FACIES ANALYSIS AND AGE RELATIONSHIPS OF LATE DEVONIAN-MIDDLE PENNSYLVANIAN CARBONATES OF THE SOUTHERN ALEXANDER TERRANE, SE ALASKA

The nature of Carboniferous carbonate deposition within the Alexander terrane of southeast Alaska is poorly understood. Previous studies in this area have largely focused on observation of varying carbonate lithologies and conodont faunas. Those data are weakly tied to stratigraphic columns and reconstruction of environmental change through time is difficult. Here we present two new stratigraphic sections of Late Devonian(?) volcaniclastic to Pennsylvanian carbonate lithologies on islands south of Craig, Alaska,

Along the eastern coast of an unnamed island, 1 kilometer west of Shelikof Island, a relatively continuous and structurally coherent outcrop spans the uppermost Port Refugio Formation (Devonian?) to the Ladrones Limestone (Pennsylvanian). This section encompasses 428 meters of Upper Devonian-Lower Mississippian to Lower Pennsylvanian strata that exhibited facies representing volcaniclastic- influenced deposition at the base and shallowed upward to inner ramp facies at the top of the section.

The second stratigraphic section lies along the northwestern shoreline of the southernmost Ladrones Islands. This section encompasses 414 meters of carbonate strata and spans the uppermost Peratrovich Formation (Mississippian) to the upper Ladrones Limestone (Middle Pennsylvanian). The base of the section consists of outer ramp facies that shallow upwards into inner ramp facies of the Ladrones Limestone.

Rare occurrences of the conodonts Bispathodus and Idiognathodus constrain a Late Devonian-Early Mississippian to Early Pennsylvanian time of deposition, and when coupled with stratigraphic analysis the succession appears to house no major hiatuses. The thickness of carbonate strata deposited over this time interval is fairly thin, indicating that the long term creation of accommodation space was minimal. The paucity of volcaniclastic debris in a region described as adjacent to intermittently active volcanism suggests that carbonate deposition may have occurred atop a mature seamount. The presence of the conodont Neolochriea in the Ladrones Limestone, only known from seamount carbonates of Japan and deep water carbonates in south China, suggests an oceanic setting.