LONG, BERNARD F., XAVIER BOESPFLUG, and NASER SABEUR, INRS-Oceanologie, Rimouski, Quebec, Canad

ABSTRACT: High Frequency Cyclicity Analysis from Scanner Density Measurements. Example of the Lower Sagamonian Formation of the St. Lawrence Estuary

The lower Sangamonian Formation (130 Ky to 100 Ky BP) is developed along the mid-estuary of the St. Lawrence on 140 m thick. It represents a regressive system track (RST) where sea level fall from + 200 m (130 Ky) to zero (100 Ky) during the glacio-eustatic rebound. The series is comprised of till, prodeltaic, deltaic and lacustrine sediments and the average sedimentation rate is 6.0 mm/y.

The high frequency cyclicity analysis has been conducted on two cores of 150 and 100 m long obtained along the St. Lawrence mid-estuary. Each core has been analyzed by a CAT Scanner (GE 9800 X Ray scanner). On this type of analysis the image resolution is 0.7 mm (pixel size) and 1536 density values are available between black and white for each pixel. A pseudo linearrelationship appears between the scanner density value (CT number) and the bulk density. A numerical serie is created after the CT value of each pixel along the longitudinal section. This very high density sampling permits a high-resolution investigation of the annual sedimentation on the mid-estuary. A sequential analysis gives, after removing the trend by an orthogonal polynomial regression, the spectral density function

The sequential analysis suggest that the thickness of the rythmicities are 6 mm, 10 mm, 26 mm, 35 mm and 123 mm. On a short sampling interval (1 m) a very precise rythmicity appears but it is due to a specific type of sedimentation characteristic of a special event. If the 6 mm rythmicity is equal to the annual cycle, the 26 mm (periodicities of 4.3 years) is interpreted as an El Nino phenomenon and the longers as the sunspot cycles.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.