Re: Chesapeake Crater

[Tommy Tyrberg <tommy.tyrberg@norrkoping.mail.telia.com>]

>> The Chesapeake Bay impact is one of several major impacts that occurred
>> close together in time near the Eocene/Oligocene border. It has been
>> hypotesized that these could be related and due to a "comet swarm"

>Which are the others? Popigay and... ?

There are at least 4, perhaps 6, microtektite layers within about a million
years in marine deposits of latest Eocene age. Two (one?) of these are
probably from Chesapeake and Popigai which are almost exactly the same age
35.6 MY BP. The origin of the others is uncertain. One may be connected to
a number of elongate structures in S Australia which probably record a
low-angle serial impact á la Comet Shoemaker-Levy (however not all of these
have been proven to be impact structures). One impact may have been in or
close to the Mediterranean where a 175 m thick (!) clastic bed in Dalmatia
has been interpreted as a case of carbonate platform collapse similar to
what happened in Chiapas at the K/T boundary.
Note that there is no 1-1 correlation between impacts and microtektite
layers. An impact by an icy object (=comet) in deep ocean would probably
not result in a microtektite layer and might in fact be very difficult to
detect at all while a serial impact like Shoemaker-Levy would show up as a
single impact layer.

>> The climate shift
>> probably had paleogeographic causes, Australia became well separated from
>> Antarctica opening a circum-Antarctic ocean which climatologically isolated
>> Antarctica, probably causing the first widespread glaciation there.

>South America, not Australia. AFAIK Australia broke off 45 million years
>ago, and Eastern Antarctica already iced over then. The circumpolar cold
>current became established when the land bridge between South America and
>Western Antarctica broke; at this time all of Antarctica froze, and
>worldwide temperatures and sea levels began to drop.

Australia and Antarctica separated in the mid Eocene. However the strait
between them was shallow at the eastern end where northern Victoria land
overlapped the Tasman rise. This part of the spreading axis was originally
a long transform fault at shallow depth and it was not until the
continental blocks had drifted far enough apart to open a deep oceanic
connection here in the late Eocene that the circum-Antarctic circulation
pattern became fully established.

>> There 
>> were other major geographic shifts att approximately the same time, for
>> example Europe which had long been an archipelago with connections with
>> North America earlier inthe Eocene became connected with Asia instead.

>Have the dating problems been solved in the meantime? AFAIK nobody knows
>whether the extinctions were coeval with one another, the impacts, the
>geographic changes and the climate shifts...

Alas no, the datings near the E/O boundary are very contentious,
particularly the land/sea correlations and the correlation of the boundary
in terrestrial deposits between continents is also disputed. Still it's
clear thet there was a remarkable concentration of impacts in the latest
Eocene.

Tommy Tyrberg