Abstract: From Plutons to Pellets: Provenance of the Upper Cretaceous (Santonian) Virgelle Member, Milk River Formation, Writing-on-Stone Provincial Park, Alberta, Canada
MEYER, RUDI, and FEDERICO F. KRAUSE, Department of Geology & Geophysics, The University of Calgary
Provenance of Virgelle Member depositional system minerals illustrates the fundamental roles of: (1) chemical and mechanical weathering of parent rocks, which are controlled by original mineralogy, climate and physiography at the source; (2) mineralogical differentiation by transport and depositional controls; and (3) syn-depositional biogenic sediment transfer at the site of accumulation. The Virgelle Member in southern Alberta was deposited as a very sand-rich progradational depositional systems tract that comprises a storm-dominated lower/middle shoreface at the base and an overlying estuarine tidal system with sandstone bars, meandering channels, and associated coastal plain mudstones. The aim of defining the parentage of framework grains and matrix of this system is to establish the provenance of the original stratigraphic distribution. This is a necessary first step to assess timing and intensity of diagenetic processes.
The database consists of: (1) Optical and electron microscope petrographic description of various sandstones within the previously established depositional model (Meyer and Krause, 1998, in press), to establish overall framework matrix and cement structure and compositions; (2) X-ray diffraction analyses of bulk and U 2-micron fractions of sandstones and interbedded mudstone lenses (n = 19), to determine clay mineral distributions; and, (3) X-ray fluorescence spectrometry of selected samples (n=7), to establish major-element geochemistry. To analyze the data and compare it to possible parent rocks we make use of Quartz-Plagioclase-Kspar (QPK) and Al2O3-CaO*+Na2O-K2O (A-CN-K) ternary diagrams and calculation of a Chemical Index of Alteration (CIA), as proposed by Nesbitt et al., 1996.
Virgelle Member sandstones are fine-to-coarse grained, lithic arkoses with an average composition of 31% feldspar with a plagioclase composition of An30-40 and a P/K-ratio = 1-2, 30% monocrystalline quartz, 22% lithic fragments including polycrystalline quartz and chert. Up to 10% detrital dolomite, 5-11% matrix clays, siderite, biotite, and common ferroan calcite cement make-up the remainder.
It is apparent from this mineral distribution that dolomite, chert and rare 2nd-cycle quartz must be sourced in Paleozoic sedimentary rocks. However, about 75% of sandstone framework minerals originate from igneous/meta-igneous rocks (feldspars, quartz, igneous and metamorphic lithic fragments). Subequal amounts of smectite and kaolinite in interbedded mudstones are consistent with a parent of intermediate composition and/or a source region of moderate rainfall. Granitic-granodioritic plutons (e.g.. Nelson batholith) and high-grade metamorphic rocks (e.g.. Shuswap) of Jurassic and Lower Cretaceous age, exposed in the Omineca belt west of the thrust front of the northern Rocky Mountains, are the most likely parents with a CIA = 48-51. Average andesine composition of the plagioclase in these rocks correlates with that of the sandstones, and the parent P/K-ratios of 2.2-2.7 were progressively reduced by preferential loss of plagioclase during weathering at the source. Volcanic fragments are a minor component of the sandstones and the P/K-ratio of presently exposed, remnant sources (e.g. Elkhorn Volcanics) is too low.
The angularity of the grains, the high proportion of plagioclase and a low CIA = 55 for the sandstones, all apper to indicate dominantly mechanical weathering at the source, induced by a relatively dry climate and rapidly changing, steep slopes, rather than extensive chemical weathering. These characteristics also rule-out prolonged along-strike alluvial transport of the sediment, parallel to the developing thrust front, as has been inferred for both younger and older elastic wedges in the basin.
Finally at the site of deposition, the clay mineral distribution is modified at the transition from estuarine channels and distal tidal bars to densely burrowed and bioturbated sandstones of the upper middle shoreface. Kaolinite constitutes 70-95% of the clays in the sandstones, and much of it is the product of diagenetic feldspar alteration. On the other hand, dense kaolinite aggregates sometimes bound by faint grain boundaries, and a change from triclinic to monoclinic kaolinite and interstratified structural types, demonstrate a detrital origin. Significantly, at the shoreface, the clay mineral proportion of kaolinite in the mudstones is reduced by an average of 30%, equivalent to a 7% bulk decrease in kaolinite. We propose that the kaolinite depletion observed in shoreface mudstones coincides with syn-depositional transfer of suspended clays into sand-size equivalent mud pellets produced by filter-feeding burrowers such as those responsible for the common Ophiomorpha sp. traces. Pellet depositional rates of 12 metric tons/km2/yr have been documented on present-day coasts for two of many filter-feeding species alone, accompanied by kaolinite enrichment of 15 to 30% and microstructural changes (Pryor, 1975). These effects are comparable to those observed at the transition of the Virgelle Member, illustrating the impact of biogenic pelletization on the sedimentation of an ancient siliciclastic shoreface.
Thus, the mineralogy of the Virgelle Member portrays a history that reflects the dominant contribution from Jurassic and Lower Cretaceous plutons and high-grade metamorphic rocks, but also biogenic mass transfer by the selective feeding activities of bottom-dwelling organisms at the site of accumulation during the Santonian.