Presentation on theme: "Mature Sandstones-Arenites QUARTZ ARENITES ► Quartz Arenites Mistakenly referred to as orthoquartzites Quartz Arenite is accepted term Quartz arenite."— Presentation transcript:
QUARTZ ARENITES ► Quartz Arenites Mistakenly referred to as orthoquartzites Quartz Arenite is accepted term Quartz arenite characteristics ► Generally white or pink; cemented with quartz Calcite cement also common Can get pressure solution between grains ► Thin blanket sands or very thick ► Monoxln qtz most common ► High proportion of undulatory quartz less common ► Can be cross bedded or contain ripple marks ► Generally sorted and rounded--end point of evolution ► May contain minor chert and heavy minerals
Varieties ► Ist cycle--more polyxln & undulatory qtz, less rounded, more feldspar, more diverse heavy minerals ► Multi-cycle— well rounded, sorted overgrowths ► Bimodality in some--selective removal of fine sand or 2 sources
► Polycrystalline quartz grain with sutured crystal-crystal boundaries. Such quartz is classified as a "unstable" grain. This quartz is sourced from low-grade metamorphic rocks(You ng, 1976).
Monocrystalline Quartz Grain
Origin ► Are supermature, sheet-like, mostly on margins of craton ► Probably have eolian history ► Look for bimodality ► Beach depositional environment possible ► Some are marine--marine fossils, interbedded with limestone & dolomite ► Most deposited in shallow marine environment, ► May be trangressive sheet sands--Therefore shallow marine--clays winnowed out ► Conditions to form sands ► i. removal of feldspars--weathering/transport ► ii. removal of clay ► iii. rounding of quartz
Beach Environment-Mussel Rock, CA ► High Energy ► Usually well sorted ► Usually well rounded ► Lacks fines
Sinuous, barchanoidal transverse dunes on a transgressive dunefield in an arid environment at Guerrero Negro, Baja California, Mexico Photograph by Patrick Hesp
Navajo Sandstone ► Coconino County, AZ ► Nearly pure quartz ► Note large scale cross beds USGS
St. Peters Sandstone ► Crossbedded St. Peter Sandstone in roadcut along State Route 39, near center of NW1/4 sec. 1, T. 4N., R. 5 E. Iowa County, Wisconsin ► Ordovician USGS
► Freshly exposed St. Peter sandstone, U.S. Silica Quarry in Pacific, MO.
Ordovician Paleogeography ► Note craton covered by shallow seas ► Note demarcation of equatore ► St Peters sandstone deposited in these shallow seas
► Cambrian quartz arenite of the Mt. Simon and Wonewoc Formation is exposed along bedding planes ► Deposited along a sandy, equatorial beach
► Must remove unstable minerals to be arenite
Quartz Cementation Two phases of cementation - This is a quartz sandstone, seen under plain light. The quartz grains have quartz overgrowths (labeled 'o'), reflecting a first phase of cementation, and then, later, a period of carbonate cementation (c) seen as a brownish material filling the pore space.
“Immature” rocks ► (Mineralogically unstable) ► Enriched in feldspars and ► Unstable rock fragments
Immature Siliciclastic Sediment ► Is usually more diverse ► (mineralogy-wise) than ► mature sediment. ► Grains include: Plagioclase feldspar Orthoclase Microcline Micas Lithics Quartz
General Arkose Description ► Typically pink & coarse grained, may be white, f.gr Polyxln quartz dominates Feldspar usually k-spar (microcline) Kaolinization of feldspars common Angular and poor to moderate sorting May have calcite cement and some matrix
Arkose ► Originally --rock with qtz and feldspar ► Re-defined-- sandstone with >25% feldspar Feldspars from granite/gneiss source ► No agreement on feldspar content 25% usually cited ► Folk--subarkose = 10-25% feldspar
► Arkose – greater than 25% Felspar ► Usually compositionally immature to submature ► Usually texturally submature ► Idicates rapid uplift, erosion, and deposition for feldspars to survive
Origin ► Mostly first cycle ► Plagioclase arkoses from volcanic source ► Need cold or arid climate for feldspar preservation humid climate bad for feldspars ► D. Need rapid uplift and erosion 1. under these conditions, feldspar preservable in humid climate
VI) Examples ► Devonian Old Red sandstone-England, Precambrian Torridon sandstone-Scotland, ► Silurian Clinton Fm-Pennsylvania Old Red Sandstone, Siccar Point 1778 Hutton studied outcrop and recognized the great expanse of geologic time Near-vertical greywackes of the Silurian Gala Group were unconformably Overlain by the younger, gently-dipping sandstones of the Devono- Carboniferous Stratheden Group (Upper Old Red Sandstone)
Interpretations ► Tyee Formation (Eocene, Oregon Coast Range) - This sandstone was deposited late during an adjacent orogeny and shows evidence of a source area that includes: quartz (q); plagioclase (f); potassium feldspar (f(k) stained here to straw yellow color); muscovite (m); and biotite (b) all of which suggest that the source area was granitic pluton that contained micas, The large Lv grain, suggests that arc volcanic rocks were also exposed in the source area
► Lookingglass Formation (Eocene, Oregon Coast Range) - This sandstone was deposited below the Tyee Formation (last slide) and shows a somewhat different source area. These grains include: quartz (q); feldsparlate during an adjacent orogeny and shows evidence of a source area that includes: quartz (q); plagioclase (f); many sedimentary lithic grains (Ls, that are likely from a shale source or a phyllite/slate source) and several heavy mineral grains (h). This composition differs from the volcanic arc derived Tyee Formation, and suggests the source area for this sandstone involved rocks that were probably laid down on the sea floor (shales) and were later uplifted into an active orogen. The heavy minerals may indicate metamorphism in the source area that would also be consistent with orogeny.