MEANDERING RIVER FACIES IN THE LOWER PART OF THE
NEW HAVEN ARKOSE, SOUTH-CENTRAL CONNECTICUT:
EARLY EVOLUTION OF THE HARTFORD BASIN IN LATE
TRIASSIC TIME
McINERNEY, D. P., Dresdner Robin Environmental
Management Inc., Jersey City, NJ 07302
HUBERT, J. F., Dept. of Geosciences, University of
Massachusetts, Amherst, MA 01003
Fluvial redbeds about 650 m above the base of the 2-km New Haven
Arkose were studied in two continuous exposures along I-84 in
Southington and I-691 in Cheshire. These 150 m of strata
accumulated in a valley produced by NW-SE extension, early in the
basin history before formation of the eastern border faults. The river
meandered over a low gradient, exiting the subtropical basin to the
south. The high plagioclase content of the arkosic sandstones suggest
that most of the detritus came from west of the valley.
Channel sandstones, 50% of the sequence, are mostly
multistory, each story fining upwards and representing passage of a
meander bend. The channel sandstones average 2.3 m, ranging from
0.6-6.5 m. Beds of floodbasin reddish brown mudstone, 25% of the
sequence, average 3.4 m, varying from 0.75-9.6 m. Reddish brown
fine sandstone to silty mudstone in beds 0.4-1.1 m in thickness
accumulated on crevasse splays and levees, forming another 25%.
Plant roots and Scoyenia burrows are abundant. Numerous caliche
paleosols imply semiaridity.
Lateral accretion cross-beds (point bars) allow calculation of
the paleotopography and paleohydrology for two channels. Data for
the second channel are in parentheses: width = 11 m (37 m), depth =
2.0 m (2.7 m), bankfull discharge = 8 cubic m per sec (47 cubic m per
sec), sinuosity = 2.2 (1.7), meander wave length = 157 m (601 m),
radius of curvature = 42 m (161 m), and river gradient = 0.0009
(0.0006).
Migration of meanders towards and away from the sections
generated two types of cycles, each with minor variations. The cycles
average 7 m, varying from 2-16 m. Each cycle starts with a channel
sandstone. Channel incision commonly ceased on a resistant caliche
layer. In type 1, the lower and middle parts of the overlying
floodbasin sequence are mudstone interbedded with levee and
crevasse-splay deposits with immature stage 1 caliche paleosols. The
upper part of the floodbasin sequence is mudstone with caliche
horizons of stage 2 or higher. Stratigraphically upwards, caliche
paleosols become closer spaced, thicker, and more mature. A new
cycle starts with arrival of a channel by river avulsion. The type 2
cycle is initially similar to type 1, but then there is a return to
interbedded mudstones and levee and crevasse-splay deposits with
immature caliche, reflecting the approach of a meandering channel,
whose arrival ends the cycle. Caliche maturity increases as the
chance of being buried in sediment decreases, so that immature
caliche occurs on levees, proximal crevasse splays, and proximal
floodbasin muds, whereas mature caliche forms on distal floodbasin
muds.
Four megacycles, from 21-28 m in thickness, are characterized
by upward increases in thickness of the channel sandstones, number
of stories in each channel sandstone, and maximum grain size in each
channel sandstone. These megacycles may be due to a climate-
forcing Milankovitch mechanism.
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