Robert L. Folk

Brazos River Bar: A Study in the Significance of Grain Size Parameters

A bar on the Brazos River near Calvert, Texas, has been analyzed in order to determine the geologic meaning of certain grain size parameters and to study the behavior of the size fractions with transport. The bar consists of a strongly bimodal mixture of pebble gravel and medium to fine sand; there is a lack of material in the range of 0.5 to 2 mm, because the source does not supply particles of this size. The size distributions of the two modes, which were established in the parent deposits, are nearly invariant over the bar because the present environment of deposition only affects the relative proportions of the two modes, not the grain size properties of the modes themselves. Two proportions are most common; the sediment either contains no gravel or else contains about 60% gravel. Three sediment types with characteristic bedding features occur on the bar in constant stratigraphic order, with the coarsest at the base. Statistical analysis of the data is based on a series of grain size parameters modified from those of Inman (1952) to provide a more detailed coverage of non-normal size curves. Unimodal sediments have nearly normal curves as defined by their skewness and kurtosis. Non-normal kurtosis and skewness values are held to be the identifying characteristics of bimodal sediments even where such modes are not evident in frequency curves. The relative proportions of each mode define a systematic series of changes in numerical properties; mean size, standard deviation and skewness are shown to be linked in a helical trend, which is believed to be applicable to many other sedimentary suites. The equations of the helix may be characteristic of certain environments. Kurtosis values show rhythmic pulsations along the helix and are diagnostic of two-generation sediments.

Travertines; depositional morphology and the bacterially constructed constituents

ABSTRACT Investigation of travertine accumulations throughout central Italy and the west-central U.S. has shown that the carbonate is precipitated in response to both inorganic and organic processes. Individual deposits range up to 85 m thick and hundreds of square kilometers in areal extent; all of the carbonate is low-magnesian calcite. Water chemistry, temperature, and morphology of the accumulation greatly influence the constituents comprising these deposits. Harsh environmental situations favor inorganic deposits, while increasingly more moderate conditions result in the formation of a greater abundance of organically precipitated material. Morphological variations of travertine deposits recognized include 1) waterfall or cascade, 2) lake-fill, 3) sloping mound, fan, or cone, 4) terraced ound, and 5) fissure ridge. Gross morphology, internal stratification, and constituents comprising these deposits vary systematically depending upon the type of accumulation and chemistry of the waters. Bacterially precipitated calcite forms a large percentage of the carbonate in many travertine accumulations, exceeding 90% of the framework grains comprising some of the lake-fill deposits. Bacteria are among the first taxa to inhabit and reproduce in harsh spring environments and produce a variety of fascinating constituents. The bacteria are primarily rods, generally 0.2 µm in diameter and less than 1.0 µm in length. The rods readily decay resulting in calcite crystals loaded with micropores. The basic building block of bacterially constructed travertine is a clump of bacteria averaging 20 µm in diameter enclosed in a single crystal of calcite. Aggregates of these crystals produce a variety of deposits including 1) crudely laminated carbonate mud, 2) finely laminat d layers of mud, 3) intraclasts, 4) foam rock, and 5) shrubs. The shrubs are most striking, commonly forming layers 1-3 cm thick but also producing bacterial pisoids. At some locales, bacterial stromatolites composed of layers of shrubs alternating with finely laminated layers of bacterial mud comprise essentially the entire deposit. The shrub layers are the result of flourishing summer growth of bacteria and, furthermore, show remarkable daily laminae 0.1-0.5 mm thick. The importance of bacteria in the formation of travertine and their universally recognized abundance in modern sediments provides an impetus for a reexamination of the role of bacteria in the origin of other types of ancient deposits.

SEM Imaging of Bacteria and Nannobacteria in Carbonate Sediments and Rocks

ABSTRACT Bacteria and nannobacteria (0.1 µm dwarfed forms) are abundant in both non-etched and gently etched travertines deposited by hot springs in Viterbo, Italy. They are also plainly visible in Bahama ooids and hardgrounds, and in Great Salt Lake aragonite ooids and cements. In addition, nannobacteria are found in Paleozoic and Mesozoic rocks. Because of their extraordinary abundance in some specimens, they probably play a prominent role in catalyzing the precipitation of carbonate materials.

Mg/Ca Ratio and Salinity: Two Controls over Crystallization of Dolomite

Carbonate precipitation is governed by both thermodynamic and kinetic considerations. The concentration and ionic composition of natural solutions influence not only the mineralogy of the precipitate, but also the crystal size and habit of both metastable and stable phases. Under marine conditions sparry calcite does not precipitate because of poisoning of sideward crystal growth by Mg, which forces Mg-calcite and aragonite to assume a micritic, steep-rhombic, or fibrous habit. Blocky calcite can form only in low Mg/Ca waters, either from primary meteoric solutions, or where Mg has been removed from marine water by dolomite or clays. In hypersaline environments precipitation is commonly rapid and, together with the high concentrations of foreign ions, it is difficult for dolomite to form because of the precise Ca-Mg ordering required. Instead, aragonite or Mg-calcite crystallizes. Dolomite can form only if the Mg/Ca ratio exceeds 5-10:1, and even then it is aphanitic and poorly ordered. At progressively reduced salinities dolomite is able to nucleate at still lower Mg/Ca ratios, approaching 1:1 in meteoric waters. The absence of foreign ions and slow crystallization, commonly under phreatic conditions, cause both dolomite and calcite to form exquisitely limpid, euhedral rhombs. Thus, dolomite probably forms most readily by a reduction in salinity, particularly in a schizohaline environment (alternating between hypersaline and near-fresh conditions, as in a floodable sabkha or a phreatic mixing zone). Flushing marine saline waters with fresh water lowers salinity but maintains a high Mg/Ca ratio; crystallization is slower and the interfering effect of foreign ions is reduced.

Length-slow Chalcedony: A New Testament for Vanished Evaporites

ABSTRACT Length-slow chalcedony, having its c-axis parallel to the fibers, is unusual in nature. Occurrences so far investigated reveal that this rare type of optically fibrous silica occurs almost exclusively in association with sulphates and evaporites. Often the evaporite minerals have been totally removed, and thus the survival of resistant length-slow chalcedony reveals the presence of former salt-flat, sabkha or sulphate-rich environments where none were before suspected. This type of chalcedony also in some instances forms in semi-arid, alkaline soils. Discovery of this simple criterion has led to development of an additional series of characteristics for identification of silicified evaporites. All siliceous deposits should be re-examined for presence of this important genetic m rker.

Shape Development On Tahiti-Nui

ABSTRACT Basalt pebble morphology was studied in nine rivers and on fourteen high- and low-wave-energy beaches around Tahiti-Nui. The study included about 15,000 measurements on 961 pebbles in the 16-256 mm size range. Tahiti was chosen because it consists predominantly of isotropically-wearing basalt, has many rivers, and numerous beaches with different sand-pebble ratios and different wave energy characteristics. The maximum projection sphericity averaged .68 for rivers, .64 for low-wave-energy beaches, and .58 for high-wave-energy beaches. Roundness averaged .38 for rivers, .47 for low-wave-energy beaches, and .55 for high-wave-energy beaches. The oblate-prolate index averaged +0.18 (prolate, rod-like) for rivers, -0.81 for low-wave-energy beaches, and -2.13 (oblate, disc-like) for high-wave-energy beaches. It appears that a sphericity value of under .66 and oblate-prolate index more negative than -1.5 distinguish beach from river pebbles. On sandy, low-wave-energy beaches the smallest pebbles are flattest, while on gravelling, high-wave-energy beaches the largest pebbles are flattest. Thus the optimum sliding size of pebble is a measure of surf vigor. A selective trapping effect causes discs to be more abundant on sandy than on gravelly beaches. Abrasion is the chief cause for the abundance of discs on the beaches; evidence is that as roundness increases from rivers to beaches, sphericity decreases and oblate-prolate index becomes more negative (disc-like). The production of discs on the beaches is predominantly a result of abrasion caused by pebbles sliding back and forth over sand or smaller pebbles in the surf zone. This change in shape is accomplished within distances of a few feet to a few hundred feet of the river mouth. Selective shape sorting is also important on some beaches with waves 1-3 feet high, and sandy beaches tend to selectively trap discs.

Carbonate Sands of Isla Perez, Alacran Reef Complex, Yucatán

Beach sediments of Isla Perez consist of about 60 per cent Halimeda algae, 25 per cent coral, and 15 per cent foraminifera and others. Six discrete sediment populations are present, distinct in skeletal origin, grain size, grain shape, and position on the island. Large blocks of massive coral occur on the southeast margin, facing the strongest waves. Sticks of staghorn coral make up extensive ramparts chiefly along the south coast, and also form a pavement with sticks oriented perpendicular to shore just below the water line. The most abundant sediment is 0ø sand, chiefly discoidal Halimeda segments; superbly sorted 2ø sand, chiefly coral grit, blankets the west coast where waves are gentlest. Gastropod fecal pellets form a thick layer at the back ends of two shallow, calm, restricted bays. Carbonate mud, resulting from abrasion of coral and Halimeda, is present in small amounts in the subtidal sediments. All the pure sediment populatons have very similar good sorting values,σranging from 0.3 to 0.6ø over a thousandfold range of mean size; these values are equivalent to those achieved on terrigenous beaches, therefore surf action tends to produce similar sorting values on pure materials no matter whether the beach material is calcareous or siliceous. Mean size, sorting, and skewness form a helical trend when coplotted for these samples. Subtidal sediments, particularly those from marine grass flats, are much more poorly sorted, give more non-normal curves, and have a much larger content of material finer than 3ø. Staghorn coral breaks down selectively into two size fractions: sticks, the size of the joints in the parent colony, and 2ø grit, the size of the crystal packets composing the skeleton. Halimeda breaks down into 0ø flakes and into 10ø dust, the size of the component microcrystals. Grains of all sizes are uniformly angular and dull except on one part of the coast, where measurable rounding and polish occur. This is probably caused by the low rate of supply of fresh coral at this site.

The “schizohaline” environment: Its sedimentary and diagenetic fabrics as exemplified by Late Paleozoic rocks of Bear Island, Svalbard

Abstract Environments characterized by fluctuating hypersaline to fresh-water conditions are defined as “schizohaline” and examples are given of situations in which “schizohaline” environments can arise. Fabrics diagnostic for the occurrence of both hyper- and hyposalinity have been recorded in Late Paleozoic rocks from Bear Island (74°30′N, 19°E): evaporite nodules, length-slow chalcedony and very finely crystalline penecontemporaneous dolomite are considered as indicators of hypersaline conditions while euhedral limpid dolomite crystals, coarse poikilitic sparry calcite and microspar calcite are thought to have originated under the hyposaline regime. Zoned dolomite crystals and euhedral crystals of authigenic mega-quartz replacing sulphates have also been recorded in the same rocks; their diagnostic importance for the schizohaline environment is, however, uncertain. Internal structural pores of some crinoid and bryozoan skeletal grains of biosparites from Bear Island are either empty or infilled with very finely crystalline dolomite. Expulsion of Mg 2+ ions from the magnesian-calcite skeletons and either: (1) “poisoning” of the micro-environment of the pores; or (2) formation of stagnant “micro-sabkha” conditions in the pores are proposed as an explanation for these phenomena.

Black Phytokarst from Hell, Cayman Islands, British West Indies

Phytokarst is a distinctive landform resulting from a curious type of biologic erosion. Filamentous algae bore their way into limestone to produce black-coated, jagged pinnacles marked by delicate, lacy dissection that lacks any gravitational orientation. Ordinary rainfall-produced karst and littoral karst are characterized by flat-bottomed pans and vertically oriented flutes, thus differing from phytokarst. Algae attack by dissolving calcite preferentially to dolomite.

Alternative origins for nannobacteria-like objects in calcite

More than 40 calcite-precipitation experiments were performed under sterile conditions in order to investigate the origins of 25–300 nm spherical-, rod-, and ovoid-shaped objects that have been widely interpreted as evidence of nanometer-scale life (i.e., nannobacteria). Individual experiments included the addition of soluble organic compounds, common species of eubacteria, or phage-induced eubacterial lysates. These experiments indicate that many of the nanometer-scale objects have inorganic or nonnannobacterial origins. In the precipitation experiments, calcite formed euhedral crystals 50–800 nm in diameter and smaller (<50 nm) anhedral or rounded particles or protocrystals. The small anhedral or rounded solids resembled nannobacteria. The relative amount of anhedral or rounded calcite was greatest in experiments with a dissolved organic component. These controlled experiments are in accord with observations that rounded nanometer-scale objects are more common in minerals formed in organic-rich environments. Bacterial fragments occur as rounded to irregularly shaped particles that included cell-wall fragments, expulsed cytoplasm, and relict capsules that also closely resembled nannobacteria. Acid etching of the large euhedral crystals produced in the precipitation experiments also resulted in the formation of nanometer-scale features that resembled nannobacteria in natural carbonates. The shapes of the etching artifacts vary as a function of the strength of the acid and the duration of etching. Much caution is advisable in interpreting the origin of rounded features <50 nm.