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Re: Age of the Navesink (Long)



In a message dated 2/15/01 8:13:53 AM Pacific Standard Time, 
znc14@TTACS.TTU.EDU writes:

<< Naturally, I am rather interested in the Navesink, as it provides the best
 specimen of a Maastrichtian hadrosaur from Appallachia. William Gallagher
 has done a great deal of work on the paleontology and stratigraphy of the
 latest Creataceous of the Atlantic Coastal Plain. Good highlights of the
 former (and, in my opinion, all too little of the latter, darned popular
 audience!) are in his book Dinosaurs of New Jersey. >>

    I decided to ask Dr. Gallagher at the New Jersey State Museum myself 
(Bill's practically next door) and he was kind enough to provide this very 
informative note for the list. Of course, be sure to check out his book, 
_When Dinosaurs Roamed New Jersey_1997, Rutgers University Press:

"The Navesink is a transgressive glauconitic unit deposited slowly in
sediment starved conditions over a period of time that may span a good part of
the Maastrichtian.  In technical terms, these transgressive glauconites (like
both the Navesink and the Hornerstown) are said to be condensed sections; that
is, in non-technical terms they are relatively thin beds that were deposited
slowly over a long perod of time.  Out west where there are nice thick
Maastrichtian beds ( the Maastrichtian section near Saltillo in Mexico is 
12,000
feet thick!) the time resolution is very good and it is easy to discern early
and late Maastrichtian, especially with the help of radiometric dates in the
interbedded bentonites; but back east here the glauconites have proven
relatively resistant to any kind of radiometric dates.  The best dates we can
get are biostratigraphic correlations by way of microfossils and ammonites, 
and
the comparatively new methodology of stable strontium isotope age estimation.
Since this is not a graduate seminar, I can't go in to the details too much, 
but
stable Sr shows some shells in the Navesink coming out with an age estimate of
65-66 million years; this would be late Maastrichtian (this is Peter 
Sugarman's
work at the NJ Geo Survey and Rutgers); a specimen of Discoscaphites gulosus
found at Inversand and recently published on by Kennedy and Cobban et al. is
correlated to the uppermost ammonite zone in the Western Interior, the
Jeletzskyites nebracensis zone of the Uppermost Fox Hills Fm., latest part of
the early Maastrichtian.  Microfossils (forams, dinoflagellates) indicate a 
late
Maastrichtian age for at least a part of the Navesink; but other fossils such 
as
some of the ammonites in the basal shellbed of the Navesink suggest an earlier
age, even acc. to Kennedy and Cobban a late Campanian age.
    I know this is all very confusing, but the bottom line is that probably 
the
Navesink spans both the early and into the late Maastrichtian.  It seems hard 
to
believe that this could be so, but most estimates of its length run well over 
3
million years, and it is only about twenty to thirty feet thick.  The key here
is that glauconitic sedimentation happens very slowly on a sediment-starved
seafloor, with fossil faunas accumulating in layers over this protracted 
period
of time.  It is not like the sedimentary regime of the Pierre or the Hell 
Creek,
where piles of sediment are being rapidly dumped off eroding incipient Rocky 
Mtn
uplift to the west
    The same is also true of the Hornerstown, which does indeed have
Maastrichtian fossils in its basal layer, just above the contact with the
Navesink.  Mosasaurs, Enchodus, Squalicorax, and ammonites of several species
are found in this basal fossil bed, which Dave Parris has named the Main
Fossiliferous Layer (MFL).  The problem with these fossils is that in the case
of the vertebrates they are always isolated elements, or at best a braincase
which is a solid fused unit of skull elements; in the case of the ammonites 
the
specimens are always very worn and abraded broken phosphatized internal molds.
These as Kennedy and Cobban have pointed out could easily have been reworked 
or
recycled from Cretaceous beds below into one of these condensed section
glauconites accumulated over a long period of time.  The microfossils 
indicate a
Danian but not very earliest Danian age for the Hornerstown.  Problem is, 
right
in with the mosasaurs and broken ammonites there are whole or partial croc and
turtle skeletons; Dave P. has even gotten a partial articulated bird wing from
the MFL.  This does fit in with a pattern of pure reworking; but it must also 
be
said that the kind of things being found as whole or partial specimens (crocs,
turtles) are survivors of the K/T thing and so not terribly informative as to
time.  The fossils in the middle of the Hornerstown are definitely Danian and
tell their own little story, but that is not your question.  So again, the
evidence is somewhat ambiguous, but it seems to point to a gap in the record 
at
the K/T bounday of short duration (my guestimate is on the order of  a couple-
several hundred thousand years) with a relict fossil assemblage containing
representatives of the last of the Mesozoic fauna in this region- possibly
"reworked' or if you prefer the French term, a  remanie- a sort of lag deposit
left to accumulate as the glauconites covered what was left after the initial
phases of the transgression eroded previously deposited beds.
    Why am I writing all this?  I just finished the arduous revision of a 
paper
on this subject presented in Vienna this past summer, and I have been driven
insane by the process.  At any rate it has forced me to consider very 
carefully
what I say on this matter, and as usual with geology, the more you look the 
more
complicated things get.  As a sort of final summation, I would say that the
Navesink is in part late Maastrichtian, but probably not latest Maastrichtian-
the dinosaur fauna might be correlative to the upper Fox Hills-lower Hell
Creek.  There is a short (by geologic standards, anyway) gap at the K/T
boundary, then a fossil assemblage that incorporates Maastrichtian elements in
the basal Hornerstown- a protracted death assemblage in the wake of the big
blast?  A tsunamtite deposit? Both combined?  Ahh, sweet speculation - now to
prove it somehow...."