A SWASH-BUILT BEACHROCK-INFLUENCED CARBONATE BERM SYSTEM: EVIDENCE OF MICROTIDAL LOW AMPLITUDE BARS?

A shallowing-upward shoreline deposit in the Lower Carters Limestone (Upper Ordovician, Nashville Dome) formed on an isolated, wave-dominated, storm-influenced carbonate strandplain. A repeated facies association of interbedded, grainstone-packstone (50-70 cm thick) and dolostone (~20 cm thick), located near the top of the deposit, is interpreted to indicate stacking of high intertidal berm and swash pool couplets. Berms typically form by shoreward migration of slip-face bars, producing a telltale, landward-dipping, steep cross-lamination on the lee side of the bar crest. Upper Carters’ berm sands exhibit only very low angle (1-2^o) to horizontal, parallel laminations. Intertidal ridge-and-runnel (aka low amplitude) bar systems may produce this type of berm deposit (i.e. lacking slip-face cross-lamination). However, RRB involve specific conditions of tidal range, fetch, sediment size and beach slope, in particular, meso- to macrotidal conditions. Three postulates therefore merit consideration: 1) these are not intertidal RRB deposits; 2) this is (at least) a low mesotidal setting; 3) under specific forcing conditions, RRB's may form in microtidal settings. Based on the totality of associated sedimentological evidence, formation via intertidal RRB’s is likely a “best-fit” model for these berm deposits, contradicting postulate #1. Strong supporting evidence for postulate #2 is lacking, although the Upper Carters Ls does host a (?microtidal) tidal flat system. Concerning postulate #3, beachrock deposits and hardgrounds may have affected morphodynamic feedback, resulting in non-parametric boundary conditions. These include reducing swash infiltration and sediment availability, and enhancing wave dynamics and sediment transport. Therefore, postulate #3 may be the most parsimonious scenario.