FLUVIAL ARCHITECTURE IN THE TRIASSIC NEW HAVEN ARKOSE, HARTFORD BASIN: EVOLUTION OF A FLUVIAL SYSTEM

In southern Connecticut, a recently exposed outcrop of the Triassic New Haven Arkose consists of two distinct laterally extensive units of fluvial deposits. Detailed measured section and lateral profile analyses document variations in lithology and architecture between the units, which reflect a transition in fluvial style.

The lower unit (>3m thick) consists of ten amalgamated channel elements, each a lenticular sheet (11-56 m wide) of poorly sorted, granular to pebbly, lithic arkose sandstone. Individual elements have meter-scale erosional relief and contain mudstone rip-up clasts, pebbles and cobbles at their base. Low-angle planar cross beds, trough cross beds and pebble stringers are present throughout. Downstream and lateral accretion (LA) elements, and mud-partings are rare. Paleoflow from troughs show SSE transport.

The upper unit (>15m thick) consists of several isolated channel elements (<35 m wide and <1 m deep) encased by overbank fines. Channels decrease in size towards the top of the outcrop. Individual channels contain LA surfaces and fine upward from coarse-grained sandstone with pebbly bases to muddy sandstone. Well-rounded, cm-size carbonate clasts are found at the base of some channels. Silty mudstone caps to some elements indicate channel abandonment. LA surfaces and troughs signify SSE transport. Overbank fines consists of mottled m-scale tabular mudstone and dm-scale, interbedded fine-grained sandstone, siltstone and mudstone beds. Bioturbation and carbonate nodules define caliche paleosols. Two well-defined paleosol horizons span the lateral extent of the outcrop. Carbon (δ¹³ C_PDB from -6 to -9.9) and oxygen (δ¹⁸ O_SMOW from -12.3 to -14.7) stable isotope analyses from the nodules support an arid climate.

Differences in channel elements and amount of floodplain fines between the two units indicate a difference in fluvial style. The lower unit was deposited in shallow, braided bedload streams with frequent channel shifting and little deposition of fine-grained sediment. The upper unit was deposited in mixed-load meandering streams with stable banks. Overbank fines were deposited as floodplain, levee and splay deposits. Transition in fluvial style is attributed to change in sedimentation rate resulting from progressive reduction in paleoslope (or discharge) over time.