WETUMPKA IMPACT STRUCTURE, ALABAMA: LARGE ORIENTED BOULDERS IN AN INTERIOR DIAMICTON

Wetumpka impact structure, Alabama, is a well-preserved, Late Cretaceous marine target feature of about 5 km in diameter. Structure-filling stratigraphy in this small but complex crater includes enigmatic impactite sands in the crater moat area and a megabreccia unit in the vicinity of the former central peak, both of which are overlain by a trans-crater slide unit. Atop the trans-crater slide unit, lies a distinctive matrix-rich, boulder and conglomerate bed, a few 10s of m thick, which is situated mainly in the central region of the crater floor. The boulder and conglomerate bed is a diamicton of impact origin, as confirmed by the presence of shocked framework silicate grains (e.g., quartz with PDFs), kink-banded micas, and other impact-affected sand grains and pebbles. Grain sizes in the matrix of the Wetumpka diamicton are clay-dominated, but there is also a significant proportion of non-clast supported pebbles, cobbles, and boulders. The grain-size distribution is strongly bi-modal, i.e., there is a gap in the size distribution of boulders from about 0.3 m to 4.5 m. In the 0.5 sq km study area, we have located 5 boulders in the size range of 4 to 6 m. Three of these boulders are schist, one is gneiss, and one is a quartzite-schist. All these boulders come from the deeper target metamorphic basement rock that underlies the Upper Cretaceous soft sediment of the upper target layer. All of these boulders have the unusual aspect of vertical orientation of foliation, which indicates a specific process at work in controlling their orientation within the diamicton. We suggest one mechanism for this may be vertically oriented planes of mass-failure in the slumping transient crater wall or the disintegrating central peak.