Carbonate deposition in an ephemeral siliciclastic alluvial system: Jurassic Shuttle Meadow Formation, Newark Supergroup, Hartford Basin, USA

Nonmarine carbonate accumulation, from a sedimentologic perspective, has multiple possible origins: clastic load, springs, groundwater discharge, soil development, biochemical seasonal changes, cave speleothems, and eolian influx. Many of these carbonates contain diagnostic textures from which origin is easy to ascertain. However, differentiating between clastic and spring/groundwater carbonates, especially in depositional systems dominated by siliciclastics, is not always simple; here criteria for recognition are developed. Subsurface hydrology and tectonic origin of the depositional basin, source rocks surrounding the basin, sedimentologic packaging and extent of the carbonates in question, as well as their sedimentary structures and fossil content, and stable isotopic analyses of the carbonates can all aid in determining carbonate origin and correctly assessing sedimentary and basin evolution. The present study focuses on a nonmarine carbonate deposit found within the Jurassic Shuttle Meadow Formation of the Hartford Basin, one of many rift basins which formed as a result of the opening of the North Atlantic Ocean during the Late Triassic to Early Jurassic. The upper 50 m of the formation at Plainville, Connecticut, is interpreted as continental in origin and is composed of five facies: red ripple cross-laminated muddy siltstones to sandstones (Fr, Sr), massive red siliciclastic mudstones (Fm), buff fine- to medium-grained horizontally laminated sandstones (Sh), red to white disrupted calcareous shales (Fl3), and rare micritic carbonates (Pm). Facies Fr, Sr, Sh, and Fm are interpreted as an alluvial plain system in an ephemeral setting with bedload sheetflooding as the dominant depositional mechanism, similar to the Channel Country drainage system of central Australia. In addition, preservation of lateral accretion sandstone sheets (accretionary benches) indicate that the Shuttle Meadow alluvial system was rapidly accreting. The anomalous existence of carbonate beds (in Facies Fl3 and Pm) in the SMF bedload-dominated siliciclastic sequence is attributed mainly to the unique nature of the clastic sediment load in the basin; no direct evidence for a spring/groundwater input is apparent. The shallow carbonate lake margin or a pond with vegetation and periodic exposure is envisioned as the depositional environment. The intraclastic texture of the micrites with rare primary relict lamination indicates early diagenetic alteration from root penetration, desiccation cracks, and burrowing.