Glaciodeltaic Sediment and Glaciofluvial Erosion Sample Clauses

Glaciodeltaic Sediment and Glaciofluvial Erosion. Glaciolacustrine sediment includes sediment deposited in a proglacial lakefloor setting as ice margin retreated towards the north from the terminal moraine position. Meltwater from the retreating glacier generally drained towards the south, filling pre-glacial topographic lows and forming Glacial Lake Block Island. Drainage to a eustatically lowered sea level was largely prevented by the terminal moraines, except at spillways (Oakley, 2012). Sediment cores from Block Island, Rhode Island and Long Island Sounds have provided physical samples of the sediment, and has been described as rhythmically layered silt and clay couplets and clay concretions (Xxxxxxx and Xxxxxx, 1966; Xxxxxxx et al., 1977). The seismic characteristics of glaciolacustrine sediment are consistent with previous studies and are easily identified by laterally continuous, closely spaced, rhythmically layered reflectors that drape the underlying topography. Glaciolacustrine sediment is seen in all the interpreted sub- bottom profiles with the exception of Figure 18. Where the underlying topography is deeply incised, glaciolacustrine sediments exceed 50 m thicknesses (Figure 19). Glaciolacustrine sediment is most ubiquitous where water depths exceed 40 m (131 ft) within Block Island Sound. The fine-grained nature of glaciolacustrine sediment makes it an area of low interest for potential sand and gravel resources.

Glaciodeltaic Sediment and Glaciofluvial Erosion. A prominent reflector separates glaciolacustrine lake deposits and overlying glaciodeltaic deposits. This reflector forms an irregular surface that is laterally continuous, with shallow v-shaped channels. This reflector is a discontinuity associated with glaciofluvial erosion during the formation of glacial lake deltas. Glaciodeltaic sediment was deposited in a prograding glacial lake delta depositional environment as sediment-laden meltwater discharged from the retreating glacier in braided streams. As sediment- laden meltwater reached Glacial Lake Block Island, the finer grained fraction of sediment remained suspended in the water column, later to be deposited by turbidity currents or settlement due to a lower flow regime as glaciolacustrine silts and clays. At the proximal lake margin, coarser sediment including sand and gravel was deposited by fluvial deposition forming the coarse topset and foreset beds. The seismic characteristics of the glaciodeltaic deposits vary depending on the location along the delta as well as the angle that the sub-bottom lines transect the deltaic deposits. In N-S sub bottom profiles, steeply dipping reflectors are interepreted as foreset beds composed of sand and gravel (Figure 13). The foreset-bottomset bedding contact represents a transition to finer grained sediments including fine sand and silt. In other N-S sub-bottom profiles (e.g. Figure 19), glaciodeltaic sediment is more acoustically transparent, with a near absence of internal reflectors. In sub-bottom lines oriented parallel to the delta front, flat-lying to low-angle dipping reflectors are observed (Figure 15). Despite some differences in seismic characteristics, the thickness of glaciodeltaic deposits in the study area is typically 33-50 ft (10 to 15 m). Glaciodeltaic sediment is the most significant source of sand and gravel resources within the study area. The seismic characteristics are consistent with deltaic deposits composed of sand and gravel, though targeted sediment cores would help to characterize the thickness and suitability for beach replenishment.