Petrography and Diagenesis of the Upper Cambrian Mt. Simon Sandstone, Southeastern Minnesota

Abstract

The Mt. Simon Sandstone (or Mt. Simon Formation) is the lowermost formation of the Dresbachian Stage (Upper Cambrian). The unit rests unconformably on a wide variety of Precambrian rocks in southeastern Minnesota and its average thickness is near 260 feet. The Mt. Simon Sandstone is dominantly a white to gray, medium- to fine-grained quartz sandstone (Q95.4F4.1L0.5 ) with minor intercalated thin shale beds. A noticeable feature of the unit is the abundance of authigenic potassium feldspar. Petrography and microprobe analysis were used to identify the framework and matrix constituents of the Mt. Simon Sandstone and to determine the diagenetic textures, paragenetic sequence and chemical composition of authigenic feldspars. Almost 95 percent of the framework grain components consist of monocrystalline (common) quartz, potassium feldspar and plagioclase, with the feldspar content higher in finer sandstones. Other minor detrital minerals include polycrystalline quartz, vein(?) quartz, recycled quartz, rock fragments, micas, collophane fossil fragments (brachiopods), and glauconite. Potassium feldspar is the most common authigenic mineral in the Mt. Simon Sandstone. The feldspar is present in almost all the samples as euhedral to subhedral overgrowths on detrital grains, ranging from 0.2 to 23.5 percent. The overgrowths are not in optical continuity with the detrital cores, and are usually pure orthoclase in composition, as evidenced by electron microprobe analysis. Minor quartz cement is present along detrital quartz contacts and in very few cases as obvious overgrowths. Authigenic dolomite exhibits two different textures: a) As euhedral, sand-sized rhombic crystals with considerable zonation showing iron-rich (red) and iron-poor dolomite (clear), and b) As rhombic finely crystalline subhedral crystals replacing illitic matrix. Poikilotopic calcite is found in a few samples, partially replacing detrital and authigenic minerals. Calcite is also present as isolated patches filling pore spaces. Kaolinite cement is most commonly found as vermicular aggregates filling pore spaces. Other authigenic minerals include hematite, pyrite and leucoxene, present as grain coatings and pore fillings. A high mineralogical maturity in the sandstones is evidenced by the presence of rounded to subrounded zircon, tourmaline, garnet and rutile. Other non-opaque detrital heavy minerals found include apatite, amphibole, pyroxene, epidote, diaspore and staurolite. The sandstone has undergone a varied diagenetic history, which includes precipitation and dissolution of authigenic minerals, dissolution of unstable detrital grains, and compaction. The paragenetic sequence in the Mt. Simon can be summarized in three stages: an early diagenesis stage marked by the presence of leucoxene and iron oxides; a burial diagenesis stage marked by the precipitation of quartz, potassium feldspar, kaolinite, illite, dolomite and siderite(?); and finally a late diagenetic stage which includes pyrite and calcite. The total average porosity for all samples is 15.5 percent; part of it is secondary porosity, as evidenced by the presence of partially dissolved grains, cements and matrix.