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Figure 4. An idealized profile of the Chesapeake Bay crater ejecta blanket tens of minutes after
the impact calculated by means of equation 1, using the coefficient derived in equation 3 and the
notional assumption that the Eocene Coastal-Plain sediments were sloping seaward at 0.5 m/km
at the time of the impact. The lithology of the target rocks follows Koeberl et al. (1996) and
Poag (1997), and the overall sediment depths were scaled with reference to the depth at which
the USGS-NASA Corehole intercepted basement granite (Horton et al., 2005). Also indicated is
the effective interference zone (Melosh, 1989) from which all materials, excluding the impactor
footprint, would have been ejected at high speeds with minimal shocking.
the impact calculated by means of equation 1, using the coefficient derived in equation 3 and the
notional assumption that the Eocene Coastal-Plain sediments were sloping seaward at 0.5 m/km
at the time of the impact. The lithology of the target rocks follows Koeberl et al. (1996) and
Poag (1997), and the overall sediment depths were scaled with reference to the depth at which
the USGS-NASA Corehole intercepted basement granite (Horton et al., 2005). Also indicated is
the effective interference zone (Melosh, 1989) from which all materials, excluding the impactor
footprint, would have been ejected at high speeds with minimal shocking.
In contrast with the gentle slope of the base of the upland deposits in Southern Maryland seen in Figure 5
and the extreme flatness of the upland deposits near Springfield and Alexandria, Va. (vide infra), we see in
Figure 7 that in Virginia the upland gravels blanket V-shaped valleys and depressions with embankments in
the direction of the crater having slopes ranging from 4 m/km on the York-Rappahannock peninsula (Fig.
7B) to 6.3 m/km just east of Richmond (Fig. 7C). This observation strongly suggests that in these cases the
upland deposits were laid down directly on top of pre-existing topography having these particular slopes.
This could easily have happened in the case of ballistic emplacement, whereas the prospects for fluvial
deposition on these slopes become even more remote than they are in Southern Maryland (vide supra). And
given the obvious armoring effects of a thick layer of coarse gravel, there is no sedimentological reason to
assume that the upland deposits are geologically young.
Although Figure 7 may raise more questions than it answers, I hope it will be found useful to the reader
wishing to test the various models. Below I speak of some of the things that I believe I see in this figure,
together with some speculations as to they may mean.
and the extreme flatness of the upland deposits near Springfield and Alexandria, Va. (vide infra), we see in
Figure 7 that in Virginia the upland gravels blanket V-shaped valleys and depressions with embankments in
the direction of the crater having slopes ranging from 4 m/km on the York-Rappahannock peninsula (Fig.
7B) to 6.3 m/km just east of Richmond (Fig. 7C). This observation strongly suggests that in these cases the
upland deposits were laid down directly on top of pre-existing topography having these particular slopes.
This could easily have happened in the case of ballistic emplacement, whereas the prospects for fluvial
deposition on these slopes become even more remote than they are in Southern Maryland (vide supra). And
given the obvious armoring effects of a thick layer of coarse gravel, there is no sedimentological reason to
assume that the upland deposits are geologically young.
Although Figure 7 may raise more questions than it answers, I hope it will be found useful to the reader
wishing to test the various models. Below I speak of some of the things that I believe I see in this figure,
together with some speculations as to they may mean.
Figure 5. Stratigraphic section of Southern Maryland reproduced from Krantz and Powars
(2000), with comparison to an idealized calculation of the original Chesapeake Bay crater ejecta
blanket. Note the gently sloping, concave-upward base of the upland deposits and the depths of
these deposits, which are comparable to the calculated idealized ejecta blanket depth.
(2000), with comparison to an idealized calculation of the original Chesapeake Bay crater ejecta
blanket. Note the gently sloping, concave-upward base of the upland deposits and the depths of
these deposits, which are comparable to the calculated idealized ejecta blanket depth.
ASSESSING THE LANDWARD EJECTA BLANKET
If the upland deposits of Figure 5 are truly Chesapeake Bay crater ejecta, then three questions immediately
come to mind: (1) Did the impact itself sculpt the gently sloping base of the Southern Maryland deposits, (2)
do the bases of the upland deposits in Virginia also slope radially in the direction of the crater, and (3) do the
contiguous terrains (Bacons Castle formation) between the upland deposits and the crater rim also harbor
vestiges of ejecta? To help answer these questions, I went to Google Earth and picked off the elevation
profiles corresponding to the crater-centric radials and sub-radials mapped in Figure 6. (The U.S.
Geological Survey sections of Southern Maryland relevant to Figure 5 are shown at the top of Fig. 6).
If the upland deposits of Figure 5 are truly Chesapeake Bay crater ejecta, then three questions immediately
come to mind: (1) Did the impact itself sculpt the gently sloping base of the Southern Maryland deposits, (2)
do the bases of the upland deposits in Virginia also slope radially in the direction of the crater, and (3) do the
contiguous terrains (Bacons Castle formation) between the upland deposits and the crater rim also harbor
vestiges of ejecta? To help answer these questions, I went to Google Earth and picked off the elevation
profiles corresponding to the crater-centric radials and sub-radials mapped in Figure 6. (The U.S.
Geological Survey sections of Southern Maryland relevant to Figure 5 are shown at the top of Fig. 6).
Figure 6. Map of the upland
deposits and Bacons Castle
formation showing the sections
for which elevation profiles are
shown in Figures 5 (Southern
Maryland) and 7 (Eastern
Virginia: Fig. 7A, B, C, and D)
as correspondingly marked
here. Note that only sections D
are truly radial to the center of
the crater. The other sections
radiate from points near the
crater center selected to
intercept long, mostly
contiguous stretches of the
upland deposits, as well as the
Bacons Castle formation.
deposits and Bacons Castle
formation showing the sections
for which elevation profiles are
shown in Figures 5 (Southern
Maryland) and 7 (Eastern
Virginia: Fig. 7A, B, C, and D)
as correspondingly marked
here. Note that only sections D
are truly radial to the center of
the crater. The other sections
radiate from points near the
crater center selected to
intercept long, mostly
contiguous stretches of the
upland deposits, as well as the
Bacons Castle formation.
In Figure 7A and B I have again superposed my calculated Chesapeake Bay crater ejecta blanket profile,
again employing gently sloping bases to represent the uppermost coastal sediments on which the ejecta likely
re-impacted. The values I selected for the slopes (~0.4 m/km) and the vertical offsets (~ 10 m below
present sea level at r = 0) of the dashed lines were based on my operating assumption that the Bacons Castle
formation may be ~10 to 20 m deep and that, like the upland deposits, it too may rest on gently a sloping
base of older sediments.
again employing gently sloping bases to represent the uppermost coastal sediments on which the ejecta likely
re-impacted. The values I selected for the slopes (~0.4 m/km) and the vertical offsets (~ 10 m below
present sea level at r = 0) of the dashed lines were based on my operating assumption that the Bacons Castle
formation may be ~10 to 20 m deep and that, like the upland deposits, it too may rest on gently a sloping
base of older sediments.
Figure 7. Elevation profiles of the upland deposits and Bacons Castle formation in Virginia along
the sections illustrated in Figure 6. A: the Potomac-Rappahannock peninsula. B: the
Rappahannock-York peninsula. C: the York-James peninsula. D: the quadrant south of the
James River. Multiple profiles on the same peninsula are arranged so that the southern most is
represented by small black squares, the central one by large hollow circles, and the northernmost
by a dash-dot line. In A and B, calculated profiles of the original Chesapeake Bay crater ejecta
blanket have been overlain. The spatial resolution of these elevation data varies in respect to the
eye altitude “flown” on Google Earth. Thus, data showing high spatial frequency of hills and
valleys were “flown” at an eye altitude typically ~10 km, while those recording only low-
frequency elevation changes were “flown” at eye altitudes of ~50 km or more. In D, high-
altitude data were supplemented low-altitude data to sharpen river valleys and scarps.
the sections illustrated in Figure 6. A: the Potomac-Rappahannock peninsula. B: the
Rappahannock-York peninsula. C: the York-James peninsula. D: the quadrant south of the
James River. Multiple profiles on the same peninsula are arranged so that the southern most is
represented by small black squares, the central one by large hollow circles, and the northernmost
by a dash-dot line. In A and B, calculated profiles of the original Chesapeake Bay crater ejecta
blanket have been overlain. The spatial resolution of these elevation data varies in respect to the
eye altitude “flown” on Google Earth. Thus, data showing high spatial frequency of hills and
valleys were “flown” at an eye altitude typically ~10 km, while those recording only low-
frequency elevation changes were “flown” at eye altitudes of ~50 km or more. In D, high-
altitude data were supplemented low-altitude data to sharpen river valleys and scarps.
In Plain Sight
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