Lee J. Suttner

Re-evaluation of the Use of Undulatory Extinction and Polycrystallinity in Detrital Quartz for Provenance Interpretation

ABSTRACT Undulosity in quartz from low rank metamorphic rocks, as studied with a universal stage, is sufficiently different from that in pluton-derived quartz to be useful in provenance interpretation. Subdivision of monocrystalline quartz of medium sand-size into two populations, one with 5° undulosity, enables efficient and reliable interpretation. Geometrical and empirical analyses of the relation between true angles of undulosity and corresponding apparent values determined on a conventional flat stage indicate that apparent values approach real values closely enough to obviate use of the universal stage for recognition of the two quartz undulosity populations in routine petrographic analysis. The amount of polycrystalline quartz and the number of crystals per grain of polycrystalline quartz in medium-sized Holocene sand also assist in provenance interpretation. Thirteen percent of the total quartz derived from plutonic rocks is polycrystalline. This compares with 29% in sand from middle and upper-rank metamorphic rocks and 53% in sand from low-rank metamorphic rocks. Greater than 75% of the polycrystalline quartz grains from plutonic and high-rank metamorphic sources contain 2-3 crystal units per grain; less than 75% of the quartz grains from low-rank metamorphic rocks contain 2-3 crystal units. Plotting of four variables, relative percentage of (1) undulose, (2) non-undulose and (3) polycrystalline quartz, and (4) number of crystal units per polycrystalline grain, on a single diamond diagram enables one to discriminate sands of plutonic, low-rank and high-rank metamorphic parentage, both for Recent and ancient sands.

Climate and the Origin of Quartz Arenites

ABSTRACT Holocene fluvial sands derived from the same type of crystalline parent rocks in humid-temperate and arid/semi-arid climates have distinct compositions. For a particular climate and parent rock, the similarity in composition of the fluvial sands and the sand-size fraction of soils collected from the associated interfluves indicates that climatically controlled weathering of a specific parent rock type is a primary control of sand composition. Published estimates of the magnitude of destruction of lithic fragments and feldspars associated with stream transport and deposition in a beach/coastal dune environment permit the simulation of quantitative approximations of compositional maturation of sand. For transport distances of <75 km the simulated maturation does not significantly modify the distinctive climatic imprints on composition. However, simulated maturation due to deposition in beach/coastal dune environment effectively destroys the climatic imprint. Although marine processes are highly capable of mechanically destroying lithic fragments and feldspar, our study suggests that large-scale production of first-cycle quartz arenites in such an environment is not probable. Only a rare, unique combination of extreme conditions of climate (tropical), relief (low), and sedimentation rate (slow) can give rise to first-cycle quartz arenites. Therefore we conclude that the bulk of ancient quartz arenite is multicycle in origin.

Alluvial Sandstone Composition and Paleoclimate, I. Framework Mineralogy

ABSTRACT Systematic variations in compositional maturity of first-cycle fluvial sandstones are found in the Cutler Formation (Permian) and Fountain Formation (Permian-Pennsylvanian) in Colorado and in the Gondwana Supergroup (Permian-Triassic) of Peninsular India. These variations reflect changing climate during deposition. Climate is considered to be the critical factor affecting maturity because source rocks (granite, granite gneiss) and tectono-environmental setting (alluvial systems associated with basement-cored block uplifts) were similar and remained relatively unchanged throughout deposition of all three of the units. The roughly 3,200-m-thick Gondwana succession, consisting of five sandstone petrofacies, is characterized by the following sequence of change (oldest to youngest) in compositional maturity expressed in Q/F/R percentages: 54:42:4 88:10:2 62:34:4 82:16:2 99:1:0. This sequence is a function of the changing paleoclimate (glacial arid temperate humid warm semiarid warm semihumid warm humid) associated with the overall global limatic change and the changing latitudinal location of India as it moved from a location close to the South Pole northward during the interval of time represented by Gondwana deposition. Lithologic and fossil evidence indicates that, during deposition of the Fountain Formation, climate gradually changed from relatively wet and warm to at least semiarid and warm. This climatic change is the main factor responsible for the difference in compositional maturity of the older Fountain sandstone (Q:F:R-65:26:9) relative to the younger (Q:F:R-54:36:10). Cutler sandstone was deposited in an arid climate and has maximum compositional immaturity (Q:F:R-49:44:7). A log/log plot of the ratio of total quartz to total feldspar plus rock fragments against the ratio of total polycrystalline quartz to to al feldspar plus rock fragments is a sensitive discriminator of first-cycle sandstones with differing climatic heritage. Bulk chemical composition data support interpretation of climate from framework mineralogy, but by themselves are not sensitive enough to be unequivocal indicators. The same is true for the ZTR index and observations of degree and abundance of solution pits/embayments on detrital quartz. Results of this study support the conclusion of earlier empirical and theoretical analyses which suggest that the optimum conditions for the production and preservation of a distinctive climatic signature on sand composition are met in extensional plate-tectonic settings. Such settings are characterized by coarse-grained crystalline parent rocks, short transport distances in low-order streams, deposition in nonroutine environments, and shallow-burial diagenesis. However, even in such optimum settings, only rarely will climate be a more important determinant of sandstone composition than the tectonic setting itself.

Alluvial sandstone composition and paleoclimate; II, Authigenic mineralogy

ABSTRACT Various empirical and theoretical arguments suggest that early formed cement in sandstone is a product of meteoric water whose chemistry is controlled by climate. The nature and distribution of early silicate cement in Gondwana, Fountain, and Cutler sandstone support this supposition. Sandstones from all three units have experienced a broadly similar diagenetic history characterized by two stages of authigenesis. Stage I was typified by neomorphic development of kaolinite, chlorite, smectite, quartz, and, additionally in the Cutler Formation, laumontite. Stage II was dominated by replacement reactions involving the production mostly of illite, iron oxide, carbonate minerals, and, mostly in the Gondwana sandstone, a second generation of quartz. Thermodynamic relations, hydrologic constraints, and radiometric dating of the late-stage illite indicate that early authigenesis took place within a few million years of deposition at burial depths measured in hundreds of meters. Consequently, the nature of the silicate cement was a function of the groundwater chemistry, as controlled by climate. During times of relative aridity, ionic concentration of the groundwater was high, and high proportions of smectite, chlorite, and, more rarely, laumontite formed. Such was the case in Petrofacies I, IV, V of the Gondwana Supergroup (for a definition of Gondwana petrofacies see part I of this study) in the Cutler Fm., and in the upper 150 m of the Fountain Fm. During times of high precipitation, pore water was dilute, which promoted authigen c formation of kaolinite and quartz. This is represented by sandstones in petrofacies II and VI of the Gondwana and by sandstones in the lower 200 m of the Fountain Fm. Illite is especially abundant as a pseudomorphic replacement of kaolinite in the older Gondwana sandstones. Illitization apparently began at burial depths of about 1,600 m at a temperature of about 75°C. Illite is not common in the Cutler Formation and is present only in subordinate amounts in the Fountain Formation from the study area. The 18O values of early clay cement in the Gondwana sandstones range systematically from base to top from 5.00 to 13.20. The systematic variation in 18O value of Gondwana clay reflects the gradual migratory drift of the Gondwana harm toward lower latitudes through time. Average 18O values for Cutler cement (13.23) and Fountain cement (19.82) are greater than the analogous values for Gondwana cements. This is consistent with the lower-latitude setting of deposition for the Cutler and Fountain relative to the latitudinal location of the Gondwana basins. However, the values are anomalously lower than what have been observed in modern-day neoformed clays in weathering profiles from low latitudes.

Relative Alteration of Microcline and Sodic Plagioclase in Semi-arid and Humid Climates

ABSTRACT Petrographic determination of the degree of alteration of Holocene detrital feldspars from first-order streams indicates that a broad overlap exists in the amount of alteration on detrital feldspars from contrasting wet and dry climates. Thus the amount of alteration of detrital feldspars from ancient sandstones is probably an invalid indicator of paleoclimate. Furthermore, regardless of climate, detrital plagioclase (Ab 80-95) generally is 15-20 percent more altered than potassium feldspar. In light of this, the paleoclimatic significance of the observed relation potassium feldspar-more-weathered-than-plagioclase in ancient sandstones (Todd, 1968) needs re-evaluation. Results of this study do suggest a possible preference of limonite plus kaolinite on detrital feldspars from humid climates versus limonite plus smectite on feldspars from semi-arid climates. However, unequivocal substantiation of this difference is still lacking. Even if the compositional difference can be rigorously demonstrated, an inability to distinguish diagenetic from climate-controlled surface weathering induced alteration of feldspar will further decrease the utility of feldspar alteration relationships in paleoclimate interpretation.

Trace fossils and marine-nonmarine cyclicity in the Fountain Formation (Pennsylvanian; Morrowan/Atokan) near Manitou Springs, Colorado

The Lower Pennsylvanian (Morrowan/Atokan) portion of the Fountain Formation in the Manitou Springs, Colorado, area commonly has been interpreted as a subaerial alluvial fan. However, approximately 12 marine-nonmarine cycles, each represented by a discrete progradational sequence, have been recognized within the lower third of the Fountain Formation in this area. The typical cycle is composed of six lithofacies: 1) transgressive-lag conglomerate; 2) offshore mudstone; 3) hummocky crossbedded sandstone; 4) planar crossbedded granular sandstone; 5) low-angle, crossbedded, coarse-grained sandstone; 6) lenticular conglomeratic sandstone. Only lithofacies 6 is nonmarine (alluvial) in origin. Two diverse trace-fossil assemblages (totalling 18 ichnogenera and 20 ichnospecies) within the Fountain Formation are restricted to the marine portions of the section, or to those portions within 1 m below marine-deposited strata. The assemblage includes Arenicolites carbonarius, ? A. ichnosp., Aulichnites parkerensis, two types of Chondrites, ? Conostichus broadheadi, ? Crossopodia ichnosp., Curvolithus manitouensis (ichnosp. nov.), Eione ichnosp., Lockeia ichnosp., Macaronichnus segregatis, Palaeophycus heberti, Palaeophycus striatus, Planolites beverleyensis, Psammichnites plummeri, Rhizocorallium irregulare, ? Skolithos ichnosp., Taenidium serpentinum, ? Teichichnus ichnosp., ? Thalassinoides ichnosp., ? Zoophycos ichnosp., and several unnamed traces. Trace fossils in the Fountain Formation can be used as indicators of sedimentary processes (e.g., rates of deposition, energy regime). Trace-fossil composition changes along a roughly south-to-north, nearshore-to-offshore gradient, generally reflecting increased influence and duration of normal-marine sedimentation. Ichnotaxa present and their distributions within marine strata in the lower part of the Fountain Formation suggest that they belong to the Curvolithus ichnoassemblage, which has been shown to indicate high sedimentaiton rates in relatively nearshore shelf settings. The Curvolithus ichnoassemblage can be subdivided into Macaronichnus -dominated and Curvolithus -dominated parts. Macaronichnus -dominated portions of the Fountain Formation indicate generally higher wave-energy sedimentation as compared with Curvolithus -dominated portions.

Paleoclimate interpretation from a petrographic comparison of Holocene sands and the Fountain Formation (Pennsylvanian) in the Colorado Front Range

ABSTRACT In at least four areas of the Colorado Front Range remarkable similarity exists in the detrital quartz populations in the Fountain Formation and Holocene sand from modern day streams draining Precambrian crystalline rocks and located close to the Fountain outcrops. Near Colorado Springs and Boulder both the Fountain and Holocene quartz populations were derived principally from granitic source rocks; in the vicinity of Loveland and Golden metamorphic rocks provided most of the quartz. Because both the Fountain sandstone and Holocene sand in these four areas came from common parent rocks and accumulated under similar conditions, plots of frequency percent resistant framework grains as a function of size for the two deposits can be used to compare Pennsylvanian and Holocene climates for he east flank of the Front Range following the approach of Young et al. (1975). In all four areas such comparisons suggest that the Fountain was deposited in a climate substantially more humid than that which has existed during the Holocene. A humid climate is consistent with the inferred near equatorial, southern hemisphere location of the Ancestral Rocky Mountains during the Permian and Pennsylvanian. An orographic effect would have been produced as warm, moist southeasterly trade winds, blowing off the mid-continent sea, ascended the Ancestral Rockies. Precipitation would have been concentrated on the east flank, but more arid conditions would have prevailed in western Colorado on the opposite flank.

Effects of Syndepositional Faulting and Folding on Early Cretaceous Rivers and Alluvial Architecture (Lakota and Cloverly Formations, Wyoming, U.S.A.)

ABSTRACT Quantitatively reconstructed paleochannel hydraulics (e.g., channel discharges, slopes, velocities), geometries (e.g., widths, depths, sinuosities, channel patterns), and alluvial architecture record the local effects of syndepositional faults and folds. These effects are evaluated from Lower Cretaceous rocks in three areas: (1) the northern Black Hills, where the axes of syndepositional folds trend roughly perpendicular to paleoflow, (2) the northwestern Black Hills, where the trend of a syndepositional fault is oblique to paleoflow, and (3) the Wind River basin, where the trends of syndepositional faults are nearly parallel to paleoflow. In each area, channel-belt deposits are laterally and vertically connected where local subsidence rates were high, producing thick, laterally extensive sandstones. Areas of lower subsidence rates are characterized by isolated or laterally connected channel-belt deposits, and thinner sandstones. Surface deformation controlled the positions of some rivers because of the development of antecedent drainages, or by directing rivers toward areas of maximum subsidence. Some reconstructed channel slopes, which range from 0.62 10-4 to 5.43 10-4, record surface deformation. The three-dimensional alluvial architecture was simulated using the model of Mackey and Bridge (1995), together with quantitatively reconstructed paleochannel parameters, in order to provide quantitative insight as to possible conditions and processes that produced the observed alluvial architecture. The effects of deformation on channel slopes, together with depositional topography, were important in controlling avulsion and resultant alluvial architecture. The most important condition that governed the alluvial architecture was comparable rates of local subsidence and sediment accumulation. Paleochannel reconstructions indicate that the rivers were 48-180 m wide and 4.4-13.6 m deep, and had discharges of 64-1073 m3s-1. Channel deposits all appear to represent point bars and associated channel fills of meandering rivers. Sinuosities were comparable, ranging from 1.1 to 1.4, and show no consistent trends with other channel parameters. Hence, with the exception of two paleochannels (out of the thirteen that were reconstructed) differences in hydraulic parameters such as slope and sediment transport rate had no discernible effect on channel pattern or sinuosity.

Late Mesozoic to Early Cenozoic Foreland Sedimentation in Southwest Montana: ABSTRACT

ABSTRACT Stratigraphic variations in the composition of Upper Jurassic through Lower Tertiary sandstones (Morrison, Kootenai, Blackleaf Formations and the Bozeman Croup) from southwest Montana, and inferred dispersal patterns and depositional environments, reflect the tectonic evolution of a foreland basin in response to gradually increasing orogenic activity. Upper Jurassic to Lower Cretaceous (Aptian) litharenites were largely derived from older miogeoclinal sedimentary rocks exposed to erosion by a combination of epeirogenic uplift over the embryonic Idaho batholith and foreland fold-thrusting in Idaho. Deposition was on a broad coastal plain. Although an active magmatic arc was present in western Idaho, first-cycle sand-size detritus derived from the arc was trapped in intra-arc basins and not transported into southwest Montana. Within the foreland basin the Belt Arch exerted progessively less influence on sedimentation from near the middle of the Jurassic to the start of the Cretaceous. By the end of the Early Cretaceous the active magmatic arc had migrated farther east in Idaho and the volcanic cover of the Idaho batholith supplied detritus for the first time for the Montana foreland basin. Transitional marine/non-marine environments of deposition dominated as a result of interplay of marine transgression and eastward progradation of an alluvial system outward from the orogenic souce areas. Within the foreland basin positive stuctures were influencing sediment dispersal patterns as early as Albian time. However, coarse-clastic facies derived from these intra-foreland structures were not deposited extensively until the Late Cretaceous. Extensional tectonics brought about a breaking up of the foreland basin in Early Cenozoic time. Immature arkosic sandstone and conglomerate were derived from nearby basement-cored block uplifts as well as from the unroofed Boulder batholith. Deposition was in intermontane basins. Cyclical variations in both the compositional maturity of sandstone and nature of the sandstone depositional environments characterize the evolution of the southwest Montana foreland basin.