Abstract: Sedimentology and Diagenesis of the Lower Cretaceous Basal Quartz Formation, Crossfield/Delacour Field, Calgary Area

Lower Cretaceous Basal Quartz Formation (BQ) sedimentology, hydrochemistry and diagenesis from T14-32, R28W4 was examined. BQ sediments record two transgression-regression cycles (BQB = lower; BQA = upper). BQB braided river deposits grading vertically and laterally northward into estuarine deposits were deposited in response to a rise in relative sea-level, and are capped by a paleosol. BQA deposits overlie and mimic the BQB cycle; braided river through estuarine deposition in response to rising sea-level. BQA deposits are unconformably overlain by marine to nearshore Ellerslie Fm. deposits.

Formation water composition and hydrodynamic head maps suggest that the Delacour/Crossfield BQ gas field and strata adjacent to the Jurassic subcrop edge are isolated from regional fluid flow. Recharge occurs from the NW in the Rocky Mountain foothills and the SE from the high-standing sweetgrass arch.

Diagenetic minerals are: abundant quartz overgrowths, minor pore-filling kaolinite, pore-rimming and bridging illite, pyrite cement and framboids, barite, and local calcite and dolomite cement, adjacent to the Jurassic subcrop edge. Three calcite precipitation events were identified through petrography and stable isotope analysis (vs. PDB): i) delta{18}O = -11, delta {13}C = +2.5 at 1.5 km, 42 degrees C from recrystallization of shells by meteoric waters; ii) delta{18}O = 11 to -16, delta {13}C = -4 to -8.5 at 2.7 km, 75 degrees C from mixing of saline waters from Paleozoic strata with fresher waters from Mesozoic strata; iii) delta{18}O = -7.5 to -11, delta {13}C = +0.5 to +1.5 at 3.5 km, 100 degrees C from an evaporated brine from Paleozoic strata. Quartz precipitated in three stages: i) less than 1.5 km, less than 42 degrees C; ii) 1.7-2.5 km, 50-65 degrees C; iii) 3.5 km, 80-90 degrees C. Kaolinite, illite and barite precipitation and secondary porosity, due to dissolution of chert grains and calcite cement, occurred during uplift. Pyrite precipitation timing is difficult to determine. Quartzose estuarine deposits are quartz cemented, while chart-rich braided river deposits contain calcite indicating that initial mineralogy controls diagenetic alteration. 

AAPG Search and Discovery Article #90940©1997 AAPG Foundation Grants-in-Aid