CHESAPEAKE IMPACT CRATER
by: Charles O’Dale
- Type: Complex
- Age ma: 35.5 ± .3 a – EOCENE
- Diameter: ~85 b km
- Location: N 37° 17’ W 76° 01’
- Shock Metamorphism: breccia matrix includes trace quantities of shocked quartz (Poag).
a The Exmore Formation breccia occurs only in subsurface in and around the Chesapeake Bay impact structure and is of late Eocene age based on analyses of planktonic foraminifers and calcareous nannofossils (Poag).
b The rim of the Chesapeake Crater is a boundary between salty ground water within the crater’s confines and fresh ground water on the outside.
About 35 million years ago a 3-5 kilometres in diameter impactor hit the western Atlantic Ocean on a shallow shelf, creating the Chesapeake Bay impact crater. At this time the sea level was much higher and the coastline was in the vicinity of Richmond, Virginia. The crater is approximately 200 km southeast of Washington, D.C. and is now buried 300-500 metres beneath the southern part of Chesapeake Bay. Analysis of seismic profiling has determined that the crater is 85km in diameter and 1.3km deep. It is a complex peak-ring crater with an inner and outer rim, a relatively flat-floored annular trough, and an inner basin that penetrates the basement. The inner basin includes a central uplift surrounded by a series of concentric valleys and ridges.
A 1.3km thick rubble bed of impact breccia fills the crater and forms a thin ejecta blanket around it. Compaction of this breccia produced a subsidence differential, causing the land surface over the breccia to remain lower than the land surface over sediments outside the crater. Another consequence of the impact is that all ground-water aquifers were truncated and excavated by the impact. In place of those aquifers is a reservoir of briny water that is 1.5 times saltier than normal seawater.
Most rivers in the area, like the Rappahannock, flow southeastward to the Atlantic. In contrast, the York and James rivers make sharp turns to the northeast where the outer rim of the crater traverses the lower York-James Peninsula. The abrupt diversions of the lower courses of the James and York Rivers (indicated by the small circles in the map above) coincide with the Chesapeake crater rim. The cause of these diversions is the differential subsidence of the outlaying country rock compared to the breccia within the Chesapeake Bay impact crater forcing a structural sag over the subsiding breccia. The river diversions are at the “rim” of this sag.
From September to December 2005, ICDP in conjunction with the United States Geological Survey drilled a deep borehole, which had a target depth of 2.2 km, into the Chesapeake Bay impact structure, Virginia, USA. Chesapeake Bay, at ca. 85-90 km diameter (Poag et al. 2004), is among the Earth’s largest and, at 35 Ma age, one of the best preserved impact structures known on Earth. It was formed within a 3-layer target, crystalline basement overlain by a well stratified sedimentary cover sequence, in turn below a shallow ocean of ca. 200 m water depth. Thus, the target sequence is very similar to that of the Chicxulub impact, although the water depth for Chesapeake Bay crater was much larger. The Chesapeake Bay structure is of interest for a number of geodisciplines. Its location on a passive continental margin has prevented post-impact tectonic disturbance. Marine deposition resumed immediately after the impact, leading to rapid burial of the impact formations and thus, good preservation. The upper part of the within-crater breccia lens has been extensively reworked by immediately post-impact environmental forces, including high-energy currents and possibly tsunami. Drilling was done into the crater moat, but close to the central uplift, to obtain as thick and as undisturbed a post-impact sequence of impactites and post-impact sediment as possible. The goal was to reach the crater floor, mainly in order to study shock barometry, hydrothermal effects below the crater, and possible breccia injections/in situ brecciation.
Coesite in suevites from the Chesapeake Bay impact structure c
John C. Jackson, J. Wright Horton Jr., I-Ming Chou, Harvey E. Belkin
The occurrence of coesite in suevites from the Chesapeake Bay impact structure is confirmed within a variety of textural domains in situ by Raman spectroscopy for the first time and in mechanically separated grains by X-ray diffraction. Microtextures of coesite identified in situ investigated under transmitted light and by scanning electron microscope reveal coesite as micrometer-sized grains (1–3 μm) within amorphous silica of impact-melt clasts and as submicrometer-sized grains and polycrystalline aggregates within shocked quartz grains. Coesite-bearing quartz grains are present both idiomorphically with original grain margins intact and as highly strained grains that underwent shock-produced plastic deformation. Coesite commonly occurs in plastically deformed quartz grains within domains that appear brown (toasted) in transmitted light and rarely within quartz of spheroidal texture. The coesite likely developed by a mechanism of solid-state transformation from precursor quartz. Raman spectroscopy also showed a series of unidentified peaks associated with shocked quartz grains that likely represent unidentified silica phases, possibly including a moganite-like phase that has not previously been associated with coesite.
c Metastable preservation of coesite and stishovite requires rapid cooling prior to amorphization. Stishovite is unstable above about 300-600°C, whereas coesite is stable up to about 1100°C, suggesting that the quartz grains studied here were quenched at relatively high postshock temperatures exceeding the stability range of stishovite, but within the stability range facilitating preservation of coesite.
Meteoritics & Planetary Science 24 March 2016
Establishing the link between the Chesapeake Bay impact structure
and the North American tektite strewn field: The Sr-Nd isotopic evidence
Alexander DEUTSCH1, and Christian KOEBERL
Abstract— The Chesapeake Bay impact structure, which is about 35 Ma old, has previously been proposed as the possible source crater of the North American tektites (NAT). Here we report major and trace element data as well as the first Sr-Nd isotope data for drill core and outcrop samples of target lithologies, crater fill breccias, and post-impact sediments of the Chesapeake Bay impact structure. The unconsolidated sediments, Cretaceous to middle Eocene in age, have ∍Srt = 35.7 Ma of +54 to +272, and ∍Ndt = 35.7 Ma ranging from −6.5 to −10.8; one sample from the granitic basement with a TNdCHUR model age of 1.36 Ga yielded an ∍Srt = 35.7 Ma of +188 and an ∍Ndt = 35.7 Ma of −5.7. The Exmore breccia (crater fill) can be explained as a mix of the measured target sediments and the granite, plus an as-yet undetermined component. The post-impact sediments of the Chickahominy formation have slightly higher TNdCHUR model ages of about 1.55 Ga, indicating a contribution of some older materials. Newly analyzed bediasites have the following isotope parameters: +104 to +119 (∍Srt = 35.7 Ma), −5.7 (∍Ndt = 35.7 Ma), 0.47 Ga (TSrUR), and 1.15 Ga (TNdCHUR), which is in excellent agreement with previously published data for samples of the NAT strewn field. Target rocks with highly radiogenic Sr isotopic composition, as required for explaining the isotopic characteristics of Deep Sea Drilling Project (DSDP) site 612 tektites, were not among the analyzed sample suite. Based on the new isotope data, we exclude any relation between the NA tektites and the Popigai impact crater, although they have identical ages within 2s̀ errors. The Chesapeake Bay structure, however, is now clearly constrained as the source crater for the North American tektites, although the present data set obviously does not include all target lithologies that have contributed to the composition of the tektites.
Meteoritics & Planetary Science 41, Nr 5, 689–703 (2006)
Confirmation of a meteoritic component in impact-melt rocks of the Chesapeake Bay impact structure, Virginia, USA – Evidence from osmium isotopic and PGE systematics
S.R. Lee, J.W. Horton Jr., and R.J. Walker
The osmium isotope ratios and platinum-group element (PGE) concentrations of impact-melt rocks in the Chesapeake Bay impact structure were determined. The impact-melt rocks come from the cored part of a lower-crater section of suevitic crystalline-clast breccia in an 823 m scientific test hole over the central uplift at Cape Charles, Virginia. The 187Os/188Os ratios of impact-melt rocks range from 0.151 to 0.518. The rhenium and platinum-group element (PGE) concentrations of these rocks are 30-270?? higher than concentrations in basement gneiss, and together with the osmium isotopes indicate a substantial meteoritic component in some impact-melt rocks. Because the PGE abundances in the impact-melt rocks are dominated by the target materials, interelemental ratios of the impact-melt rocks are highly variable and nonchondritic. The chemical nature of the projectile for the Chesapeake Bay impact structure cannot be constrained at this time. Model mixing calculations between chondritic and crustal components suggest that most impact-melt rocks include a bulk meteoritic component of 0.01-0.1% by mass. Several impact-melt rocks with lowest initial 187Os/188Os ratios and the highest osmium concentrations could have been produced by additions of 0.1%-0.2% of a meteoritic component. In these samples, as much as 70% of the total Os may be of meteoritic origin. At the calculated proportions of a meteoritic component (0.01-0.1% by mass), no mixtures of the investigated target rocks and sediments can reproduce the observed PGE abundances of the impact-melt rocks, suggesting that other PGE enrichment processes operated along with the meteoritic contamination. Possible explanations are 1) participation of unsampled target materials with high PGE abundances in the impact-melt rocks, and 2) variable fractionations of PGE during syn- to post-impact events.
The Meteoritical Society, 2006.
Petrography, mineralogy, and geochemistry of deep gravelly sands in the Eyreville B core, Chesapeake Bay impact structure
Katerina Bartosova, Susanne Gier, J. Wright Horton Jr., Christian Koeberl, Dieter Mader, and Henning Dypvik
The ICDP–USGS Eyreville drill cores in the Chesapeake Bay impact structure reached a total depth of 1766 m and comprise (from the bottom upwards) basement-derived schists and granites/pegmatites, impact breccias, mostly poorly lithified gravelly sand and crystalline blocks, a granitic slab, sedimentary breccias, and postimpact sediments. The gravelly sand and crystalline block section forms an approximately 26 m thick interval that includes an amphibolite block and boulders of cataclastic gneiss and suevite. Three gravelly sands (basal, middle, and upper) are distinguished within this interval. The gravelly sands are poorly sorted, clast supported, and generally massive, but crude size-sorting and subtle, discontinuous layers occur locally. Quartz and K-feldspar are the main sand-size minerals and smectite and kaolinite are the principal clay minerals. Other mineral grains occur only in accessory amounts and lithic clasts are sparse (only a few vol%). The gravelly sands are silica rich (~80 wt% SiO2). Trends with depth include a slight decrease in SiO2 and slight increase in Fe2O3. The basal gravelly sand (below the cataclasite boulder) has a lower SiO2 content, less K-feldspar, and more mica than the higher sands, and it contains more lithic clasts and melt particles that are probably reworked from the underlying suevite. The middle gravelly sand (below the amphibolite block) is finer-grained, contains more abundant clay minerals, and displays more variable chemical compositions than upper gravelly sand (above the block). Our mineralogical and geochemical results suggest that the gravelly sands are avalanche deposits derived probably from the nonmarine Potomac Formation in the lower part of the target sediment layer, in contrast to polymict diamictons higher in the core that have been interpreted as ocean-resurge debris flows, which is in agreement with previous interpretations. The mineralogy and geochemistry of the gravelly sands are typical for a passive continental margin source. There is no discernible mixing with marine sediments (no glauconite or Paleogene marine microfossils noted) during the impact remobilization and redeposition. The unshocked amphibolite block and cataclasite boulder might have originated from the outer parts of the transient crater.
Meteoritics and Planetary Science 2010
The Toms Canyon structure, New Jersey outer continental shelf: A possible late Eocene impact crater
C.Wylie PoagL.J. Poppe
The Toms Canyon structure (~ 20–22 km wide) is located on the New Jersey outer continental shelf beneath 80–100 m of water, and is buried by ~ 1 km of upper Eocene to Holocene sedimentary strata. The structure displays several characteristics typical of terrestrial impact craters (flat floor; upraised faulted rim; brecciated sedimentary fill), but several other characteristics are atypical (an unusually thin ejecta blanket; lack of an inner basin, peak ring, or central peak; being nearly completely filled with breccia). Seismostratigraphic and biostratigraphic analyses show that the structure formed during planktonic foraminiferal biochron P15 of the early to middle late Eocene. The fill unit is stratigraphically correlative with impact ejecta cored nearby at Deep Sea Drilling Project (DSDP) Site 612 and at Ocean Drilling Program (ODP) Sites 903 and 904 (22–35 km southeast of the Toms Canyon structure). The Toms Canyon fill unit also correlates with the Exmore breccia, which fills the much larger Chesapeake Bay impact crater (90-km diameter; 335 km to the southwest). On the basis of our analyses, we postulate that the Toms Canyon structure is an impact crater, formed when a cluster of relatively small meteorites approached the target site bearing ~N 50 °E, and struck the sea floor obliquely.
Deep Sea Drilling Project Site 612 bolide event: New evidence of a late Eocene impact-wave deposit and a possible impact site, US east coast
W. Wei, C. Wylie Poag, Lawrence J. Poppe, David W. Folger, David S. Powars, Robert B. Mixon, Lucy E. Edwards, andScott Bruce
A remarkable >60-m-thick, upward-fining, polymictic, marine boulder bed is distributed over >15 000 km2 beneath Chesapeake Bay and the surrounding Middle Atlantic Coastal Plain and inner continental shelf. The wide varieties of clast lithologies and microfossil assemblages were derived from at least seven known Cretaceous, Paleocene, and Eocene stratigraphic units. The supporting pebbly matrix contains variably mixed assemblages of microfossils along with trace quantities of impact ejecta. The youngest microfossils in the boulder bed are of early-late Eocene age. On the basis of its unusual characteristics and its stratigraphic equivalent to a layer of impact ejecta at Deep Sea Drilling Project (DSDP) Site 612. It is postulated that this boulder bed was formed by a powerful bolide-generated wave train that scoured the ancient inner shelf and coastal plain of southeastern Virginia.
- D.S. Powars and T.S. Bruce, USGS, Feb. 2000; THE EFFECTS OF THE CHESAPEAKE BAY IMPACT CRATER ON THE GEOLOGICAL FRAMEWORK AND CORRELATION OF HYDROGEOLOGIC UNITS OF THE LOWER YORK-JAMES PENINSULA, VIRGINIA
- Poag C. Wylie 1999, Chesapeake Invader
- C. Wylie Poag, Christian Koeberl, and Wolf Uwe Reimold; The Chesapeake Bay Crater: Geology and geophysics of a Late Eocene submarine impact structure USGS