 | |  | |  | |  | |  | |  | |  | |  | |  | |  | |  |
Figure 11. Cumulative particle size distributions for various erosive environments. The solid
black data points are after Emery (1955) and the open circles are due Schlee (1957); in both of
these cases the distributions were recorded as percentages of the total number of samples.
Schlee’s samples were pre-selected to have median sizes larger than 10 mm, thus omitting the
loam member of the "upland deposits." The dashed curves were taken from the work of Attal
and Lavé (2006), who first sorted by lithology and then weighed particles coarser than 10 mm in
the field; the size distributions of their finer fractions were determined in the laboratory.
black data points are after Emery (1955) and the open circles are due Schlee (1957); in both of
these cases the distributions were recorded as percentages of the total number of samples.
Schlee’s samples were pre-selected to have median sizes larger than 10 mm, thus omitting the
loam member of the "upland deposits." The dashed curves were taken from the work of Attal
and Lavé (2006), who first sorted by lithology and then weighed particles coarser than 10 mm in
the field; the size distributions of their finer fractions were determined in the laboratory.
Established stratigraphic columns are not subject to reshuffling
Another objection I heard at the Penrose Conference was that, whereas fossil biozones may occasionally
need to be re-dated when more accurate constraints are imposed (usually by radiometric dating of volcanic-
ash or impact layers; e.g., Smit, 1999), it is inconceivable that any accepted sequence of biozones in a long-
established stratigraphic column should ever be compelled to exchange positions across a time plane. I will
give my answer to that below.
But first the background: Figure 5 is an adaptation of a U.S. Geological Survey section of the Potomac-
Patuxent peninsula extending north-northwestward, passing just eastward of the District of Columbia. The
upland deposits are clearly shown as resting on their scarp-free base, gently sloping in the general direction
of the Chesapeake Bay crater. The Calvert formation (darkest band to the right) and the Choptank and St.
Marys formations that successively overlie it comprise the base of the upland deposits. Therefore the
upland deposits must be younger than those underlying formations, thus contradicting my hypothesis if one
accepts their historical dating as Miocene or more recent. However, “the plot thickens” when one begins to
contemplate that there is also an in-the-crater Calvert formation (e.g., Poag, 1997).
In the following discussion, I will refer informally to the extensive upland Calvert formation as “Calvert I”
and to the relatively tiny in-crater Calvert formation as “Calvert II.” These two Calverts are diachronous in
anyone’s model, not just mine. In the currently accepted stratigraphic column they are diachronous across
a time plane including hiatuses summing to as much as 28 m.y. (Poag, 1977). My operating premise is that
Calvert II was deposited soon after the impact and owes its preservation to the fact that the crater floor has
been continuously subsiding due to compaction of the Exmore breccias under the weight of post-impact
sedimentation. Poag (1997): “The evidence suggests that the thick breccia lens inside the crater continued
to compact and subside more rapidly than deposits outside the crater... and may continue even today.”
Thus, in my view, sediments entering the crater in the first few million years of its existence are sure to still
be there, whereas sediments deposited more recently would have been subject to removal and replacement
by ever younger sediments subsequent to the eventual breach of the crater rim by the many rivers that
presently converge there. In particular, a “Calvert II” envisioned as being 28 m.y. younger than the crater
should not have survived fluvial erosion during the lowstands of the Quaternary.
In my model, Calverts I and II are diachronous across a time plane of only about 2 million years. The
microfossils found in both Calverts are neritic species. Therefore, in anyone’s model Calvert I must have
been deposited during sea level transgressions equal to or greater than ~79 m (the highest point in Calvert I
that I’ve been able to locate), whereas Calvert II would have been deposited during an exceptional
regression, almost certainly the one that occurred around the Eocene-Oligocene boundary. Hallam (1984)
has estimated eustatic sea levels of about 155 m during most of the Late Eocene, vis-à-vis a comparatively
brief peak of about 65 m around 10 Ma. Any doubts that the sea levels were that much lower in the
Miocene than in the Late Eocene should be dispelled by recent oxygen isotope data (e.g., Zachos et al.,
2001), which prove that ice-caps were small to nonexistent before the Chesapeake Bay impact, whereas a
substantial Antarctic cap has been continuously in place ever since.
Summary
In addition to supplanting the untenable fluvial model for the upland deposits, my new model for the
stratigraphic column of the U.S. Middle-Atlantic Coastal Plain is (1) in accord with the physics and geology
of impact cratering and (2) matches far better the emerging picture of Cenozoic eustatic sea level variations
than does the currently accepted one. Moreover, the diachronism that I invoke – crossing a time plane
spanning a likely-hiatus-free 2 m.y. – is far less problematic on its face than the hiatus-riddled column
historically predicated on a Miocene-age “Calvert I.”
CONCLUSIONS RELATING TO THE LATE EOCENE EARTH
The consequences of this study for non-impact geologists whose prime concern is the Late Eocene Earth
can be summarized in this way: If the essence of my thesis should turn out to be correct, then those
concerned with neritic biota of the Late Eocene would then need to bring “under their tent” a whole lot of
additional species, that is, those species that I argue have been misdated as Miocene. If this should be found
out to be the case, then a number of vexing anomalies in the fossil record would almost certainly be cleared
up in the process. Moreover, comparison of the species present in “Calvert II,” which I propose to have
been deposited during the Early Oligocene lowstand, with those present within “Calvert I,” which I argue
was deposited sometime during the Mid Eocene highstand, would finally give a the correct picture of exactly
which extinctions may or may not have resulted from the Late Eocene impacts. Finally, studies of the
survival rates of the “Calvert-II” species would provide an excellent test of Hallam’s (1984) proposition that
lowstands should bring about extinctions of neritic species endemic to highstand ecosystems.
In any event, it is worth noting that Pälike et al. (2006) have computer modeled the paleoclimatic record of
the entire Oligocene preserved in the Pacific Ocean Site 1218 Ocean Drilling Project core, showing among
other things that the Oi-1 glaciation can have resulted from long-term orbital-forcing cycles only in
combination with some unspecified terrestrial trigger, with this trigger indicated by their simulations to have
occurred between 35.2 and 36.3 Ma (see their Fig. 3). Since these dates bracket the radiometric ages of the
Chesapeake Bay and Popigai impacts, they support the proposition of Fawcett and Boslough (2002) that this
climate-change trigger – instead of being some cryptically extreme removal of CO2 from the atmosphere –
could have been the winter-hemisphere cooling resulting from the shadow of an equatorial debris ring
thrown up by one or the other of the two major Late Eocene impact events.
REFERENCES CITED
Allday, J., 2000, Apollo in Perspective: Spaceflight Then and Now: CRC Press, p. 320.
Attal, M., and Lavé, J., 2006, Changes of bedload charactistics along the Marsyandi River (central Nepal):
Alvarez, W., 1996, Trajectories of ballistic ejecta from the Chicxulub crater: in Ryder, G., Fastovsky, D.,
Alvarez. L.W., Alvarez, W., Asaro, F., Michel, H.V., 1980, Extraterrestrial cause for the Cretaceous-
Baker, V.R., 2002, The study of superfloods: Science, v. 295, p. 2379-2380.
Bradt, R.C., and Tressler, R.E., 1994, Fractography of Glass: New York, Springer, 310 p.
Chesterman, C.W., and Loew, K.E., 1978, The Audubon Society Field Guide to North American Rocks and
Deer, W.A., Howie, R.A., and Zussman, J., 1966, An Introduction to Rock Forming Minerals: New York,
Dragon Run Steering Committee, 2003, The state of Dragon Run: Status of natural resources: Virginia
Eiseley, L., 1960, The Firmament of Time: (reprinted 1999) Lincoln, University of Nebraska Press, 183 p.
Eiseley, L., 1961, The Man Who Saw Through Time: New York, Charles Scribner’s Sons, 125 p.
Emery, K.O., 1955, Grain size of marine beach gravels: Journal of Geology, v. 63, p. 39-49.
Fawcett, P.J., and Boslough, B.E., 2002, Climatic effects of an impact-induced equatorial debris ring:
Another objection I heard at the Penrose Conference was that, whereas fossil biozones may occasionally
need to be re-dated when more accurate constraints are imposed (usually by radiometric dating of volcanic-
ash or impact layers; e.g., Smit, 1999), it is inconceivable that any accepted sequence of biozones in a long-
established stratigraphic column should ever be compelled to exchange positions across a time plane. I will
give my answer to that below.
But first the background: Figure 5 is an adaptation of a U.S. Geological Survey section of the Potomac-
Patuxent peninsula extending north-northwestward, passing just eastward of the District of Columbia. The
upland deposits are clearly shown as resting on their scarp-free base, gently sloping in the general direction
of the Chesapeake Bay crater. The Calvert formation (darkest band to the right) and the Choptank and St.
Marys formations that successively overlie it comprise the base of the upland deposits. Therefore the
upland deposits must be younger than those underlying formations, thus contradicting my hypothesis if one
accepts their historical dating as Miocene or more recent. However, “the plot thickens” when one begins to
contemplate that there is also an in-the-crater Calvert formation (e.g., Poag, 1997).
In the following discussion, I will refer informally to the extensive upland Calvert formation as “Calvert I”
and to the relatively tiny in-crater Calvert formation as “Calvert II.” These two Calverts are diachronous in
anyone’s model, not just mine. In the currently accepted stratigraphic column they are diachronous across
a time plane including hiatuses summing to as much as 28 m.y. (Poag, 1977). My operating premise is that
Calvert II was deposited soon after the impact and owes its preservation to the fact that the crater floor has
been continuously subsiding due to compaction of the Exmore breccias under the weight of post-impact
sedimentation. Poag (1997): “The evidence suggests that the thick breccia lens inside the crater continued
to compact and subside more rapidly than deposits outside the crater... and may continue even today.”
Thus, in my view, sediments entering the crater in the first few million years of its existence are sure to still
be there, whereas sediments deposited more recently would have been subject to removal and replacement
by ever younger sediments subsequent to the eventual breach of the crater rim by the many rivers that
presently converge there. In particular, a “Calvert II” envisioned as being 28 m.y. younger than the crater
should not have survived fluvial erosion during the lowstands of the Quaternary.
In my model, Calverts I and II are diachronous across a time plane of only about 2 million years. The
microfossils found in both Calverts are neritic species. Therefore, in anyone’s model Calvert I must have
been deposited during sea level transgressions equal to or greater than ~79 m (the highest point in Calvert I
that I’ve been able to locate), whereas Calvert II would have been deposited during an exceptional
regression, almost certainly the one that occurred around the Eocene-Oligocene boundary. Hallam (1984)
has estimated eustatic sea levels of about 155 m during most of the Late Eocene, vis-à-vis a comparatively
brief peak of about 65 m around 10 Ma. Any doubts that the sea levels were that much lower in the
Miocene than in the Late Eocene should be dispelled by recent oxygen isotope data (e.g., Zachos et al.,
2001), which prove that ice-caps were small to nonexistent before the Chesapeake Bay impact, whereas a
substantial Antarctic cap has been continuously in place ever since.
Summary
In addition to supplanting the untenable fluvial model for the upland deposits, my new model for the
stratigraphic column of the U.S. Middle-Atlantic Coastal Plain is (1) in accord with the physics and geology
of impact cratering and (2) matches far better the emerging picture of Cenozoic eustatic sea level variations
than does the currently accepted one. Moreover, the diachronism that I invoke – crossing a time plane
spanning a likely-hiatus-free 2 m.y. – is far less problematic on its face than the hiatus-riddled column
historically predicated on a Miocene-age “Calvert I.”
CONCLUSIONS RELATING TO THE LATE EOCENE EARTH
The consequences of this study for non-impact geologists whose prime concern is the Late Eocene Earth
can be summarized in this way: If the essence of my thesis should turn out to be correct, then those
concerned with neritic biota of the Late Eocene would then need to bring “under their tent” a whole lot of
additional species, that is, those species that I argue have been misdated as Miocene. If this should be found
out to be the case, then a number of vexing anomalies in the fossil record would almost certainly be cleared
up in the process. Moreover, comparison of the species present in “Calvert II,” which I propose to have
been deposited during the Early Oligocene lowstand, with those present within “Calvert I,” which I argue
was deposited sometime during the Mid Eocene highstand, would finally give a the correct picture of exactly
which extinctions may or may not have resulted from the Late Eocene impacts. Finally, studies of the
survival rates of the “Calvert-II” species would provide an excellent test of Hallam’s (1984) proposition that
lowstands should bring about extinctions of neritic species endemic to highstand ecosystems.
In any event, it is worth noting that Pälike et al. (2006) have computer modeled the paleoclimatic record of
the entire Oligocene preserved in the Pacific Ocean Site 1218 Ocean Drilling Project core, showing among
other things that the Oi-1 glaciation can have resulted from long-term orbital-forcing cycles only in
combination with some unspecified terrestrial trigger, with this trigger indicated by their simulations to have
occurred between 35.2 and 36.3 Ma (see their Fig. 3). Since these dates bracket the radiometric ages of the
Chesapeake Bay and Popigai impacts, they support the proposition of Fawcett and Boslough (2002) that this
climate-change trigger – instead of being some cryptically extreme removal of CO2 from the atmosphere –
could have been the winter-hemisphere cooling resulting from the shadow of an equatorial debris ring
thrown up by one or the other of the two major Late Eocene impact events.
REFERENCES CITED
Allday, J., 2000, Apollo in Perspective: Spaceflight Then and Now: CRC Press, p. 320.
Attal, M., and Lavé, J., 2006, Changes of bedload charactistics along the Marsyandi River (central Nepal):
- implications for understanding hillslope sediment supply, sediment load evolution along fluvial
networks, and denudation in active orogenic belts: in Willett, S.D., Hovius, N., Brandon, M.T., and
Fisher, D.M., eds., Tectonics, Climate, and Landscape Evolution: Geological Society of America
Special Paper 398, Penrose Conference Series, p. 143-171.
Alvarez, W., 1996, Trajectories of ballistic ejecta from the Chicxulub crater: in Ryder, G., Fastovsky, D.,
- and Gartner, S., eds., The Cretaceous-Tertiary Event and Other Catastrophes in Earth History:
Boulder, Colorado, Geological Society of America Special Paper 307, p. 141-150.
Alvarez. L.W., Alvarez, W., Asaro, F., Michel, H.V., 1980, Extraterrestrial cause for the Cretaceous-
- Tertiary extinction: Science, v. 208, p. 1095-1108.
Baker, V.R., 2002, The study of superfloods: Science, v. 295, p. 2379-2380.
Bradt, R.C., and Tressler, R.E., 1994, Fractography of Glass: New York, Springer, 310 p.
Chesterman, C.W., and Loew, K.E., 1978, The Audubon Society Field Guide to North American Rocks and
- Minerals: Tokyo, Alfred A. Knopf, 850 p.
Deer, W.A., Howie, R.A., and Zussman, J., 1966, An Introduction to Rock Forming Minerals: New York,
- Longman,.
Dragon Run Steering Committee, 2003, The state of Dragon Run: Status of natural resources: Virginia
- Department of Conservation and Recreation (Report, December, 2003), 34 p.
Eiseley, L., 1960, The Firmament of Time: (reprinted 1999) Lincoln, University of Nebraska Press, 183 p.
Eiseley, L., 1961, The Man Who Saw Through Time: New York, Charles Scribner’s Sons, 125 p.
Emery, K.O., 1955, Grain size of marine beach gravels: Journal of Geology, v. 63, p. 39-49.
Fawcett, P.J., and Boslough, B.E., 2002, Climatic effects of an impact-induced equatorial debris ring:
- Journal of Geophysical Research, v. 107, p. 10.1029/2001JD001230.
In Plain Sight
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