Abstract

Petrographic, chemical and isotopic studies of marine sediment-hosted septarian concretions from Lower Jurassic Whiteaves Formation in the Queen Charlotte Islands, B.C., Canada, reveal a complex diagenetic history that has resulted from bacterially mediated reactions at shallow burial depth as well as hydrothermally modified meteoric waters. The general diagenetic sequence in cement-filled septaria cracks includes: 1) fibrous calcite, 2a) bladed calcite or 2b) ferroan "saddle-like" calcite, 3) blocky calcite with early ferroan and late non-ferroan stages, and 4) barite. Each cement generation has a characteristic chemical and isotopic signature. Oxidation of upward-diffused methane contributed to an extremely negative value ofdelta C (ca. -35 per thousand ) in the early generation of fibrous calcite. The delta O (ca. -3.0 per thousand ) and 87 Sr/ 86 Sr (0.70683) values of these calcites, however, reflect Jurassic seawater composition. Plutonism during Late Jurassic to Early Cretaceous time provided hydrothermal discharge that altered meteoric waters. This process became the main factor for the precipitation of "saddle-like" and bladed calcites. These two generations of carbonate cement are characterized by relative depletion in delta 18 O (-11.5 to -17.4 per thousand ) and less radiogenic 87 Sr/ 86 Sr (0.70532 to 0.70575) signatures. Progressive modification in pore water chemistry and increasing depth of burial occurred during the formation of late ferroan blocky calcite.

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