--> Abstract: Carbon and Oxygen Isotopes for the Abiod Formation, Tunisia, by T. C. O'Hearn, R. T. J. Moody, and K. Ratcliffe; #90987 (1993).

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O'HEARN, TERRENCE C., Texaco EPTD, Houston, TX; R. T. J. MOODY and K. RATCLIFFE, Kingston Geological Services, Dominic House, Kingston-upon-Thames, UK

ABSTRACT: Carbon and Oxygen Isotopes for the Abiod Formation, Tunisia

Stable carbon and oxygen isotope data are reported for chalk matrix and fracture-filling calcites from the Abiod Formation (Campanian-Maastrichtian) in central and northern Tunisia. The Abiod is a very clean foraminiferal biomicrite with an average total clay content of 2.6%.

Surface chalk matrix samples have an average (isotope){18}O value of -3.13 o/oo, and an average (isotope){13}C value of +1.38 o/oo (all values relative to PDB). Average (isotope){18}O values for subsurface chalk matrix samples range from -3.03 to -4.93 o/oo, and become progressively lighter with depth. Average (isotope){13}C values for subsurface chalk matrix vary from +2.07 to +2.22 o/oo. Oxygen isotope data indicate progressive lithification of the Abiod during burial. The lack of significant shifts in both carbon and oxygen isotopes precludes meteoric influence during Abiod diagenesis.

Fracture-filling calcite cement within an early fracture set has a mean (isotope){18}O value of -7.22 o/oo, and a mean (isotope){13}C value of +2.40 o/oo. Fracture-filling calcite cement within a late fracture set has a mean (isotope){18}O value of -8.40 o/oo, and a mean (isotope){13}C value of +2.19 o/oo. Carbon values are similar for both chalk matrix and fracture-filling calcite, and have not changed significantly during diagenesis, reflecting control by pre-diagenetic processes. Calcite cements within fractures are sourced internally by pressure solution processes, and are not related to external basinal or meteoric waters. Chalk lithification and fracture-filling calcite precipitation occurred within a closed to semi-closed diagenetic system.

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