| Abstract |
- The Middle and Upper Devonian Guilmette Formation was deposited within the inner shelf region of the Cordilleran miogeosyncline. Twelve distinct lithofacies are recognized in the Guilmette Formation
and represent deposition from various peritidal environments including quiet-water lagoons, organic buildups, a barrier bar/beach complex, and intertidal through supratidal flats. The relative sea-level curve determined from the vertical
succession of lithofacies in the lower Guilmette Formation defines a period of relatively constant subtidal sedimentation punctuated by clusters of abrupt shallowing and deepening events; no well-defined
shallowing-upward sequences are observed. Six distinct shallowing-upward cycles are recognized in the middle and upper Guilmette Formation; deepening events at the base of each cycle are abrupt, commonly local in extent, and progressively stronger in
magnitude upsection, suggesting that crustal instabilities related to Anlter orogenic activity affected the inner shelf region prior to formation of the Pilot basin. Quartz-rich lithofacies (lithofacies 3, 8, arid 9) were deposited in intertidal and supratidal environments arid underwent very early diagenetic alteration, including silica and carbonate cementation, neomorphism, arid syndepositional dolomitization. Syndepositional dolomitization occurred as the result of surface-derived, downward-flowing fluids, possibly from hypersaline brines. Because of rapid subsidence arid sedimentation rates, subtidal. carbonate lithofacies (lithofacies 1, 2, 4, 5, 6, 7, 10, 11, and 12) were transported into moderate arid deep burial environments prior to significant alteration by shallow phreatic diagenetic processes. Diagenetic alteration inc1iii early marine cementation, neomorphism, mineral stabilization, calcite cementation,
siicification, and late diagenetic dolomitization; late dolanitization affects less than 25% of the Guilmette Formation. The lack of any fresh or brackish water diagenetic textures, the limited and patchy distribution of late diagenetic dolomite, the
lack of dolomite associated with the upper Guilmette Formation disconformity, and the association of dolomite with abundant
stylolites preclude dolomitization by mixing-zone processes. The same criteria suggest that dolomitization occurred during moderate to deep burial from internally-derived Mg-ions arid fluids moving along stylolites. The major source for Mg-ions was supplied by interbedded syndepositional dolomite beds arid the underlying Simonson
Dolomite. Late diagenetic dolanitization of the Guilmette Formation was indirectly paleogeographically controlled because inner shelf tidal flats are the most common environment for syndepositional dolomitization.
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