CAT-SCAN IN MARINE STRATIGRAPHY - A QUANTITATIVE APPROACH

Computed axial tomography (CAT-scan) gives a pseudo-3D representation of sedimentary cores. Sediments may be described and numerically proce ssed from digitized high resolution images. A 150 m long borehole was drilled and sampled at Ile-aux-Coudres, middle estuary of the St. Lawr ence, Quebec, and analysed by a tomodensitometer (CAT-scanner). This b orehole represents mainly the Illinoian-Sangamonian transition series (isotopic stage 5e, 130-80 kyr B.P.) of a post-glacial glacioisostatic marine invasion and the subsequent regressive prodelta. Four represen tative samples, 900 mm long, were chosen for analysis from a transitio n zone between glacial and prodeltaic sediments. They have been descri bed from tomographic longitudinal and transversal sections, from CT nu mber series, and also from classical analysis such as grain size, orga nic matter and carbonates. CT numbers reflect sedimentary characterist ics and are closely linked to grain size and to organic matter. Carbon ates are only weakly correlated, probably because of its low concentra tion. Bed thickness and numerical processing of CT number series are u sed to characterize lamina, and allow for depicting sub-annual, annual and longer periodicities. Clay facies, fine and thick rhythmites, and isolated diamicton beds were identified. From bottom to top, the firs t sample contains homogeneous clay rhythmites, typical of a distal env ironment; the second is composed of diamictons and thin, alternating l ight-dark annual lamina sets; in the third sample, annual light-dark r hythmites are slightly thicker and display subtle semi-annual cyclicit ies; in the fourth sample, annual rhythmites are several centimetres i n thickness and contain preserved sub-annual events, of possible seaso nal cyclicity. This succession of facies and associated stratal period icities may be typical of transgressive sedimentation in a basin such as the St. Lawrence estuary during the transition from glacial to non- glacial conditions.