DEPOSITIONAL PROCESSES AND STRATIGRAPHY OF ALLUVIAL DEPOSITS IN THE JURASSIC PORTLAND FORMATION, CONNECTICUT

The upper Portland Formation contains thick packages of amalgamated sandstone and conglomerate beds that have traditionally been interpreted as alluvial fan and braided stream deposits laid down along the edge (Eastern Border Fault) of the Hartford Rift Basin. Examination of new outcrops revealed four lithofacies that reflect deposition under a range of flow conditions. These facies are vertically stacked into cycles that reflect a combination of autocyclic and allocyclic (tectonic and climatic) depositional controls.

Facies are: 1) plane-bedded conglomeratic sandstone, 2) trough cross-bedded conglomeratic sandstone, 3) massive sandstone, and 4) siltstone. The plane-bedded facies and trough cross-bedded facies both consist of medium- to coarse-grained sandstone in beds (0.5 to 2 m thick) that typically have discontinuous conglomeratic lenses at their bases. Bedding surfaces are low-relief, but can have small, shallow channels. These facies are interpreted to represent sand bars deposited during periods of high flow rates in braided streams. The nature of cross-bedding (plane-bedding or trough cross-bedding) most likely reflects varying flow conditions or deposition in different locations along a channel bar. The massive facies is medium- to coarse-grained sandstone beds (1 to 2 m thick) with isolated gravel and cobbles . Massive beds are interpreted as hyperconcentrated flow deposits from periods of high flow rates and rapid deposition in braided stream channels. The siltstone facies occurs in thin beds (typically less than 20 cm), and is interpreted to represent deposition under reduced flow velocities, probably in floodplains.

Where siltstone is present, vertical cycles of the facies are apparent. Each cycles begin with several plane-bedded, trough cross-bedded, and/or massive sandstone beds (braided stream channel deposits) and fines upward into siltstone and thin cross-bedded sandstone beds (channel margins and floodplains). These cycles are interpreted to reflect autocyclic processes such as channel avulsion. Where siltstone is rare, individual cycles cannot be identified. These intervals may reflect deposition under lower accommodation or higher sediment supply conditions than the intervals that contain more siltstone.