CHARACTERISTICS AND INTERPRETATION OF UPPER-OFFSHORE DEPOSITS

Upper-offshore deposits consist of laterally persistent, yellowish green and gray parallel-laminated mudstone with thin, tabular, typically erosively-based, light grey, very fine-grained, silty sandstone beds (Figure 4.1-4.2). Parallel lamination, combined-flow ripple cross-lamination, and symmetrical to near-symmetrical ripples with rounded tops are common. Microhummocky and hummocky cross-stratification, current-rippled cross-lamination, gutter casts, small load casts, and tool marks occur locally. Fragments of brachiopods (including lingulids), trilobites and crinoids, as well as conodonts, are present in shell lags. Articulated and semi-articulated trilobites are locally common within the mudstone units (Waisfeld and Vaccari 2003).

Upper-offshore deposits represent alternating background suspension fallout in a low-energy setting punctuated by distal storm deposition. The presence of micro-hummocky cross-stratification, combined-flow ripple cross-lamination, symmetrical to near-symmetrical ripples and gutter casts indicates periodic disruption by storm flows in an otherwise low-energy environment dominated by slow accumulation of silt-sized particles during fairweather conditions (Buatois and Mángano 2003).