부산대학교 도서관

During Late Mississippian and Early Pennsylvanian time, a change in relative sea level caused marine invasion of ancient stream valleys cut into the top of the Redwall Limestone. These deposits are now exposed as isolated lens-shaped patches up to 120 m thick and 370 m wide in the walls of the western Grand Canyon. The basal unit of the valley fill consists of a fining-upward sequence from chert-pebble conglomerate to ripple-laminated and flat-bedded sandstone and siltstone, suggesting a transition from fluvial domination to tidal domination. The middle unit is a yellow-brown, cliff-forming skeletal limestone with a diverse marine fauna. This limestone unit has a large, distinct channel outline and commonly exhibits trough cross-strata having bimodal current directions. The dominant flow direction is upstream, suggesting a flood-tidal channel that filled with skeletal debris. Skeletal grainstone beds are commonly paired with a dark purple-brown sandy limestone, interpreted to be the ebb-tidal channel. Diagenetic patterns within the grainstone show good reserve possibilities within the flood-tidal channel. Above the limestone are mud-cracked and bimodally ripple-laminated sandstone and siltstone with interbedded algal, ostracodal limestone that suggest restricted intertidal conditions. Within the conglomerates and limestones are opaque hematitic clasts and two types of quartz grains that suggest early ferruginization and early silicification of evaporites on the newly exposed Redwall platform.