Tuesday, March 26, 2002
S4. Studies of Depositional
Systems and Sedimentary
Rocks: In Honor of Edward
Scudder Belt
(Cosponsored by NE SEPM)
Sheraton Springfield, Mahogony
1:05 PM Olsen, Paul E.
STRATIGRAPHY AND AGE OF THE EARLY JURASSIC
PROTLAND FORMATION OF
CONNECTICUT AND MASSACHUSETTS: A CONTRIBUTION
TO THE TIME SCALE OF THE
EARLY JURASSIC
OLSEN. Paul E., Lamont-Doherty Earth Observatory of Columbia Univ., Rt.
9W, Palisades,
NY 10964-8000, polsen@ldeo.columbia.edu; KENT, Dennis V., Rutgers Univ.,
610 Taylor
Rd., Piscataway. N.J. 08854-8066; and LETOURNEAU, Peter M., Lamont-Doherty
Earth
Observatory of Columbia Univ., Rt. 9W, Palisades, NY 10964-8000
The stratigraphy of the more than 4 km
thick Portland Formation of the Hartford basin has
remained relatively poorly known despite
nearly two centuries of study. Within the last three
years, however, we have been able to piece
together a fairly complete outline of the stratigraphy
of the lower 2 km of the formation which,
together with that of the underlying East Berlin and
Shuttle Meadow formations and coeval strata
in the Newark basin, allow an astronomical cal-
ibration of the time scale of the Hettangian
Age.
Cyclical lacustrine
strata, similar in to that seen in the East Berlin Formation, characterize
the
lower 2 km of the Portland. Most of the
hierarchy of climatic precession-related cycles are present
including the 20 ky Van Houten cycle,
the ~100 ky short modulating cycle, the 406 ky McLaughlin
cycle, and the ~1.75 my long modulating
cycle. Van Houten cycles reflect the climatic precession
cycle of ~20 ky, and are about 20 m thick
over much of the Hartford basin, thickening towards the
eastern border fault. Varve, cyclostratigraphic
and paleomagnetic correlations show that individual
Van Houten cycles can be traced over most,
if not all, of the basin, allowing the scattered outcrops
to be compiled to a common cyclostratigraphic
scale. Four full McLaughlin cycles are present in
the lower Portland, with one continuing
from the underlying East Berlin Formation; we propose to
recognize these mappable units as members
as follows (from the bottom up): "Northampton",
"East Granby", "South Hadley Falls", "Mittinegue",
and "Stony Brook" members. Correlation to the
paleomagnetic reversal stratigraphy of
the Paris basin marine section suggests that the zones of
reversed polarity in the "Mittinegue"
and "Stony Brook" members in the upper part of the lower
Portland are of Sinemurian age. Thus,
the Hettangian Age, encompassing in the Hartford basin
the duration of the uppermost New Haven
through lower Portland formations, is about 2 million
years long, consistent with estimates
from marine cyclostratigraphy. Simple extrapolation of the
accumulation rate into the upper, largely
fluvial, Portland (~2 km) suggests that there is an add-
itional 2 million years represented, probably
all of Sinemurian age.
From: Olsen, P. E., Kent,
D. V., and LeTourneau, 2002, Stratigraphy and Age
of the Early Jurassic
Portland Formation of Connecticut and Massachusetts:
A Contribution to the
Time Scale of the Early Jurassic. Geological Society of
America, Abstracts with
Programs, v. 34, no. 2, p. A-61.