CHARACTERIZATION AND ORIGIN OF THE GARRARD SILTSTONE (UPPER ORDOVICIAN), CENTRAL AND EASTERN KENTUCKY (STORM DEPOSITS)

The Caradocian Garrard Siltstone is the only major siliciclastic formation among the fossiliferous Ordovician carbonates in Kentucky. The Garrard consists of quartzose siltstones interbedded with arenaceous shales and calcirudite limestones; it is restricted to the central and eastern parts of Kentucky. The formation was deposited as part of an overall coarsening-upward sequence associated with the warning phases of the Taconic Orogeny within a relatively shallow-water, outer-shelf depositional system. Sedimentary features suggest that both storm and fair-weather processes were involved in its formation.;The Garrard is characterized by "hummocky sequences" which reflect storm activity. The typical Garrard hummocky sequence includes in ascending order: (1) an erosional base with localized tool marks; (2) a basal lag of bioclasts and/or lithoclasts; (3) hummocky stratification; (4) flat laminae; (5) wave-ripple laminae; and (6) a thin, capping shale. Storm-generated allochthonous calcirudites commonly occur as the basal portion of limestone-siltstone storm couplets. The above sequence reflects oscillatory flow and waning-flow deposition initiated by high-energy storm events, whereas the intervening units of shale represent a return to fair-weather, hemipelagic deposition. Reworking by tidal and/or normal marine currents produced megaripples and sand waves on structurally controlled topographic highs. Soft-sediment deformation structures occur extensively in the siltstones and formed largely by liquefaction and foundering into underlying muds. A brachiopod community dominated by the opportunistic genus Rafinesquina characterizes the autochthonous Garrard faunas, which pioneered newly formed substrates and produced firmgrounds that succeeding organisms colonized.;The mineralogy of the siltstones suggests the initial source was a granitic terrane located to the southeast, however, the immedate sources were probably pre-existing shelf sediments from which northwestwardly moving storm-surge currents entrained and transported the sediment to Garrard outer-shelf environments.;Coarsening and thickening southeasterly, the Garrard attains its maximum thickness near the Kentucky-Virginia border where it merges into the Martinsburg Formation. The westward and northward limits of the unit largely coincide with basement faults, suggesting that growth faults reactivated during the Taconic Orogeny probably controlled the distribution of the unit. Changes in thickness and facies distribution also were probably related to structurally controlled bottom topography.