Depositional and diagenetic history of the lower Silurian 'basal transgressive sandstone' in the Southern Appalachians

The boundary between Upper Ordovician (Ashgillian) tidal-flat redbeds and Lower Silurian (Llandoverian) marine shelf clastic strata in the southern Appalachians is marked by a distinct 0.6-3 m thick, very fine- to medium-grained, quartzose to sublitharenitic, transgressive sandstone body that can be traced across the Copper Creek, Wallen Valley, Whiteoak Mountain, Pine Mountain, and Sequatchie thrust sheets. It is interpreted to have been deposited in a beach face to inner shelf setting. This 'Basal Transgressive Sandstone (BTS) can be divided into three distinct facies variants. The wave-rippled to horizontally cross-stratified variant (WL) only occurs along the Copper Creek thrust sheet in eastern Tennessee, and it represents deposition in a beach face to upper shoreface setting. The cross-bedded sandstone variant (CBS) occurs along the Whiteoak Mountain thrust sheet and is interpreted as representing channel sequences that filled in existing tidal channels as the sea slowly transgressed over tidal flat deposits of the Upper Ordovician Juniata and Sequatchie formations. The fine-grained bioturbated variant (FGB) occurs along the Pine Mountain, Wallen Valley, Whiteoak Mountain, and Sequatchie thrust sheets. This variant is considered to represent the lower shoreface to inner shelf setting where deposition was slow and continuous to discontinuous, depending upon the degree of bioturbation displayed. The physical features observed within the BTS are strongly suggestive of the occurrence of an unconformity at the Ordovician-Silurian boundary. The sandstone overlies Upper Ordovician Lingula -bearing tidal-flat deposits that were laid down during a relative sea-level lowstand. Evidence for prolonged subaerial exposure occurs in the form of paleosol horizons which occur directly beneath the BTS at the easternmost exposures (Copper Creek thrust), the most proximal setting of the BTS. Phosphatic pebble lags occur there and in all the more distal deposits, and are interpreted to represent ravinement surfaces which formed through the process of erosional shoreface retreat during a relative sea-level rise. The BTS is always overlain by a distal terrigenous shelf sequence consisting of thin tempestites interbedded with gray-green shale, further indicating a relative sea-level rise. Within the westernmost exposures, a distinctive color change from red to gray occurs almost directly beneath the BTS. All of these features point to the existence of an unconformity. There is question, however, as to the duration of time for which there are no preserved strata between the BTS and the underlying Ashgillian redbeds. Biostratigraphic evidence for an unconformity is lacking due to the paucity of body fossils in the Lx>wer Silurian-age strata and the lack of any chronstratigraphic indicators of narrow range among the species collected. Therefore, this unconformity represents a hiatus of unknown duration. Point-count data indicate that the most proximal deposits of the BTS, i.e. the WL variant, underwent a paragenetic sequence of deposition, mechanical compaction through burial, chemical compaction through intergranular pressure solution, and quartz cementation. Very minor dolomitization occurred late in the burial history. Rocks of the CBS and FGB variants probably consisted of a large amount of framework carbonate grains that were later completely replaced by ferroan dolomite and ankerite. This interpretation arises from the fact that, in thin-section, the detrital grains appear to be "floating" in ferroan carbonate material such that grain-to grain contacts are extremely rare among framework quartz and rock fragments, with the exception of some of the phosphatic pebble lags. Coarse, sand-sized quartz grains have rare overgrowths within these lags. These precursor carbonate cements have subsequently been replaced, through burial diagenesis, by ferroan dolomite and ankerite. Some precursor ferroan calcite still remains within the phosphatic pebble lags and has been partially replaced by ferroan dolomite and ankerite. Finally, dedolomite has been observed in thin-sections of samples collected from the most weathered outcrop exposures. The occurrence of this phase is apparently a weathering phenomenon.