DEPOSITIONAL CONTROLS ON A HIERARCHY OF CYCLES PRESERVED IN LOWER JURASSIC FLUVIAL SEDIMENTS (UPPER PORTLAND FORMATION, HARTFORD RIFT BASIN, CT)

Braided fluvial sediments of the upper Portland Formation (Lower Jurassic) were deposited during waning phases of rifting in the Hartford Basin, CT. These strata record several orders of cyclic packaging based on grain size trends and the facies distributions. The cycles are interpreted to reflect changes in accommodation and sediment supply related to both autogenic and allogenic processes.

Individual channel-fill sequences represent the smallest of these elements. They range from less than 0.5 to about 2 meters in thickness and are typically composed of planar-bedded /cross-bedded conglomerate and sandstone that fines upward. They are bounded below by a surface that records an abrupt grain size increase and often shows evidence of erosion. In some places the channel-fill elements are capped by a thin, discontinuous mudstone layer that represents channel abandonment. These channel-fill elements amalgamate both laterally and vertically to form larger-scale multistory channel belts. The channel belts range between 1.5 and 11 meters in thickness, and generally either fine-upwards, or first coarsen, then fine-upwards. Although channel-fill elements thicken and thin irregularly within a channel belt element, the channel belts maintain a relatively uniform thickness. Each channel belt is bounded below by an erosional surface with up to 1 meter of relief. Thicker (typically > 5 cm), continuous mudstone layers with evidence of pedogenic alteration cap the channel belt elements. Finally, several channel belt elements stack into outcrop-scale (30+ meters), fining-upward packages, representing the largest scale of cyclicity. They are capped by thicker (meters) floodplain mudstone intervals.

The smallest scale channel-fill elements are likely caused by autocyclic migration of channels and bars within a braided river system. The larger channel belts likely represent autogenic avulsions within the overall fluvial environment. Alternatively, they may have resulted from a cyclic climate control that influenced the capacity of the rivers to transport bedload or affected accommodation. The outcrop-scale fining upward trend may be controlled by allogenic factors such as climate, tectonics, or changes in base level that resulted in an increase in accommodation.