Using CT to Image Storm-Generated Stratigraphy in Sandy Sediment Off Fort Walton Beach, Florida, USA

摘要

High-resolution measurements of X-ray attenuation with NRL's HD-500 microfocus Computed Tomography (CT) imager have been accomplished on 5.9cm- diameter diver-collected cores from an acoustic experiment conducted off Fort Walton Beach, Florida. The purpose of this imagery is ultimately to quantify sediment heterogeneity for modeling acoustic scattering. Cores, mainly consisting of medium quartz sand, were scanned with X-rays at 114-micrometer intervals downcore and the images were reconstructed to create a three- dimensional image of sediment density variations from the sediment-water interface to approximately 20 cm sediment depth. The small voxel size used in the image reconstruction allowed unusually detailed images to be acquired that reveal storm-generated sedimentary structure that include mud flasers, laminae from migrating sand ripples and depositional lag layers. Mud flasers are lens- shaped mud inclusions created by the deposition of fine-grained mud onto storm- generated ripples, followed by migration over the mud inclusions of sand ripples mobilized by subsequent storms. The size and shape of the flasers indicate that they were formed from mud immobilized in ripple troughs as the sand ripple crests advanced across the sea floor. Thin, planar, sub-horizontal laminations presumably due to climbing ripples are also evident from the CT images. Above or below sets of planar laminae are comparatively thick layers of homogeneous sand deposition or, infrequently, a coarse, shell fragment-infused lag layer presumably due to an increase and subsequent decrease in the bottom stress regime.