Harvey Blatt

Proportions of Exposed Igneous, Metamorphic, and Sedimentary Rocks

Of the rocks exposed on the Earth9s surface, 66 percent (±3.5 percent) are sedimentary and 34 percent are crystalline, at the 95 percent confidence level. Extrusive igneous rocks average about one-fourth of all crystalline rock outcrops, with the highest percentages in Asia and South America. Less than 5 percent of all Precambrian rocks are mapped as sedimentary. The relationship between the geologic age of a sedimentary rock and its outcrop area is lognormal and is well described by a decay curve with a half-life of 130 × 10 6 yr. That is, one-half of all existing sedimentary rocks are younger than Jurassic. Such a short half-life indicates that the rate of sedimentary recycling must be very rapid.

Undulatory Extinction in Quartz of Igneous and Metamorphic Rocks and Its Significance in Provenance Studies of Sedimentary Rocks

ABSTRACT Thin-sections of 119 igneous and metamorphic rocks and 44 clastic rocks were examined in an attempt to evaluate the use of undulatory extinction and polycrystallinity in detrital quartz grains for the determination of provenance of sediments. Non-undulatory quartz is uncommon in plutonic igneous rocks, schists, and gneisses. For 101 rocks of these types examined, the average amount of total quartz which is of the non-undulatory type is only 14.9 percent. Differences in the percentages of non-undulatory quartz among these three groups of rocks are not statistically significant. The quartz in extrusive igneous rocks, however, is almost entirely (91%) of the non-undulatory type; it is probable, however, that quartz-bearing extrusive rocks supply only very small quantities of sand-sized quartz grains into the sedimentary cycle. Mineralogically immature clastic sedimentary rocks contain low percentages of non-undulatory quartz, similar to the potential source rocks examined (excluding extrusive igneous rocks). Mineralogically mature rocks, however, are characterized by high percentages of non-undulatory quartz; the average amount of total quartz which is of the non-undulatory type in the 20 orthoquartzites examined was 43.1 percent; values ranged from 14 to 80 percent. The undulatory grains appear to be destroyed selectively by mechanical and chemical agencies during successive sedimentary cycles; this might be expected on thermodynamic grounds, since strained (undulatory) grains have higher free energies than strain-free ones. Consequently, and abundance of non-undulatory quartz grains in a sedimentary rock robably indicates that the assemblage of grains has passed through several sedimentary cycles, rather than derivation from either plutonic igneous or metamorphic source rocks. It appears likely that large quantities of polycrystalline quartz grains are derived from both plutonic igneous and metamorphic rocks. Large amounts of polycrystalline quartz grains in the total quartz of a sedimentary rock probably indicate that a significant proportion of the quartz grains have been derived from primary source rocks relatively late in the history of the assemblage of quartz grains. Detrital polycrystalline quartz grains are less stable than either the undulatory or non-undulatory varieties, and are extremely rare in mineralogically mature sandstones. It is concluded that the presence or absence of undulatory extinction and polycrystallinity in clastic quartz grains is of very limited usefulness in determining the provenance of sediments.

Intergranular pressure solution and cementation of the Tuscarora orthoquartzite

ABSTRACT Intergranular pressure solution is believed by many sandstone petrologists to be the major source of silica for the cementation of orthoquartzites, although there is no quantitative supporting evidence from thin-section petrology. The white Tuscarora orthoquartzite, often cited as a good example of cementation by pressure solution, has been studied using luminescence petrology to evaluate this hypothesis. Point counts were made to determine the amount of pressure solution and authigenic silica in 183 thin sections of samples of the Tuscarora orthoquartzite from Pennsylvania, Virginia, and West Virginia. Luminoscope measurements show large amounts of pressure solution to be an uncommon phenomenon in the Tuscarora throughout the study area. Pressure solution can account for only 30-35% of the pore-filling quartz cement. Proximity to folds, grain size, sorting and clay content are factors that have been suggested as controls on the amount of pressure solution in orthoquartzites. Of these, only clay appears related to the occurrence of pressure solution in the Tuscarora. Furthermore, the well-cemented samples show less pressure solution than more friable samples, but contain more pore-filling cement. We conclude that initiation of cementation by silica during early diagenesis has prevented widespread development of intergranular pressure solution by equalizing the distribution of stress along grain boundaries. Of the many possible sources of silica other than pressure solution, transport of H4SiO4 in surface-derived ground water seems the most likely source of the majority of the cement found in the Tuscarora.

Provenance Determinations and Recycling of Sediments

ABSTRACT The most important problem in the interpretation of the micropetrology of sandstones is the evaluation of the abundance of detritus of first-cycle as opposed to polycyclic origin. At present few tools or data with which to make this distinction exist, largely because of the lack of study by sedimentary petrologists of igneous rocks, metamorphic rocks, and soils. Also, studies of relative rates of abrasion of sand-sized rock fragments and feldspar are rare. Most igneous and metamorphic rocks are equilibrium or near equilibrium assemblages of minerals. The mineral species present in such rocks are controlled by the laws of physical chemistry applied to the bulk chemical composition of the magma or pre-existing rocks. In contrast, sandstones are invariably non-equilibrium mineral assemblages and, therefore, the mineralogic character and origin of each sand grain must be considered as independent of surrounding grains. For this reason sedimentary petrology is a more difficult field of study than igneous or metamorphic petrology. The interpretation of ultimate provenances of sandstones has suffered from over-application of generalizations and under-application of detailed mineralogic analyses and interpretations. New ways of analyzing data concerning the mineral composition of sandstones are needed.

Original characteristics of clastic quartz grains

Granular gravel and sand-sized quartz fragments in untransported naturally disintegrated detritus were examined from nineteen massive plutonic rocks, sixteen gneisses, and six schists located in the desert areas of southern Arizona and southern California. The mean size and standard deviation of each quartz type derived from each rock group reveal: (1) the mean size of total quartz released from massive plutonic rocks and gneisses is identical (coarse sand) and is one and one-half to two times courser than total quartz released from schists; (2) monocrystalline quartz grains with undulatory extinction are courser in size than non-undulatory grains. Therefore, grain size must be specified when the percentages of non-undulatory quartz in sandstones are compared. The angular difference between c-axes in medium sand-sized quartz grains with undulatory extinction averages four to five degrees with standard deviation of two to three degrees and is independent of the type of rock from which the grain was derived. Measurements of this feature are probably useless as indicators of provenance. Polycrystalline quartz grains are yielded in abundance by each of the groups of rocks examined. Higher proportions of these grains occur in the coarser size fractions. Therefore, the proportion of polycrystalline quartz released from each type of rock depends, in part, upon the size frequency distribution of total quartz released from the rock. Polycrystalline quartz grains of granular gravel to medium sand size are not derived almost entirely from foliated metamorphic rocks. Polycrystalline quartz grains from the several types of rocks examined possess some distinguishing features. (1) For any grain size other than very fine sand, polycrystalline quartz from gneisses will be formed of a greater number of quartz crystals than will grains from massive plutonic rocks or schists. (2) Quartz crystals in polycrystalline grains from gneisses and schists commonly show preferred crystallographic orientation. (3) The grain size distribution of quartz crystals forming polycrystalline grains from gneisses and schists is frequently bimodal. Intragranular suturing and moderate grain elongation are not reliable as indicators of provenance. Monocrystalline quartz grains between granular gravel and medium sand size are derived in ranch greater abundance from massive plutonic rocks than from gneisses or schists. However, a terrain exposing foliated rocks (gneisses plus schists) will yield an amount of monocrystalline quartz approximately equal to that yielded by a terrain of massive plutonic rocks. Comparison of the character of first-cycle quartz, as observed in this study, with quartz as seen in many mineralogically mature sandstones provides strong evidence that quartz grains are reduced in size by sedimentary processes. Size for size, the more mature sediments contain lower proportions of polycrystalline quartz in their total quartz, higher proportions of non-undulatory quartz, lower proportions of grains in the 1-4 mm size range, and fewer elongate quartz grains.

Perspectives; Oxygen isotopes and the origin of quartz

Published data indicate that in igneous rocks the delta 18 O of quartz averages about +9 per thousand ; in metamorphic rocks, +13 to +14 per thousand ; in sandstones, +11 per thousand ; in shales, +19 per thousand ; quartz overgrowths in sandstones, +20 per thousand ; and cherts, +28 per thousand . Either a very large proportion of the silt and clay-size quartz in shales (about 90% of the total quartz in shales) is secondary, or the isotopic data are incomplete. It is concluded that the latter interpretation is correct because published isotopic analyses of metamorphic rocks consider only the coarser-grained rocks. Phyllites and slates, whose quartz is nearly all of silt and clay size and has more positive delta 18 O values than quartz from schists and gneisses, probably supplies the bulk of quartz to mudrocks.

Determination of Mean Sediment Thickness in the Crust: A Sedimentologic Method

Mean sediment thickness, volume of detntal quartz, and grain size distribution of dctrital quartz have been calculated for the total sediment mass of the earths crust. Sedimentologic and petrographic data were used rather than geochemical data. Results are in good agreement with the earlier estimates based on geochemical arguments. Mean sediment thickness is computed to be 2690 feet and represents the erosion of 7640 feet of crystalline rocks. The percent of detrital quartz in the sediment mass is estimated to be 21.0 percent. The detntal quartz has a mean grain size of 4.0ϕ its frequency-distribution is log-normal with standard deviation of 2.5ϕ. As earlier work has indicated the mean size of quartz from newly disintegrated crystalline rocks to be 0.67 mm, the mean size of 0.06 mm determined for the earths detrital quartz represents a size decrease of 90 percent by sedimentary processes.

Provenance Studies and Mudrocks

ABSTRACT Provenance studies based on the mineralogy of sandstones seem to be falling rapidly into disrepute as a result of the increasing emphasis on the effects of diagenesis on the original detrital mineral composition. Feldspars in sandstones commonly are dissolved, altered to clay minerals, and albitized by widely occurring diagenetic processes. Accessory minerals all too frequently are confined in ancient sandstones to ZTR. Mudrocks, in contrast, have the potential of greater preservation of feldspars and accessory minerals because of their lesser permeabilities. Few studies have attempted to use the nonclay mineralogy of mudrocks for provenance work, and these few have had promising but mixed results. Mudrocks deserve more attention in provenance studies than they have received.

Intrastratal solution and non-opaque heavy minerals in shales

ABSTRACT Non-opaque heavy minerals have been examined from contemporaneous sandstone-shale pairs at six horizons in Tertiary sediments of the Texas Gulf coastal Plain. The total weight of heavy minerals in the shales is much less than in their associated sandstones but the same grain sizes of heavy minerals are present in both rock types. Sediment in the coarse and medium silt sizes contains the same variety of minerals as the very fine sand size. In these relatively young shales the variety of minor minerals (those forming 0.1-5.0 percent of the non-opaque suite) is only slightly larger than in the contemporaneously deposited sandstones. however, the number of species present only in trace amounts (one or two grams among the 5400 grains counted) is significantly larger in the shales than in contemporaneously deposited sandstones. Also, the sandstones contain much larger percentages of altered grains than do the shales. These results apparently are due to the greater permeability of the sandstones and suggest that shales rather than sandstones should be examined in heavy-mineral studies. unstable species post-depositionally dissolved in sandy beds may be preserved in contemporaneously deposited shales.

Polycrystallinity: Effect on the Durability of Detrital Quartz

ABSTRACT Experimental studies using a tumbling barrel apparatus reveal the following sequence of mechanical durabilities for varieties of quartz: finely-polycrystalline quartz > coarsely-polycrystalline quartz > monocrystalline quartz. Published field studies indicate that monocrystalline quartz has a greater survival potential than polycrystalline quartz. This suggests that chemical stability is more important than inherent mechanical durability in determining the survival potential of detrital grains during their predepositional history.