ABSTRACT
On the basis of a literature review and impact-geologic theoretical considerations, it is argued that
the loams and gravels that blanket ~10,000 km2 of the U.S. Middle-Atlantic Coastal Plain in an arc
about the buried Late-Eocene, 90-km-diameter Chesapeake Bay structure display morphologic,
lithologic, and stratigraphic features that are consistent with their being ejecta from that crater and
are absolutely incompatible with the currently held belief that they are the work of rivers in Late-
Miocene or more recent times. Also supporting this conclusion is a wide variety of unusual clasts
found within the upland deposits that both individually and as a group have no other interpretation
than as being ejecta from a crater whose target area included (1) minor pelagic limestone overlying
(2) a deep accumulation of siliciclastic sediments ranging from the finest silts to well rounded
pebbles, cobbles, and boulders largely comprised of Paleozoic quartzite and (3) a granite component
within the crystalline basement.
INTRODUCTION
From the moment of the buried structure’s discovery, the conventional wisdom has been that the ejecta
blanket of the 35.5-Ma, 90-km-diameter Chesapeake Bay crater (Poag et al., 1994; Koeberl et al., 1996;
Poag 1997; Poag et al., 2004; Gohn et al., 2008) had long since been removed by erosion from the surface
of the U.S. Middle-Atlantic Coastal Plain and the abutting Piedmont province. All seminal papers, books,
and authoritative reviews of the subject tacitly imply that not even a scrap of ejecta has ever been found on
the present-day surface.
Notwithstanding, there did arise one “voice crying in the wilderness.” For 9 years and counting I have
been gathering evidence that ~5,000 km2 of “upland deposits” mapped in Southern Maryland, Virginia, and
the District of Columbia are far better interpreted as crater ejecta than as fluvial deposits (Griscom, 1999,
2001, 2002, 2003a, 2007). Most significantly, Griscom et al. (2003a) employed a wide range of solid-state-
physical methods to demonstrate that a type of iron ore endemic to the “upland deposits” is a peculiar
nanocrystalline form of “ferric oxyhydroxide” characterized by atomic arrangements similar to Goethite but
hardness closer to that of quartz. In retrospect, this material might turn out to be better described as a
unique form of ferrihydrite (Michel et al., 2007). Thin-section photomicroscopy has revealed these hard
ferric oxyhydroxides (ferrihydrites?) to comprise the matrix material of a type of matrix-supported breccia
ubiquitous to the upland deposits, leading Griscom et al. (2003a) to argue that (1) they must be melt-matrix
breccias because iron oxide precipitates too slowly from solution to solidify around free falling clasts and
therefore that (2) they must be impactoclastic in origin because there are no known igneous rocks
composed of 95% pure iron oxide.
I begin the present paper with a review of the lithology of the “upland deposits” and critically discuss their
geomorphogeny in relation to the uniformitarian explanation historically assigned to them. I then compare
their materials properties and geomorphography to quantitative predictions of Chesapeake-Bay-crater ejecta-
blanket thicknesses and properties expected on the basis of well-established experimental, observational, and
theoretical principles of impact geology (Melosh, 1989). In addition, I report field studies of a diamict
siliciclastic stratum discovered topographically below the upland deposits in Northern Virginia and several
unusual clasts found therein, which I argue can only be interpreted as crater ejecta. And finally I describe a
unique granite boulder found within the upland deposits, which I argue is a meteorite from the Earth
(Gladman et al., 1996) – almost certainly a fragment of basement rock ejected from the interference zone of
the Chesapeake Bay impactor.
THE UPLAND DEPOSITS: NOMENCLATURE AND HISTORY
The “upland deposits,” sometimes termed “upland gravels” or “upland terrace gravel,” are not consistently
designated by a single formation name, despite their having been mapped over an area as large as 5,000
km2. Therefore, I will employ the nomenclature “upland deposits” without the quotation marks to denote
the particular lithofacies to be described below, wherever they may have been mapped (Fig. 1), irrespective
of any other names that may have been historically assigned to them. The upland deposits of the Patuxent-
Potomac peninsula of Southern Maryland are denoted the Brandywine formation (formerly the Lafayette),
whereas lithologically similar upland deposits blanketing lower terraces have traditionally been assigned
different formation names according to their relative elevations, the most common example being the
Sunderland at 52 m (e.g., Schlee, 1957). Ironically, the scarp-free Brandywine formation of Southern
Maryland slopes gently from its highest elevation of ~82 m east of the District of Columbia down to just
~30 m about 100 km to the southeast.
On the basis of a literature review and impact-geologic theoretical considerations, it is argued that
the loams and gravels that blanket ~10,000 km2 of the U.S. Middle-Atlantic Coastal Plain in an arc
about the buried Late-Eocene, 90-km-diameter Chesapeake Bay structure display morphologic,
lithologic, and stratigraphic features that are consistent with their being ejecta from that crater and
are absolutely incompatible with the currently held belief that they are the work of rivers in Late-
Miocene or more recent times. Also supporting this conclusion is a wide variety of unusual clasts
found within the upland deposits that both individually and as a group have no other interpretation
than as being ejecta from a crater whose target area included (1) minor pelagic limestone overlying
(2) a deep accumulation of siliciclastic sediments ranging from the finest silts to well rounded
pebbles, cobbles, and boulders largely comprised of Paleozoic quartzite and (3) a granite component
within the crystalline basement.
INTRODUCTION
From the moment of the buried structure’s discovery, the conventional wisdom has been that the ejecta
blanket of the 35.5-Ma, 90-km-diameter Chesapeake Bay crater (Poag et al., 1994; Koeberl et al., 1996;
Poag 1997; Poag et al., 2004; Gohn et al., 2008) had long since been removed by erosion from the surface
of the U.S. Middle-Atlantic Coastal Plain and the abutting Piedmont province. All seminal papers, books,
and authoritative reviews of the subject tacitly imply that not even a scrap of ejecta has ever been found on
the present-day surface.
Notwithstanding, there did arise one “voice crying in the wilderness.” For 9 years and counting I have
been gathering evidence that ~5,000 km2 of “upland deposits” mapped in Southern Maryland, Virginia, and
the District of Columbia are far better interpreted as crater ejecta than as fluvial deposits (Griscom, 1999,
2001, 2002, 2003a, 2007). Most significantly, Griscom et al. (2003a) employed a wide range of solid-state-
physical methods to demonstrate that a type of iron ore endemic to the “upland deposits” is a peculiar
nanocrystalline form of “ferric oxyhydroxide” characterized by atomic arrangements similar to Goethite but
hardness closer to that of quartz. In retrospect, this material might turn out to be better described as a
unique form of ferrihydrite (Michel et al., 2007). Thin-section photomicroscopy has revealed these hard
ferric oxyhydroxides (ferrihydrites?) to comprise the matrix material of a type of matrix-supported breccia
ubiquitous to the upland deposits, leading Griscom et al. (2003a) to argue that (1) they must be melt-matrix
breccias because iron oxide precipitates too slowly from solution to solidify around free falling clasts and
therefore that (2) they must be impactoclastic in origin because there are no known igneous rocks
composed of 95% pure iron oxide.
I begin the present paper with a review of the lithology of the “upland deposits” and critically discuss their
geomorphogeny in relation to the uniformitarian explanation historically assigned to them. I then compare
their materials properties and geomorphography to quantitative predictions of Chesapeake-Bay-crater ejecta-
blanket thicknesses and properties expected on the basis of well-established experimental, observational, and
theoretical principles of impact geology (Melosh, 1989). In addition, I report field studies of a diamict
siliciclastic stratum discovered topographically below the upland deposits in Northern Virginia and several
unusual clasts found therein, which I argue can only be interpreted as crater ejecta. And finally I describe a
unique granite boulder found within the upland deposits, which I argue is a meteorite from the Earth
(Gladman et al., 1996) – almost certainly a fragment of basement rock ejected from the interference zone of
the Chesapeake Bay impactor.
THE UPLAND DEPOSITS: NOMENCLATURE AND HISTORY
The “upland deposits,” sometimes termed “upland gravels” or “upland terrace gravel,” are not consistently
designated by a single formation name, despite their having been mapped over an area as large as 5,000
km2. Therefore, I will employ the nomenclature “upland deposits” without the quotation marks to denote
the particular lithofacies to be described below, wherever they may have been mapped (Fig. 1), irrespective
of any other names that may have been historically assigned to them. The upland deposits of the Patuxent-
Potomac peninsula of Southern Maryland are denoted the Brandywine formation (formerly the Lafayette),
whereas lithologically similar upland deposits blanketing lower terraces have traditionally been assigned
different formation names according to their relative elevations, the most common example being the
Sunderland at 52 m (e.g., Schlee, 1957). Ironically, the scarp-free Brandywine formation of Southern
Maryland slopes gently from its highest elevation of ~82 m east of the District of Columbia down to just
~30 m about 100 km to the southeast.
| In plain sight: The Chesapeake Bay crater ejecta blanket David L. Griscom impactGlass research international, 3938 E Grant Rd #131, Tucson, Arizona 85712-2559, USA |
Figure 1. Map of the upland deposits of Southern Maryland, Eastern Virginia, and the District
of Columbia and of the Bacons Castle formation of Eastern Virginia, displayed in relation to the
buried Chesapeake Bay crater. Mapped land surfaces were taken from Schlee (1957) and Frye
(1986). The gravity map of the impact structure with the crater rim indicated by dashes was
taken from Koeberl et al. (1996). Dashed circle at 115 km radius from the crater center at Cape
Charles, Va., is provided for visual reference.
of Columbia and of the Bacons Castle formation of Eastern Virginia, displayed in relation to the
buried Chesapeake Bay crater. Mapped land surfaces were taken from Schlee (1957) and Frye
(1986). The gravity map of the impact structure with the crater rim indicated by dashes was
taken from Koeberl et al. (1996). Dashed circle at 115 km radius from the crater center at Cape
Charles, Va., is provided for visual reference.
One good reason for not using the word “gravel” alone to characterize this particular upland unit is the
fact that the upland deposits comprise a bilayer consisting of a lower gravel member and an upper loam
member (Hack, 1955). The overall thickness of the upland deposits normally ranges from ~3 to ~10 m
(Hack, 1955), although exceptional thicknesses up to 15 m have been noted. According to Schlee (1957),
the gravel member is fairly well stratified and sedimentation units range from ~2 m down to ~5 cm in
thickness, while the contact with the overlying loam is gradational over ~0.5 to 1 m.
The origin of these unusual deposits had been controversial for well over a century when Schlee (1957)
interpreted his important new data in terms of Hack’s (1955) version of the fluvial-deposition model. In a
handy overview of the geology of Virginia, Frye (1986) describes the upland deposits in terms of the Hack
(1955) model, including in particular the conclusion that these deposits were laid down by rivers in
Miocene, Pliocene, or Pleistocene times, given that they overlie clay terraces long-ago dated as Miocene (see
also, Hack, 1955; Schlee, 1957). A recent check of web sites treating the U.S. Mid-Atlantic Coastal Plain
suggests that the Hack-Schlee model has not changed in the slightest since the discovery of the Chesapeake
Bay crater. Notably, however, the upland deposits themselves are generally devoid of fossils and therefore
have been dated solely on the basis of the inferred ages of the fossils in the shallow-water clays that
immediately underlie them ...without modern radiometric confirmation.
The loam member
According to Schlee (1957), the “massive” upper loam member consists chiefly of compacted silt with
some sand, clay, and scattered pebbles, and ranges in color from pale-yellowish-brown to grayish-orange to
moderate-yellowish-brown. Hack (1955) reported this loam to be about 90% quartz silt, with 2-5% heavy
minerals. My personal experience from living in a home perched on a steep embankment composed of this
upland loam is that it is so coherent that grass roots can scarcely penetrate it, yet even a steeply-sloped
surface partially denuded of grass showed no noticeable sign of pluvial erosion in 30 years.
The gravel member
Schlee (1957) dedicated his field work to elucidating the lithology of the gravel member of the upland
deposits, mainly in Southern Maryland (88 sites). However, he also sampled the District of Columbia (7
sites) and Northern Virginia (3 sites ~10 km west of Washington, D.C.). The samples analytically studied
by Griscom et al. (2003a), as well as several that I will illustrate for the first time below, were collected ~10
km south of Washington in Springfield, Va., and near Alexandria, Va. (roughly 15 km southeast of Schlee’s
3 Northern Virginia sites).
According to Schlee (1957), the gravel member is pale-to-dark-yellowish-orange to mottled pale-yellowish-
brown poorly sorted gravel and sandy gravel, as well as very pale-orange gravelly clay. “The gravel
member is fairly well stratified, and sedimentation units [ranging from 2 m to 5 cm thick] are easily
recognized.” “Where bedding is poor, sedimentation units up to [5 m] thick are present. Laterally the beds
range from [30 to 60 m] or more down to [3 m] or less in the case of sand lenses.”
Schlee (1957) continues “The gravel has a bimodal size distribution and consists of a gravel framework,
averaging 61% of the whole, and a matrix of sand, silt, and clay (emphasis added).” This fact led him to
conclude that the bed load of heavy gravel and the suspended load of sand, clay and silt were likely to have
been deposited simultaneously (very unusual, if not impossible for rivers). And he was perfectly right. His
carefully determined average framework of 61% is surely statistically valid, and this is significantly less than
the theoretical maximum for random close packing of spheres, i.e., 64% (Jaeger and Nagel, 1992).
fact that the upland deposits comprise a bilayer consisting of a lower gravel member and an upper loam
member (Hack, 1955). The overall thickness of the upland deposits normally ranges from ~3 to ~10 m
(Hack, 1955), although exceptional thicknesses up to 15 m have been noted. According to Schlee (1957),
the gravel member is fairly well stratified and sedimentation units range from ~2 m down to ~5 cm in
thickness, while the contact with the overlying loam is gradational over ~0.5 to 1 m.
The origin of these unusual deposits had been controversial for well over a century when Schlee (1957)
interpreted his important new data in terms of Hack’s (1955) version of the fluvial-deposition model. In a
handy overview of the geology of Virginia, Frye (1986) describes the upland deposits in terms of the Hack
(1955) model, including in particular the conclusion that these deposits were laid down by rivers in
Miocene, Pliocene, or Pleistocene times, given that they overlie clay terraces long-ago dated as Miocene (see
also, Hack, 1955; Schlee, 1957). A recent check of web sites treating the U.S. Mid-Atlantic Coastal Plain
suggests that the Hack-Schlee model has not changed in the slightest since the discovery of the Chesapeake
Bay crater. Notably, however, the upland deposits themselves are generally devoid of fossils and therefore
have been dated solely on the basis of the inferred ages of the fossils in the shallow-water clays that
immediately underlie them ...without modern radiometric confirmation.
The loam member
According to Schlee (1957), the “massive” upper loam member consists chiefly of compacted silt with
some sand, clay, and scattered pebbles, and ranges in color from pale-yellowish-brown to grayish-orange to
moderate-yellowish-brown. Hack (1955) reported this loam to be about 90% quartz silt, with 2-5% heavy
minerals. My personal experience from living in a home perched on a steep embankment composed of this
upland loam is that it is so coherent that grass roots can scarcely penetrate it, yet even a steeply-sloped
surface partially denuded of grass showed no noticeable sign of pluvial erosion in 30 years.
The gravel member
Schlee (1957) dedicated his field work to elucidating the lithology of the gravel member of the upland
deposits, mainly in Southern Maryland (88 sites). However, he also sampled the District of Columbia (7
sites) and Northern Virginia (3 sites ~10 km west of Washington, D.C.). The samples analytically studied
by Griscom et al. (2003a), as well as several that I will illustrate for the first time below, were collected ~10
km south of Washington in Springfield, Va., and near Alexandria, Va. (roughly 15 km southeast of Schlee’s
3 Northern Virginia sites).
According to Schlee (1957), the gravel member is pale-to-dark-yellowish-orange to mottled pale-yellowish-
brown poorly sorted gravel and sandy gravel, as well as very pale-orange gravelly clay. “The gravel
member is fairly well stratified, and sedimentation units [ranging from 2 m to 5 cm thick] are easily
recognized.” “Where bedding is poor, sedimentation units up to [5 m] thick are present. Laterally the beds
range from [30 to 60 m] or more down to [3 m] or less in the case of sand lenses.”
Schlee (1957) continues “The gravel has a bimodal size distribution and consists of a gravel framework,
averaging 61% of the whole, and a matrix of sand, silt, and clay (emphasis added).” This fact led him to
conclude that the bed load of heavy gravel and the suspended load of sand, clay and silt were likely to have
been deposited simultaneously (very unusual, if not impossible for rivers). And he was perfectly right. His
carefully determined average framework of 61% is surely statistically valid, and this is significantly less than
the theoretical maximum for random close packing of spheres, i.e., 64% (Jaeger and Nagel, 1992).
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v5.1 12/09/08
In Plain Sight
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