia (1)
-
-
-
-
Atlantic Ocean
-
North Atlantic (1)
-
-
biogeography (1)
-
Cenozoic
-
Quaternary
-
Pleistocene (4)
-
-
Tertiary
-
Neogene
-
Miocene
-
Calvert Formation (4)
-
lower Miocene (1)
-
middle Miocene
-
Choptank Formation (3)
-
-
Saint Marys Formation (3)
-
upper Miocene
-
Eastover Formation (2)
-
-
-
Pliocene
-
Yorktown Formation (1)
-
-
-
Paleogene
-
Eocene
-
lower Eocene
-
Aquia Formation (2)
-
-
middle Eocene
-
Lutetian (1)
-
-
upper Eocene
-
Piney Point Formation (1)
-
-
-
Oligocene
-
lower Oligocene (1)
-
-
Paleocene (3)
-
Paleocene-Eocene Thermal Maximum (2)
-
-
-
-
Chordata
-
Vertebrata
-
Tetrapoda
-
Mammalia
-
Theria
-
Eutheria
-
Cetacea
-
Archaeoceti (1)
-
Odontoceti (1)
-
-
-
-
-
-
-
-
continental shelf (1)
-
continental slope (1)
-
Deep Sea Drilling Project
-
IPOD
-
Leg 95
-
DSDP Site 612 (2)
-
-
-
-
faults (1)
-
ichnofossils (2)
-
Invertebrata
-
Arthropoda
-
Mandibulata
-
Crustacea
-
Ostracoda (1)
-
-
-
-
Mollusca (2)
-
Protista
-
Foraminifera (2)
-
-
-
isotopes
-
stable isotopes
-
Sr-87/Sr-86 (1)
-
-
-
Mesozoic
-
Cretaceous
-
Potomac Group (2)
-
Upper Cretaceous
-
Maestrichtian (1)
-
Senonian (1)
-
-
-
-
metals
-
alkaline earth metals
-
strontium
-
Sr-87/Sr-86 (1)
-
-
-
-
metamorphic rocks
-
impactites
-
impact breccia
-
suevite (2)
-
-
-
-
metamorphism (1)
-
North America
-
Gulf Coastal Plain (1)
-
-
oceanography (1)
-
paleoclimatology (2)
-
paleoecology (2)
-
paleogeography (1)
-
Paleozoic (3)
-
palynomorphs
-
acritarchs (1)
-
Dinoflagellata (9)
-
miospores
-
pollen (2)
-
-
-
Plantae
-
algae
-
nannofossils (3)
-
-
-
Precambrian
-
upper Precambrian
-
Proterozoic (3)
-
-
-
sea-level changes (2)
-
sedimentary rocks
-
bone beds (1)
-
-
sedimentation (4)
-
sediments
-
clastic sediments
-
boulders (1)
-
-
marine sediments (2)
-
-
stratigraphy (6)
-
tectonics (3)
-
tektites (1)
-
United States
-
Alabama (1)
-
Alaska (2)
-
Atlantic Coastal Plain
-
Central Atlantic Coastal Plain (1)
-
-
Chesapeake Bay (7)
-
Delaware
-
Sussex County Delaware
-
Bethany Beach Delaware (1)
-
-
-
Delmarva Peninsula (1)
-
Maryland
-
Calvert County Maryland (1)
-
Dorchester County Maryland (1)
-
Talbot County Maryland (1)
-
-
New Jersey (1)
-
Potomac River (2)
-
Salisbury Embayment (1)
-
South Carolina
-
Dorchester County South Carolina (1)
-
-
Virginia
-
Mathews County Virginia (1)
-
New Kent County Virginia (1)
-
Northampton County Virginia (9)
-
Pamunkey River (1)
-
Westmoreland County Virginia (1)
-
-
-
-
rock formations
-
Canning Formation (1)
-
Sagavanirktok Formation (1)
-
-
sedimentary rocks
-
sedimentary rocks
-
bone beds (1)
-
-
-
sedimentary structures
-
borings (1)
-
-
sediments
-
sediments
-
clastic sediments
-
boulders (1)
-
-
marine sediments (2)
-
-
Front Matter
Chesapeake Bay Impact Structure—Development of “Brim” Sedimentation in a Multilayered Marine Target
ABSTRACT The late Eocene Chesapeake Bay impact structure was formed in a multilayered target of seawater underlain sequentially by a sediment layer and a rock layer in a continental-shelf environment. Impact effects in the “brim” (annular trough) surrounding and adjacent to the transient crater, between the transient crater rim and the outer margin, primarily were limited to the target-sediment layer. Analysis of published and new lithostratigraphic, biostratigraphic, sedimentologic, petrologic, and mineralogic studies of three core holes, and published studies of a fourth core hole, provided information for the interpretation of the impact processes, their interactions and relative timing, their resulting products, and sedimentation in the brim. Most studies of marine impact-crater materials have focused on those found in the central crater. There are relatively few large, complex marine craters, of which most display a wide brim around the central crater. However, most have been studied using minimal data sets. The large number of core holes and seismic profiles available for study of the Chesapeake Bay impact structure presents a special opportunity for research. The physical and chronologic records supplied by study of the sediment and rock cores of the Chesapeake Bay impact indicate that the effects of the initial, short-lived contact and compression and excavation stages of the impact event primarily were limited to the transient crater. Only secondary effects of these processes are evident in the brim. The preserved record of the brim was created primarily in the subsequent modification stage. In the brim, the records of early impact processes (e.g., outgoing tsunamis, overturned flap collapse) were modified or removed by later processes. Transported and rotated, large and small clasts of target sediments, and intervals of fluidized sands indicate that seismic shaking fractured and partially fluidized the Cretaceous and Paleogene target sediments, which led to their inward transport by collapse and lateral spreading toward the transient crater. The succeeding inward seawater-resurge flow quickly overtook and interacted with the lateral spreading, further facilitating sediment transport across the brim and into the transient crater. Variations in the cohesion and relative depth of the target sediments controlled their degree of disaggregation and redistribution during these events. Melt clasts and shocked and unshocked rock clasts in the resurge sediments indicate fallout from the ejecta curtain and plume. Basal parautochthonous remnant sections of target Cretaceous sediments in the brim thin toward the collapsed transient crater. Overlying seawater-resurge deposits consist primarily of diamictons that vary laterally in thickness, and vertically and laterally in maximum grain size. After cessation of resurge flow and re-establishment of pre-impact sea level, sandy sediment gravity flows moved from the margin to the center of the partially filled impact structure (shelf basin). The uppermost unit consists of stratified sediments deposited from suspension. Postimpact clayey silts cap the crater fill and record the return to shelf sedimentation at atypically large paleodepths within the shelf basin. An unresolved question involves a section of gravel and sand that overlies Neoproterozoic granite in the inner part of the brim in one core hole. This section may represent previously unrecognized, now parautochthonous Cretaceous sediments lying nonconformably above basement granite, or it may represent target sediments that were moved significant distances by lateral spreading above basement rocks or above a granite megaclast from the overturned flap. The Chesapeake Bay impact structure is perhaps the best documented example of the small group of multilayer, marine-target impacts formed in continental shelves or beneath epeiric seas. The restriction of most impact effects to the target-sediment layer in the area outside the transient cavity, herein called the brim, and the presence of seawater-resurge sediments are characteristic features of this group. Other examples include the Montagnais (offshore Nova Scotia, Canada) and Mjølnir (offshore Norway) impact structures.
Chesapeake Bay Impact Structure—Development of “Brim” Sedimentation in a Multilayered Marine Target
Geology and biostratigraphy of the Potomac River cliffs at Stratford Hall, Westmoreland County, Virginia
Abstract The cliffs along the Potomac River at Stratford Hall display extensive exposures of Miocene marine strata that belong successively to the Calvert, Choptank, St. Marys, and Eastover Formations. Within the lower part of this sequence, in the Calvert and Choptank Formations, there is well-developed cyclic stratigraphy. Above the Miocene units lies the marginal marine to deltaic Pleistocene Bacons Castle Formation, which is the highest and youngest formation exposed in the cliffs. The goals of this field trip guide are to (1) show the Miocene formations exposed in the cliffs and discuss the paleoenvironments within which they formed, (2) demonstrate the cyclicity in the Miocene marine formations and discuss its origin, (3) compare and contrast the section exposed at the Stratford and Nomini Cliffs with the classic Miocene Calvert Cliffs sequence exposed to the northeast in Calvert County, Maryland, and the Miocene sequence recovered in the Haynesville cores to the southeast in Richmond County, Virginia, (4) discuss and explain why a detailed correlation among these three places has been so difficult to attain, and (5) show typical lithologies of the Bacons Castle Formation and discuss the paleoenvironments in which they formed.
A new specimen of Agorophius pygmaeus (Agorophiidae, Odontoceti, Cetacea) from the early Oligocene Ashley Formation of South Carolina, USA
Abstract The Salisbury embayment is a broad tectonic downwarp that is filled by generally seaward-thickening, wedge-shaped deposits of the central Atlantic Coastal Plain. Our two-day field trip will take us to the western side of this embayment from the Fall Zone in Washington, D.C., to some of the bluffs along Aquia Creek and the Potomac River in Virginia, and then to the Calvert Cliffs on the western shore of the Chesapeake Bay. We will see fluvial-deltaic Cretaceous deposits of the Potomac Formation. We will then focus on Cenozoic marine deposits. Transgressive and highstand deposits are stacked upon each other with unconformities separating them; rarely are regressive or lowstand deposits preserved. The Paleocene and Eocene shallow shelf deposits consist of glauconitic, silty sands that contain varying amounts of marine shells. The Miocene shallow shelf deposits consist of diatomaceous silts and silty and shelly sands. The lithology, thickness, dip, preservation, and distribution of the succession of coastal plain sediments that were deposited in our field-trip area are, to a great extent, structurally controlled. Surficial and subsurface mapping using numerous continuous cores, auger holes, water-well data, and seismic surveys has documented some folds and numerous high-angle reverse and normal faults that offset Cretaceous and Cenozoic deposits. Many of these structures are rooted in early Mesozoic and/or Paleozoic NE-trending regional tectonic fault systems that underlie the Atlantic Coastal Plain. On Day 1, we will focus on two fault systems (stops 1-2; Stafford fault system and the Skinkers Neck-Brandywine fault system and their constituent fault zones and faults). We will then see (stops 3-5) a few of the remaining exposures of largely unlithified marine Paleocene and Eocene strata along the Virginia side of the Potomac River including the Paleocene-Eocene Thermal Maximum boundary clay. These exposures are capped by fluvial-estuarine Pleistocene terrace deposits. On Day 2, we will see (stops 6-9) the classic Miocene section along the ~25 miles (~40 km) of Calvert Cliffs in Maryland, including a possible fault and structural warping. Cores from nearby test holes will also be shown to supplement outcrops.
Abstract Miocene strata exposed in the Calvert Cliffs, along the western shore of the Chesapeake Bay, Maryland, have a long history of study owing to their rich fossil record, including a series of spectacular shell and bone beds. Owing to increasingly refined biostratigraphic age control, these outcrops continue to serve as important references for geological and paleontological analyses. The canonical Calvert, Choptank, and St. Marys Formations, first described by Shattuck (1904), are generally interpreted as shallowing-up, from a fully marine open shelf to a variety of marginal marine, coastal environments. More detailed paleoenvironmental interpretation is challenging, however, owing to pervasive bioturbation, which largely obliterates diagnostic physical sedimentary structures and mixes grain populations; most lithologic contacts, including regional unconformities, are burrowed firmgrounds at the scale of a single outcrop. This field trip will visit a series of classic localities in the Calvert Cliffs to discuss the use of sedimentologic, ichnologic, taphonomic, and faunal evidence to infer environments under these challenging conditions, which are common to Cretaceous and Cenozoic strata throughout the U.S. Gulf and Atlantic Coastal Plains. We will examine all of Shattuck‚s (1904) original lithologic “zones” within the Plum Point Member of the Calvert Formation, the Choptank Formation, and the Little Cove Point Member of the St. Marys Formation, as well as view the channelized “upland gravel” that are probably the estuarine and fluvial equivalents of the marine upper Miocene Eastover Formation in Virginia. The physical stratigraphic discussion will focus on the most controversial intervals within the succession, namely the unconformities that define the bases of the Choptank and St. Marys Formations, where misunderstanding would mislead historical analysis.
Dinocyst taphonomy, impact craters, cyst ghosts and the Paleocene–Eocene thermal maximum (PETM)
An Occurrence of the Protocetid Whale “ Eocetus ” wardii in the Middle Eocene Piney Point Formation of Virginia
Two cores at the outer margin of the Chesapeake Bay impact structure show significant structural and depositional variations that illuminate its history. Detailed stratigraphy of the Watkins School core reveals that this site is outside the disruption boundary of the crater with respect to its lower part (nonmarine Cretaceous Potomac Formation), but just inside the boundary with respect to its upper part (Exmore Formation and a succession of upper Eocene to Pleistocene postimpact deposits). The site of the U.S. Geological Survey–National Aeronautics and Space Administration Langley core, 6.4 km to the east, lies wholly within the annular trough of the crater. The Potomac Formation in the Watkins School core is not noticeably impact disrupted. The lower part of crater unit A in the Langley core represents stratigraphically lower, but similarly undeformed material. The Exmore Formation is only 7.8 m thick in the Watkins School core, but it is over 200 m thick in the Langley core, where it contains blocks up to 24 m in intersected diameter. The upper part of the Exmore Formation in the two cores is a polymict diamicton with a stratified zone at the top. The postimpact sedimentary units in the two cores have similar late Eocene and late Miocene depositional histories and contrasting Oligocene, early Miocene, and middle Miocene histories. A paleochannel of the James River removed Pliocene deposits at the Watkins School site, to be filled later with thick Pleistocene deposits. At the Langley site, a thick Pliocene and thinner Pleistocene record is preserved.
The International Continental Scientific Drilling Program (ICDP)–U.S. Geological Survey (USGS) Eyreville drill cores from the Chesapeake Bay impact structure provide one of the most complete geologic sections ever obtained from an impact structure. This paper presents a series of geologic columns and descriptive lithologic information for the lower impactite and crystalline-rock sections in the cores. The lowermost cored section (1766–1551 m depth) is a complex assemblage of mica schists that commonly contain graphite and fibrolitic sillimanite, intrusive granite pegmatites that grade into coarse granite, and local zones of mylonitic deformation. This basement-derived section is variably overprinted by brittle cataclastic fabrics and locally cut by dikes of polymict impact breccia, including several suevite dikes. An overlying succession of suevites and lithic impact breccias (1551–1397 m) includes a lower section dominated by polymict lithic impact breccia with blocks (up to 17 m) and boulders of cataclastic gneiss and an upper section (above 1474 m) of suevites and clast-rich impact melt rocks. The uppermost suevite is overlain by 26 m (1397–1371 m) of gravelly quartz sand that contains an amphibolite block and boulders of cataclasite and suevite. Above the sand, a 275-m-thick allochthonous granite slab (1371–1096 m) includes gneissic biotite granite, fine- and medium-to-coarse–grained biotite granites, and red altered granite near the base. The granite slab is overlain by more gravelly sand, and both are attributed to debris-avalanche and/or rockslide deposition that slightly preceded or accompanied seawater-resurge into the collapsing transient crater.
During 2005–2006, the International Continental Scientific Drilling Program and the U.S. Geological Survey drilled three continuous core holes into the Chesapeake Bay impact structure to a total depth of 1766.3 m. A collection of supplemental materials that presents a record of the core recovery and measurement data for the Eyreville cores is available on CD-ROM at the end of this volume and in the GSA Data Repository. The supplemental materials on the CD-ROM include digital photographs of each core box from the three core holes, tables of the three coring-run logs, as recorded on site, and a set of depth-conversion programs. In this chapter, the contents, purposes, and basic applications of the supplemental materials are briefly described. With this information, users can quickly decide if the materials will apply to their specific research needs.
The U.S. Geological Survey (USGS) acquired two 1.4-km-long, high-resolution (~5 m vertical resolution) seismic-reflection lines in 2006 that cross near the International Continental Scientific Drilling Program (ICDP)–USGS Eyreville deep drilling site located above the late Eocene Chesapeake Bay impact structure in Virginia, USA. Five-meter spacing of seismic sources and geophones produced high-resolution images of the subsurface adjacent to the 1766-m-depth Eyreville core holes. Analysis of these lines, in the context of the core hole stratigraphy, shows that moderate-amplitude, discontinuous, dipping reflections below ~527 m correlate with a variety of Chesapeake Bay impact structure sediment and rock breccias recovered in the cores. High-amplitude, continuous, subhorizontal reflections above ~527 m depth correlate with the uppermost part of the Chesapeake Bay impact structure crater-fill sediments and postimpact Eocene to Pleistocene sediments. Reflections with ~20–30 m of relief in the uppermost part of the crater-fill and lowermost part of the postimpact section suggest differential compaction of the crater-fill materials during early postimpact time. The top of the crater-fill section also shows ~20 m of relief that appears to represent an original synimpact surface. Truncation surfaces, locally dipping reflections, and depth variations in reflection amplitudes generally correlate with the lithostrati-graphic and sequence-stratigraphic units and contacts in the core. Seismic images show apparent postimpact paleochannels that include the first possible Miocene paleochannels in the Mid-Atlantic Coastal Plain. Broad downwarping in the postim-pact section unrelated to structures in the crater fill indicates postimpact sediment compaction.
An unusually thick section of sedimentary breccias dominated by target-sediment clasts is a distinctive feature of the late Eocene Chesapeake Bay impact structure. A cored 1766-m-deep section recovered from the central part of this marine-target structure by the International Continental Scientific Drilling Program (ICDP)–U.S. Geological Survey (USGS) drilling project contains 678 m of these breccias and associated sediments and an intervening 275-m-thick granite slab. Two sedimentary breccia units consist almost entirely of Cretaceous nonmarine sediments derived from the lower part of the target sediment layer. These sediments are present as coherent clasts and as autoclastic matrix between the clasts. Primary (Cretaceous) sedimentary structures are well preserved in some clasts, and liquefaction and fluidization structures produced at the site of deposition occur in the clasts and matrix. These sedimentary breccias are interpreted as one or more rock avalanches from the upper part of the transient-cavity wall. The little-deformed, unshocked granite slab probably was transported as part of an extremely large slide or avalanche. Water-saturated Cretaceous quartz sand below the slab was transported into the seafloor crater prior to, or concurrently with, the granite slab. Two sedimentary breccia units consist of polymict diamictons that contain cobbles, boulders, and blocks of Cretaceous nonmarine target sediments and less common shocked-rock and melt ejecta in an unsorted, unstratified, muddy, fossiliferous, glauconitic quartz matrix. Much of the matrix material was derived from Upper Cretaceous and Paleogene marine target sediments. These units are interpreted as the deposits of debris flows initiated by the resurge of ocean water into the seafloor crater. Interlayering of avalanche and debris-flow units indicates a partial temporal overlap of the earlier avalanche and later resurge processes. A thin unit of stratified turbidite deposits and overlying laminated fine-grained deposits at the top of the section represents the transition to normal shelf sedimentation.
Collapse and inward slumping of unconsolidated sedimentary strata expanded the Chesapeake Bay impact structure far beyond its central basement crater. During crater collapse, sediment-loaded water surged back to fill the crater. Here, we analyze clast frequency and granulometry of these resurge deposits in one core hole from the outermost part of the collapsed zone (i.e., Langley) as well as a core hole from the moat of the basement crater (i.e., Eyreville A). Comparisons of clast provenance and flow dynamics show that at both locations, there is a clear change in clast frequency and size between a lower unit, which we interpret to be dominated by slumped material, and an upper, water-transported unit, i.e., resurge deposit. The contribution of material to the resurge deposit was primarily controlled by stripping and erosion. This includes entrainment of fallback ejecta and sediments eroded from the surrounding seafloor, found to be dominant at Langley, and slumped material that covered the annular trough and basement crater, found to be dominant at Eyreville. Eyreville shows a higher content of crystalline clasts than Langley. There is equivocal evidence for an anti-resurge from a collapsing central water plume or, alternatively, a second resurge pulse, as well as a transition into oscillating resurge. The resurge material shows more of a debris-flow–like transport compared to resurge deposits at some other marine target craters, where the ratio of sediment to water has been relatively low. This result is likely a consequence of the combination of easily disaggregated host sediments and a relatively shallow target water depth.
Biostratigraphic analysis of sedimentary breccias and diamictons in the Chesa-peake Bay impact structure provides information regarding the timing and processes of late-stage gravitational crater collapse and ocean resurge. Studies of calcareous nannofossil and palynomorph assemblages in the International Continental Scientific Drilling Program (ICDP)–U.S. Geological Survey (USGS) Eyreville A and B cores show the mixed-age, mixed-preservation microfossil assemblages that are typical of deposits from the upper part of the Chesapeake Bay impact structure. Sparse, poorly preserved, possibly thermally altered pollen is present within a gravelly sand interval below the granite slab at 1392 m in Eyreville core B, an interval that is otherwise barren of calcareous nannofossils and dinocysts. Gravitational collapse of water- saturated sediments from the transient crater wall resulted in the deposition of sediment clasts primarily derived from the nonmarine Cretaceous Potomac Formation. Collapse occurred before the arrival of resurge. Low pollen Thermal Alteration Index (TAI) values suggest that these sediments were not thermally altered by contact with the melt sheet. The arrival of resurge sedimentation is identified based on the presence of diamicton zones and stringers rich in glauconite and marine microfossils at 866.7 m. This horizon can be traced across the crater and can be used to identify gravitational collapse versus ocean-resurge sedimentation. Glauconitic quartz sand diamicton dominates the sediments above 618.2 m. Calcareous nannofossil and dino-flagellate data from this interval suggest that the earliest arriving resurge from the west contained little or no Cretaceous marine input, but later resurge pulses mined Cretaceous sediments east of the Watkins core in the annular trough. Additionally, the increased distance traveled by resurge to the central crater in turbulent flow conditions resulted in the disaggregation of Paleogene unconsolidated sediments. As a result, intact Paleogene clasts in Eyreville cores are rare, but clasts of semilithified Potomac Formation silts and clays are common.
The Eyreville core holes provide the first continuously cored record of postimpact sequences from within the deepest part of the central Chesapeake Bay impact crater. We analyzed the upper Eocene to Pliocene postimpact sediments from the Eyreville A and C core holes for lithology (semiquantitative measurements of grain size and composition), sequence stratigraphy, and chronostratigraphy. Age is based primarily on Sr isotope stratigraphy supplemented by biostratigraphy (dinocysts, nannofossils, and planktonic foraminifers); age resolution is approximately ±0.5 Ma for early Miocene sequences and approximately ±1.0 Ma for younger and older sequences. Eocene–lower Miocene sequences are subtle, upper middle to lower upper Miocene sequences are more clearly distinguished, and upper Miocene–Pliocene sequences display a distinct facies pattern within sequences. We recognize two upper Eocene, two Oligocene, nine Miocene, three Pliocene, and one Pleistocene sequence and correlate them with those in New Jersey and Delaware. The upper Eocene through Pleistocene strata at Eyreville record changes from: (1) rapidly deposited, extremely fine-grained Eocene strata that probably represent two sequences deposited in a deep (>200 m) basin; to (2) highly dissected Oligocene (two very thin sequences) to lower Miocene (three thin sequences) with a long hiatus; to (3) a thick, rapidly deposited (43–73 m/Ma), very fine-grained, biosiliceous middle Miocene (16.5–14 Ma) section divided into three sequences (V5–V3) deposited in middle neritic paleoenvironments; to (4) a 4.5-Ma-long hiatus (12.8–8.3 Ma); to (5) sandy, shelly upper Miocene to Pliocene strata (8.3–2.0 Ma) divided into six sequences deposited in shelf and shoreface environments; and, last, to (6) a sandy middle Pleistocene paralic sequence (~400 ka). The Eyreville cores thus record the filling of a deep impact-generated basin where the timing of sequence boundaries is heavily influenced by eustasy.
The Eyreville and Exmore, Virginia, core holes were drilled in the inner basin and annular trough, respectively, of the Chesapeake Bay impact structure, and they allow us to evaluate sequence deposition in an impact crater. We provide new high-resolution geochronologic (<1 Ma) and sequence-stratigraphic interpretations of the Exmore core, identify 12 definite (and four possible) postimpact depositional sequences, and present comparisons with similar results from Eyreville and other mid-Atlantic core holes. The concurrence of increases in δ 18 O with Chesapeake Bay impact structure sequence boundaries indicates a primary glacioeustatic control on deposition. However, regional comparisons show the differential preservation of sequences across the mid-Atlantic margin. We explain this distribution by the compaction of impactites, regional sediment-supply changes, and the differential movement of basement structures. Upper Eocene strata are thin or missing updip and around the crater, but they thicken into the inner basin (and offshore to the southeast) due to rapid crater infilling and concurrent impactite compaction. Oligocene sequences are generally thin and highly dissected throughout the mid-Atlantic region due to sediment starvation and tectonism, except in southeastern New Jersey. Regional tectonic uplift of the Norfolk Arch coupled with a southward decrease in sediment supply resulted in: (1) largely absent Lower Miocene sections around the Chesapeake Bay impact structure compared to thick sections in New Jersey and Delaware; (2) thick Middle Miocene sequences across the Delmarva Peninsula that thin south of the Chesapeake Bay impact structure; and (3) upper Middle Miocene sections that pinch out just north of the Chesapeake Bay impact structure. Conversely, the Upper Miocene–Pliocene section is thick across Virginia, but it is poorly represented in New Jersey because of regional variations in relative subsidence.
Impact effects and regional tectonic insights: Backstripping the Chesapeake Bay impact structure
Origin and emplacement of impactites in the Chesapeake Bay impact structure, Virginia, USA
The late Eocene Chesapeake Bay impact structure, located on the Atlantic margin of Virginia, may be Earth's best-preserved large impact structure formed in a shallow marine, siliciclastic, continental-shelf environment. It has the form of an inverted sombrero in which a central crater ∼40 km in diameter is surrounded by a shallower brim, the annular trough, that extends the diameter to ∼85 km. The annular trough is interpreted to have formed largely by the collapse and mobilization of weak sediments. Crystalline-clast suevite, found only in the central crater, contains clasts and blocks of shocked gneiss that likely were derived from the fragmentation of the central-uplift basement. The suevite and entrained megablocks are interpreted to have formed from impact-melt particles and crystalline-rock debris that never left the central crater, rather than as a fallback deposit. Impact-modified sediments in the annular trough include megablocks of Cretaceous nonmarine sediment disrupted by faults, fluidized sands, fractured clays, and mixed-sediment intercalations. These impact-modified sediments could have formed by a combination of processes, including ejection into and mixing of sediments in the water column, rarefaction-induced fragmentation and clastic injection, liquefaction and fluidization of sand in response to acousticwave vibrations, gravitational collapse, and inward lateral spreading. The Exmore beds, which blanket the entire crater and nearby areas, consist of a lower diamicton member overlain by an upper stratified member. They are interpreted as unstratified ocean-resurge deposits, having depositional cycles that may represent stages of inward resurge or outward anti-resurge flow, overlain by stratified fallout of suspended sediment from the water column.