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Did the impact sculpt the base of the upland deposits? I believe it entirely possible and even probable that
the jetting phase of an impact into shallow coastal waters would erase any pre-existing scarps imprinted on
soft coastal-plain sediments as former shorelines. That is, I suggest that the answer to question (1) above is
probably yes, but as yet I have no proof.
Do the Virginia upland deposits mimic those of Southern Maryland in a crater-centric way? The
following passage from Poag (1997) suggests that the answer to question number (2) is definitely yes:
“Above the [presumed] lower Miocene unit, coarser siliciclastic units of middle Miocene to Quaternary age
[i.e., the upland deposits and Bacons Castle fm.] are widespread throughout southeastern Virginia. Outside
the crater, these units thicken gradually as they dip gently to the southeast. But where they cross the crater
rim, the units abruptly thicken (moderately to slightly) and sag into the annular trough (emphasis and
bracketed words added).” The passage that I’ve italicized would certainly describe the Chesapeake Bay
crater ejecta blanket per se in its last stages of existence ...and I suggest that Homo sapiens in fact appeared
on the scene just in the nick of time to find it in exactly this condition!
Elsewhere Poag (1997) remarks: “The modern topography of the Chesapeake Bay region also appears to
reflect the buried crater’s influence (Peebles, 1984; Mixon, 1985; Mixon et al., 1989). For example, the
middle Pleistocene-upper Pleistocene contact approximates the position of the Suffolk scarp, a feature of 11-
22 m relief, which parallels the western rim of the crater (emphasis added).” This undisputed fact raises an
interesting question: Just how does a deeply buried 35.5-Ma crater exercise structural control over the
geomorphography of coastal-plain sediments supposedly emplaced by rivers 28 m.y. later and subsequently
subjected to the sea level transgressions and regressions of the Quaternary?
Is the Bacons Castle formation Chesapeake Bay crater ejecta? Figure 7 gives no clue, except possibly for
the seaward rising scarp seen in Figure 7D, which is unexpected for a coastal-plain terrace but is consistent
with its being a vestige of an originally (vastly) higher crater rim. But that is pure speculation. On the other
hand, the following information gleaned from Goodwin and Johnson (1970) is solid fact:
“One strong contrast noted by Wentworth (1928) between the Brandywine gravels [i.e., the Eastern Virginia
upland deposits] and lower, younger gravels [i.e., the Bacons Castle formation] was the absence of striated
boulders and cobbles in the higher level gravels and the presence of such striated boulders and cobbles in the
lower gravels.”
Apropos of that, I note that relatively rare striated rocks have been found in outcrops of Chicxulub-crater
ejecta slightly inside of 5 crater radii (Ocampo et al., 1996; Pope et al., 1999; King and Petruny, 2003),
although nothing is known of their radial distribution since no outcrops closer to the Chicxulub crater have
been found. King et al. (1997) have argued that such striations result from hypervelocity interactions among
clasts during excavation and ejection – consistent with the fact that ejected fragments “seldom interact with
one another” in the ejecta curtain (Melosh, 1989). Therefore, since the excavation-flow crater ejecta is
volumetrically so much greater than, and kinetically so much slower than the interference-zone ejecta
(which I will argue are the origin of the coarse upland gravels), striated rocks should be an increasingly
common occurrence in ejecta blankets as the crater rim is approached from great distance along a radial.
Indeed, the Bacons Castle formation, which is known to commonly contain striated cobbles and boulders
(Wentworth, 1928), occupies a ~120° segment of the 1-to-2.5-crater-radii annulus that can be drawn about
the center of the Chesapeake Bay structure (Fig. 1).
In Early Oligocene times, the rivers would have found courses circumferential to the ejecta blanket.
Presently, we see that the North Anna and Mattaponi Rivers are flowing perpendicular to the most southerly
radial section reproduced in Figure 7B (small black squares). Likewise, for the James and Chickahominy
Rivers in Figure 7C. And the same thing again for the Nottoway and Blackwater Rivers in Figure 7D. In
fact, all six of these rivers are, at least at their respective intersections with one of these radials, flowing
circumferentially about the Chesapeake Bay crater. Of course, most of these rivers do not flow very far
circumferentially before turning back in the direction of the crater. The exception is the Nottoway River,
which follows a 115-km radius for fully 60 km (Fig. 1)!
Note also that a V-shaped valley coincident with the mouth of the Chickahominy River extends all the way
across the James-York peninsula (Fig. 7C) where it directly faces the mouth of the Mattaponi River on the
York-Rappahannock peninsula (Fig. 7B). I propose that this groove could represent a paleochannel of the
York River (defined as the confluence of the Pamunkey and Mattaponi Rivers) dating to the period when it
had not yet breached the landward portion of the Chesapeake Bay crater rim (dashed arrow in Fig. 8A). If I
am correct in this, then the western crater rim would have survived for at least 2 m.y. until the Early-
Oligocene lowstand finally permitted the rivers to begin searching anew for their old channels.
Since all six of the rivers flowing circumferentially to the crater in Figure 7 happen to be moving in the
counterclockwise sense, one might guess that this may be the result of the Corriolis force acting on
southward flowing rivers. However, there may be an alternative explanation: an impenetrable radial barrier
on the York-Rappahannock peninsula. Remembering that my calculated ejecta-blanket profile neglects
dunes, hummocks, ridges, and rays, one must admit to the possibility of an extra-deep ray running up one of
the peninsulas radial to the crater center. But is there any evidence for this? Well, yes, it appears that there
is:
The Dragon Run watershed (Fig. 8A) is a raised-rim depression, which evidently has never during its
existence allowed a major river to enter it from the north or west – a true oddity, given the canonical view
that the U.S. Middle-Atlantic Coastal Plain gravels were all deposited by rivers within the past 10 m.y. (If
rivers should have a way to self-construct barriers around depressions, then New Orleans could be secured
without the Army Corps of Engineers!) It is seen in Figure 8A (Dragon Run Steering Committee, 2003) that
the Dragon Run watershed is almost perfectly radial to the center of the Chesapeake Bay crater and its
outline looks as though it could be decomposed into a sequence overlapping of ellipses – very similar to the
string of secondary craters of the lunar crater Copernicus reproduced in Figure 8B.
the jetting phase of an impact into shallow coastal waters would erase any pre-existing scarps imprinted on
soft coastal-plain sediments as former shorelines. That is, I suggest that the answer to question (1) above is
probably yes, but as yet I have no proof.
Do the Virginia upland deposits mimic those of Southern Maryland in a crater-centric way? The
following passage from Poag (1997) suggests that the answer to question number (2) is definitely yes:
“Above the [presumed] lower Miocene unit, coarser siliciclastic units of middle Miocene to Quaternary age
[i.e., the upland deposits and Bacons Castle fm.] are widespread throughout southeastern Virginia. Outside
the crater, these units thicken gradually as they dip gently to the southeast. But where they cross the crater
rim, the units abruptly thicken (moderately to slightly) and sag into the annular trough (emphasis and
bracketed words added).” The passage that I’ve italicized would certainly describe the Chesapeake Bay
crater ejecta blanket per se in its last stages of existence ...and I suggest that Homo sapiens in fact appeared
on the scene just in the nick of time to find it in exactly this condition!
Elsewhere Poag (1997) remarks: “The modern topography of the Chesapeake Bay region also appears to
reflect the buried crater’s influence (Peebles, 1984; Mixon, 1985; Mixon et al., 1989). For example, the
middle Pleistocene-upper Pleistocene contact approximates the position of the Suffolk scarp, a feature of 11-
22 m relief, which parallels the western rim of the crater (emphasis added).” This undisputed fact raises an
interesting question: Just how does a deeply buried 35.5-Ma crater exercise structural control over the
geomorphography of coastal-plain sediments supposedly emplaced by rivers 28 m.y. later and subsequently
subjected to the sea level transgressions and regressions of the Quaternary?
Is the Bacons Castle formation Chesapeake Bay crater ejecta? Figure 7 gives no clue, except possibly for
the seaward rising scarp seen in Figure 7D, which is unexpected for a coastal-plain terrace but is consistent
with its being a vestige of an originally (vastly) higher crater rim. But that is pure speculation. On the other
hand, the following information gleaned from Goodwin and Johnson (1970) is solid fact:
“One strong contrast noted by Wentworth (1928) between the Brandywine gravels [i.e., the Eastern Virginia
upland deposits] and lower, younger gravels [i.e., the Bacons Castle formation] was the absence of striated
boulders and cobbles in the higher level gravels and the presence of such striated boulders and cobbles in the
lower gravels.”
Apropos of that, I note that relatively rare striated rocks have been found in outcrops of Chicxulub-crater
ejecta slightly inside of 5 crater radii (Ocampo et al., 1996; Pope et al., 1999; King and Petruny, 2003),
although nothing is known of their radial distribution since no outcrops closer to the Chicxulub crater have
been found. King et al. (1997) have argued that such striations result from hypervelocity interactions among
clasts during excavation and ejection – consistent with the fact that ejected fragments “seldom interact with
one another” in the ejecta curtain (Melosh, 1989). Therefore, since the excavation-flow crater ejecta is
volumetrically so much greater than, and kinetically so much slower than the interference-zone ejecta
(which I will argue are the origin of the coarse upland gravels), striated rocks should be an increasingly
common occurrence in ejecta blankets as the crater rim is approached from great distance along a radial.
Indeed, the Bacons Castle formation, which is known to commonly contain striated cobbles and boulders
(Wentworth, 1928), occupies a ~120° segment of the 1-to-2.5-crater-radii annulus that can be drawn about
the center of the Chesapeake Bay structure (Fig. 1).
In Early Oligocene times, the rivers would have found courses circumferential to the ejecta blanket.
Presently, we see that the North Anna and Mattaponi Rivers are flowing perpendicular to the most southerly
radial section reproduced in Figure 7B (small black squares). Likewise, for the James and Chickahominy
Rivers in Figure 7C. And the same thing again for the Nottoway and Blackwater Rivers in Figure 7D. In
fact, all six of these rivers are, at least at their respective intersections with one of these radials, flowing
circumferentially about the Chesapeake Bay crater. Of course, most of these rivers do not flow very far
circumferentially before turning back in the direction of the crater. The exception is the Nottoway River,
which follows a 115-km radius for fully 60 km (Fig. 1)!
Note also that a V-shaped valley coincident with the mouth of the Chickahominy River extends all the way
across the James-York peninsula (Fig. 7C) where it directly faces the mouth of the Mattaponi River on the
York-Rappahannock peninsula (Fig. 7B). I propose that this groove could represent a paleochannel of the
York River (defined as the confluence of the Pamunkey and Mattaponi Rivers) dating to the period when it
had not yet breached the landward portion of the Chesapeake Bay crater rim (dashed arrow in Fig. 8A). If I
am correct in this, then the western crater rim would have survived for at least 2 m.y. until the Early-
Oligocene lowstand finally permitted the rivers to begin searching anew for their old channels.
Since all six of the rivers flowing circumferentially to the crater in Figure 7 happen to be moving in the
counterclockwise sense, one might guess that this may be the result of the Corriolis force acting on
southward flowing rivers. However, there may be an alternative explanation: an impenetrable radial barrier
on the York-Rappahannock peninsula. Remembering that my calculated ejecta-blanket profile neglects
dunes, hummocks, ridges, and rays, one must admit to the possibility of an extra-deep ray running up one of
the peninsulas radial to the crater center. But is there any evidence for this? Well, yes, it appears that there
is:
The Dragon Run watershed (Fig. 8A) is a raised-rim depression, which evidently has never during its
existence allowed a major river to enter it from the north or west – a true oddity, given the canonical view
that the U.S. Middle-Atlantic Coastal Plain gravels were all deposited by rivers within the past 10 m.y. (If
rivers should have a way to self-construct barriers around depressions, then New Orleans could be secured
without the Army Corps of Engineers!) It is seen in Figure 8A (Dragon Run Steering Committee, 2003) that
the Dragon Run watershed is almost perfectly radial to the center of the Chesapeake Bay crater and its
outline looks as though it could be decomposed into a sequence overlapping of ellipses – very similar to the
string of secondary craters of the lunar crater Copernicus reproduced in Figure 8B.
Figure 8. A: Location of the Dragon Run watershed (Dragon Run Steering Committee, 2003)
relative to the Chesapeake Bay impact structure (the latter being indicated by means of the
gravity map of Koeberl et al., 1996). B: A linear string of secondary impacts associated with the
lunar crater Copernicus (Lunar Orbiter photograph LO V M-144). The Dragon Run watershed
is a similar raised rim depression. Its boundary in green is a fluvial-transport divide; all streams
inside it flow inward. The dashed arrow indicates a proposed circumferential paleochannel of
the York River before it succeeded in breached the crater rim.
relative to the Chesapeake Bay impact structure (the latter being indicated by means of the
gravity map of Koeberl et al., 1996). B: A linear string of secondary impacts associated with the
lunar crater Copernicus (Lunar Orbiter photograph LO V M-144). The Dragon Run watershed
is a similar raised rim depression. Its boundary in green is a fluvial-transport divide; all streams
inside it flow inward. The dashed arrow indicates a proposed circumferential paleochannel of
the York River before it succeeded in breached the crater rim.
Melosh (1989, p. 95) states: “Nearly every large crater or basin seems to have one or more especially
prominent radial troughs extending from the rim out to nearly one crater diameter.” “Originally thought to
be tectonic features because of their often impressively straight walls, it is now believed that they are created
by lines of coalescing secondary craters...” While I believe that this is precisely the correct explanation of
the Dragon Run watershed, here is some truth in advertising: (1) by “large crater” Melosh (1989) is referring
to those in the 100-to-200 km range on the Moon, Mars, and Mercury, and (2) the secondary crater chain
associated with the 93-km-diameter crater Copernicus shown in Figure 8B begins at ~4 crater radii, not
one. That said, even our knowledge of Martian craters excavated into sediments likely containing some
liquid water at depth are insufficient guidance for what to expect for equivalently large impacts into ~600 m
of water-saturated sediments lying beneath ~150 m of sea water on the Earth. The only way to know
anything for sure about such a crater on the Earth is to discover one with a sufficiently well preserved ejecta
blanket ...and study it!
IN PLAIN SIGHT: SECONDARY CRATERS POCK THE MIDLOTHIAN UPLANDS!
Goodwin and Johnson (1970): “The flat, undissected upland surface of the Midlothian gravels is marked by
numerous elliptical to subcircular depressions or basins. These basins were first reported by Johnson and
Goodwin (1967). Over twenty such basins have been recognized in this area... The basins have formed on
the Midlothian gravels and the basin sediments are immediately underlain by coarse gravels. The elevation of
the basin floors is between [105 and 110 m] above sea level and a low ridge or rim surrounds the basins...
The basins range in size from [~50 m] to more than [1 km] and where elliptical, their major axis trends from
N 60° W to 80° W.” N.B. The direction to the Chesapeake Bay crater center from these sites is N 78° W.
Goodwin and Johnson (1970) further mentioned that the 1.5-to-5-m-high rims of the Midlothian upland
basins are “...most commonly best developed on the south and east sides of the basins,” i.e., the crater side.
N.B., Melosh (1989) states that “Secondary craters are typically asymmetric, having steeper slopes on the
side closest to the primary crater.”
Goodwin and Johnson (1970) led a field trip to one of these “Carolina Bays” where a drainage ditch had not
long before been dug from the interior through the rim. Quoting from their guide book:
“In general the section shows that the rim is underlain dominantly by sand containing some pebbles but only
very minor amounts of clay. Rarely cobbles or pebbles occur within the sand. A pronounced decrease in
sand occurs from the rim toward the bay's interior and massive, brownish-gray, silty clay with a few
scattered quartz pebbles becomes the dominant sediment. This clay is in direct contact with the underlying
gravels (emphasis added). The lateral transition from sand to clay is gradational but occurs within a
distance of less than [60 m] from the rim.”
Elsewhere Goodwin and Johnson (1970) remark that “A reconnaissance survey of the surrounding region
revealed that these basins are restricted to the upland surfaces of the Midlothian gravels. No evidence was
found of similar basins in Piedmont areas without a gravel veneer or in areas which have been severely
dissected by erosion. Apparently the development of the bays is in no way related to the Piedmont bedrock
which underlies the Midlothian gravels. Some of the bays have formed on gravels overlying granite, and
some have developed on gravels overlying Triassic shale, sandstone, and coal measures.”
prominent radial troughs extending from the rim out to nearly one crater diameter.” “Originally thought to
be tectonic features because of their often impressively straight walls, it is now believed that they are created
by lines of coalescing secondary craters...” While I believe that this is precisely the correct explanation of
the Dragon Run watershed, here is some truth in advertising: (1) by “large crater” Melosh (1989) is referring
to those in the 100-to-200 km range on the Moon, Mars, and Mercury, and (2) the secondary crater chain
associated with the 93-km-diameter crater Copernicus shown in Figure 8B begins at ~4 crater radii, not
one. That said, even our knowledge of Martian craters excavated into sediments likely containing some
liquid water at depth are insufficient guidance for what to expect for equivalently large impacts into ~600 m
of water-saturated sediments lying beneath ~150 m of sea water on the Earth. The only way to know
anything for sure about such a crater on the Earth is to discover one with a sufficiently well preserved ejecta
blanket ...and study it!
IN PLAIN SIGHT: SECONDARY CRATERS POCK THE MIDLOTHIAN UPLANDS!
Goodwin and Johnson (1970): “The flat, undissected upland surface of the Midlothian gravels is marked by
numerous elliptical to subcircular depressions or basins. These basins were first reported by Johnson and
Goodwin (1967). Over twenty such basins have been recognized in this area... The basins have formed on
the Midlothian gravels and the basin sediments are immediately underlain by coarse gravels. The elevation of
the basin floors is between [105 and 110 m] above sea level and a low ridge or rim surrounds the basins...
The basins range in size from [~50 m] to more than [1 km] and where elliptical, their major axis trends from
N 60° W to 80° W.” N.B. The direction to the Chesapeake Bay crater center from these sites is N 78° W.
Goodwin and Johnson (1970) further mentioned that the 1.5-to-5-m-high rims of the Midlothian upland
basins are “...most commonly best developed on the south and east sides of the basins,” i.e., the crater side.
N.B., Melosh (1989) states that “Secondary craters are typically asymmetric, having steeper slopes on the
side closest to the primary crater.”
Goodwin and Johnson (1970) led a field trip to one of these “Carolina Bays” where a drainage ditch had not
long before been dug from the interior through the rim. Quoting from their guide book:
“In general the section shows that the rim is underlain dominantly by sand containing some pebbles but only
very minor amounts of clay. Rarely cobbles or pebbles occur within the sand. A pronounced decrease in
sand occurs from the rim toward the bay's interior and massive, brownish-gray, silty clay with a few
scattered quartz pebbles becomes the dominant sediment. This clay is in direct contact with the underlying
gravels (emphasis added). The lateral transition from sand to clay is gradational but occurs within a
distance of less than [60 m] from the rim.”
Elsewhere Goodwin and Johnson (1970) remark that “A reconnaissance survey of the surrounding region
revealed that these basins are restricted to the upland surfaces of the Midlothian gravels. No evidence was
found of similar basins in Piedmont areas without a gravel veneer or in areas which have been severely
dissected by erosion. Apparently the development of the bays is in no way related to the Piedmont bedrock
which underlies the Midlothian gravels. Some of the bays have formed on gravels overlying granite, and
some have developed on gravels overlying Triassic shale, sandstone, and coal measures.”
In Plain Sight
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