Coated Grains

The term “coated grains” was coined by Wolf (1960) as a substitute for Folk’s (1959) “oolites”, “to include other concentrically formed materials such as pisolites” (Wolf 1960, p. 1415). Among the different terms proposed for specific kinds of these grains the term “ooid” has unquestionably the longest tradition in geological literature, going back to the 18th century, although the orthography as used here is of later date, Having been introduced by Kalkowsky (1908). As ooids, “small egg-like grains, resembling the roe of a fish, each of which has usually a small fragment of sand as a nucleus, around which concentric layers of calcareous matter have accumulated” (Lyell 1855, p. 12) have been named. In 1916 Heim introduced a new term: “oncoid” (in original German orthography: Onkoid; “onchos” in Greek means “nodules”), for denoting a type of coated grain found by him in Mesozoic deposits of the eastern Swiss Alps which differed from an ooid in that the coated grain called oncoid possessed (Heim 1916, p. 566): (1) non-concentric overlapping of separate envelopes, (2) not exclusively rounded form, (3) compact structure, with or without nucleus, and (4) sharp or indistinct boundaries. As noted by Heim (1916), transitions between ooids and oncoids exist, and this is expressed in encrustation of ooids by irregular calcareous envelopes.

In his paper on Classification of coated grains, Peryt (this vol.) subdivides the coated grains in ooids, vadoids, oncoids, and rhodoids. In my opinion, such a classification mixing descriptive (ooids, oncoids) with genetic terms of the same rank (vadoids, rhodoids) is inconvenient. Moreover, Peryt considers the “hot spring ooids” of Richter and Besenecker (this vol.) and some of the ooids discussed by Richter (this vol.) — thermal ooids, cave ooids, aqueduct ooids, and power plant ooids — as vadoids, though this is not justified in my opinion. This was the reason for preparing the following comment.

Coated grains, which may have skeletal cores, are non-skeletal carbonate and non-carbonate grains formed in marine as well as non-marine environments. These grains are classified according to their structure and environmental parameters (Peryt, this vol.). Peloids are micritic grains without recognizable structures and are represented by individual grains and aggregates. Ooids, oncoids, rhodoids, and vadoids are coated grains with distinct structural features.

Coated grains form in various environments, some of which have been characterized by their carbon and oxygen isotopic composition. The origin and history of ancient carbonate rocks may be inferred by studying these compositions. The carbon and oxygen isotopes in carbonate minerals have both been used extensively in the studies of environment of deposition, diagenesis and burial history (Gross 1964, Land 1970, Tan and Hudson 1974, Weber et al. 1976, Hudson 1977, Campos and Hallam 1979).

Neritic coated grains of macroid size grow below the tidal zone over restricted surfaces of the sea floor reflecting thus the narrowness of the equilibria conditioning their genesis. As the growth of the grains is a time dependent process, the abiotic and biotic factors involved in the genesis of coated grains must be time-dependent also. The equilibria between the factors are therefore dynamic.

“It cannot give a better description of its (the cloud’s) shape than by comparing it with that of a pine tree; it rushed to a great height, straight and smooth as the trunk of a tree which divides into branches at the top.” This is the description of the initial ash cloud of the eruption of Mount Vesuvius on August 24, 79 A. D. as given by C. Plinius (the younger) in a letter to the historian P.C. Tacitus. Had he shown the same scientific interest as his uncle C. Plinius (the elder) who died during the eruption, probably from respirational problems, he might have discovered in the ash layers peculiar pellets frequently exhibiting concentric textures. Such bodies are now generally referred to as accretionary lapilli, in preference over terms as volcanic hailstones, mud balls, etc.

In 1855, Lyell gave an appropriate definition of “oolitic grains” (p. 12): “The variety of limestone called ‘oolite’ is composed of numerous small egg-like grains, resembling the roe of a fish, each of which has usually a small fragment of sand as a nucleus, around which concentric layers of calcareous matter have accumulated.” Also Sorby’s detailed description of calcareous “oolitic grains” from 1879 is still modern. He distinguished between the following three types: (1) “Concentric structure” characterizes the aragonitic oolitic grains of the hot springs at Karlsbad (CSSR) and of the modern sediments in the Bahamas. “Concentric” means, according to Sorby, layers of tangentially oriented aragonite crystals. (2) “Radiate structure” refers to the radial-fibrous structure of many calcitic fossil oolitic grains. Sorby recognized this structure as primary. This was neglected, at least for marine ooids, from Cayeux (1935) to Shearman et al. (1970) and has been reestablished by Simone (1974), Sandberg (1975) and Wilkinson and Landing (1978) only in the last ten years. (3) “Recrystallized structure” is developed in calcitized former aragonitic oolitic grains, in which “the larger irregular crystals formed in them have either no special orientation or are arranged with the principal axis converging towards the centre”.

Ferriferous ooids are a common feature in several banded cherty iron formations as well as in chert-poor iron formations (Kimberley 1978). Ooids in both cherty and chert-poor iron formations are similar morphologically to Recent calcareous ooids (Markun and Randazzo 1980, Rohrlich 1974). Ferriferous oolitic layers are always concentric and do not display the pervasive radial structure that cuts across concentric layers in many ancient calcitic ooids. Ancient calcitic ooids with just a small proportion of iron minerals also display well-preserved concentric layering (Figs. 1 A, 2 C).

Calcareous ooids from modern and ancient marine and non-marine settings are highly variable owing to differences in initial cortical composition and fabric. Compositionally, modern ooids consist of low-magnesian calcite (0%–5% MgCO₃), high-magnesian calcite (10%–18% MgCO₃) and aragonite with more than one phase commonly occurring within an individual cortex (e.g, Land et al. 1979). The primary mineralogy of ancient (now low-magnesian calcite) cortices was also variable (e.g., Kettenbrink and Manger 1971), although the initial magnesium content of ancient calcite ooids is undocumented and the relative abundance of aragonite versus calcite phases has not been constant throughout Phanerozoic time (e.g., MacKenzie and Pigott 1981).

Recent studies along the shallow margins of the northern Bahama Banks have strongly indicated that sand transport has been and continues to be one of the dominant processes determining the character of these platform edges and distal lagoons (Hubbard et al. 1976, Hine 1977, Hine and Neumann 1977, Harris 1979, Palmer 1979, Hine et al. 1981a, b, Wilber 1981). The recent acquisition and analysis of high quality space and aerial imagery (see acknowledgements) show extensive fields of relict, large-scale bedforms. These large bedforms (maximum size: spacing — 1 km, length — 10 km, height — 5 m) are generally covered and stabilized with sparse to dense benthic flora and are thus termed “relict” — that is, the original sediments and depositional topography are no longer in equilibrium with the modern environment. This suggests that sand movement was more vigorous and pervasive earlier in the Holocene on the bank tops and margins than at present.

Although Recent (Holocene) carbonate sands have been deposited during an extremely brief epoch of geologic time, their subsurface facies relationships may be complex. The development of ooid shoals in the Bahamas has been examined by coring and seismic profiling by Ball (1967), Buchanan (1970), Hine (1977), Harris (1979), Palmer (1979) and Hine et al. (1981). The studies document facies, sedimentary structures, vertical sequences and geometry of bank-margin ooid accumulations. It is these sedimentary features that are critical in our interpretation of ancient oolite deposits seen in outcrop or borehole cores.

Since the pioneering work of Sorby (1879), considerable attention has been directed toward the examination of compositions and fabrics exhibited by modern and ancient ooids. Although volumetrically important in modern settings and in the rock record, marine ooids show little variation from locality to locality, and therefore represent a small sample of compositions and fabrics possessed by coated grains. Although volumetrically insignificant, non-marine ooids from both high- and low-salinity systems are important in understanding the physical and chemical processes which give rise to variations in coated grains.

Aragonitic coated grains < 1 mm in diameter with a layered cortex formed by crystals of tangential orientation are characteristic of ooids from the Bahamas (Illing 1954, Newell et al. 1960). This structure is normal for recent ooids from marine environments (for exceptions see Land et al. 1979). The µm-sized aragonite crystals are rod- or baton-shaped (Fabricius and Klingele 1970, Loreau 1970).

Coated grains form in a number of environments. In the modern marine environment, the cortex of coated grains contains individual aragonite crystals with their long axes oriented tangential to the nucleus. In the cortex of some coated grains, the long axes or aragonite (and calcite) crystals are normal to the rims forming ooids with a radial fabric. Modern coated grains having this fabric occur in hypersaline waters such as the Great Salt Lake of Utah; in Baffin Bay, off Laguna Madre, Texas; in lakes in the Kulundin Steppe; or in thick accumulations of algal mats, as found in sea-marginal pools of the modern Red Sea, and along the west coast of Australia (Vital 1948, Friedman et al. 1973, Kahle 1974, Sandberg 1975, Davies and Martin 1976, Halley 1977, Friedman and Sanders 1978, p. 52–53).

The Liassic carbonate platform in the Veneto is rich enough in oolitic rock-types to justify the fact that one of its components, an Upper Lias carbonate formation, is called the S. Vigilio Oolite.

Sichuan Basin was an epicontinental sea in the late (Aurenig) of the Early Triassic. The sea was bounded to the north and west by land masses, and to the south by a submarine high, while to the east lay the open sea (Fig. 1). These paleogeographic features controlled sedimentation in the basin.

The Givetian-Frasnian Portilla Limestone Formation is well exposed in the Cantabrian Mountains in N.W. Spain (Fig. 1). It is sandwiched between the siliciclastic underlying Huergas and overlying Nocedo Formations. The member subdivision is area-dependable (Fig. 2). Six facies have been recognised (Reijers 1972); all reflect deposition in a shallow marine environment on an extensive carbonate platform and its slope. One facies is siliciclastic. It predominates in one member (C in the central and eastern and B in the western part of the study area; Fig. 2) and straddles the Givetian-Frasnian boundary. This unit reflects a change in the sedimentation. It is the result of a Givetian-Frasnian pulse of reactivation of the hinterland supply area. Neither this member nor this facies will further be considered in the present paper.

The Boomplaas Formation represents a newly recognized carbonate platform deposit (Beukes 1978) below the Campbellrand carbonate platform sequence in the lower part of the Transvaal Supergroup on the Kaapvaal craton in Griqualand West, South Africa (Fig. 1A). Deposition of the Transvaal Supergroup commenced more than 2300 m.y. ago (Button 1976) making the Boomplaas Formation one of the oldest known carbonate platform deposits in the world.

Nodules composed of coralline algae have been described since the latter part of the eighteenth centuary (Pallas 1766, Ellis and Solander 1786). The first extensive descriptive work on free-living corallines was undertaken by Foslie (1894), Lemoine (1910) and later by Cabioch (1966, 1969). Bosellini and Ginsburg (1970) were the first to recognise the palaeoecological significance of rhodoliths and they suggested a descriptive terminology. They proposed the term rhodolite for “... nodules and detached branch growths with a nodular form composed principally of coralline algae ...”. Unfortunately the term rhodolite has priority for a variety of garnet (Binda 1973) and therefore “rhodolith” has now become the accepted term (Toomey 1975). More recently “rhodoid” has been suggested (Peryt, this vol.) but this is not preferred by the author because (a) an acceptable term is already in use and (b) its etymology is unsuitable (Rhodolith-rhodophycean or red stone, rhodoid-rhodophycean or red-like).

Rhodoliths (= rhodolites, coralline algal nodules) are nodules and unattached branched growths with a nodular form composed principally of coralline algae.

Some genera of the calcareous red algae (Phylum rhodphyta) have an encrusting habit which may produce laminated nodules termed rhodolites by Bosellini and Ginsburg (1971) and which here are termed rhodoids. Rhodoids have been recognized from many modern and ancient environments and increasing use is being made of their paleoecology for paleo-environmental reconstruction (Bosellini and Ginsburg 1971, Adey and MacIntyre 1972, Milliman 1974, 1977, Bosence 1976, Buchbinder 1977, Studencki 1979). The importance of rhodolites in temperate carbonates is being increasingly recognized and this paper describes a number of occurrences from New Zealand Cenozoic sediments deposited under warm temperate conditions at about 45 °S latitude (Kennett et al. 1975, Molnar et al. 1975, Nelson 1978a).

The Sacramento Mountains of southcentral New Mexico (Fig. 1) outcrop at the eastern edge of the basin-and-range province, and are a fault-block range. The mountains are aligned in an approximate north-south direction, are tilted about one degree to the east, and on the west, are bounded by a gravity fault zone situated close to the present escarpment (Pray 1961). It has been estimated that the minimum displacement along the western escarpment for a distance of 29 km is on the order of 2330 m, but this decreases to about half this amount towards the northern end of the range where the Early Permian section is exposed. Most of the salient structural features were formed by major tectonic movements during latest Upper Carboniferous and Early Permian time, with only relatively little structural complication to the Permian sequence. The major uplift of the Sacramento Mountains fault-block is believed to have occurred in late Tertiary time. Significantly, Pray (1961, p. 5) reports that fault scarps seen in Recent alluvium suggest that the uplift of the mountains is still continuing.

Oncoids are a group of algally (red algae excepted), cyanobacterially and bacteri-ally coated grains which originate(d) in marine and freshwater phreatic zones, although some workers prefer to use the term oncoids for all coated grains which are not ooids. Recent oncoids have been described from the lacustrine, fluvial and marine environments. The marine oncoids, occurring in the intertidal and shallow subtidal environments, are non-lithified, as opposed to hard freshwater oncoids. As noticed by Monty (1972), recent freshwater forms are closely similar to the ancient marine oncoids (see Flügel 1978, Tables 15 and 16). This in turn may be related to the replacement of porostromate oncoids by spongiostromate oncoids in marine settings in the Jurassic (see Peryt 1981, with discussion). At that time, porostromate oncoids (and mainly Girvanella oncoids — see Peryt 1981, Tables 1 and 2, and papers by Biddle, Bowman, Catalov, Poncet, and Tichy, this vol.) and spongiostromate oncoids existed in different environmental settings.

A wide variety of calcareous nodules have been regarded as being probably formed by the activity of cyanophytes. Stromatolites too, are generally assumed to be mainly of cyanophyte origin. But it is only possible to be confident about these interpretations if the deposits contain direct internal evidence of the algae involved. In the Recent this can be provided by the presence of the living algae themselves, but in ancient material the evidence must be in the form of mineralized fossil remains or distinctive petrographic fabrics. In many cases it must be admitted that a cyanophyte origin for stromatolites and oncoids is inferred only from general similarities in form and structure between them and Recent cyanophyte mats which have trapped and bound particulate sediment. Specific evidence for the type of algae (or other microorganisms) involved is usually lacking. However, some fossil examples of stromatolites and oncoids contain convincing evidence of their cyanophyte origin due to the presence of mineralized algal remains within them. Many of the Precambrian silicified microfloras, which provide valuable information about the early history of life on Earth, occur within stromatolites. Examples include the microfloras from the Transvaal Dolomite, Gunflint Iron Formation, Belcher Group, Paradise Creek Formation, Bungle Bungle Dolomite, Beck Spring Dolomite, Bitter Springs Formation, and many others (Schopf 1977, Table 2).

Two types of lacustrine coated grains from an oligotrophic freshwater lake (Attersee, Austria) are described in this paper: 1.One type are the so-called “Krusten- and Furchensteine”, lacustrine encrusted and furrowed stones. They have been known for more than 120 years. Schimper (1857) reported them from Lake Geneva (published by Forel 1878). Later on they were discussed more thoroughly, e.g., by Boysen-Jensen (1909), Kann (1941) and Schneider (1977). The carbonate crusts are structured like cauliflowers and build micro-“patch-reefs” on every solid substrate (inorganic and biologic). The crusts are biogenic growth structures which in addition incorporate trapped detritus and inorganically precipitated calcite (Schneider and Schröder 1980). Furrows occur on carbonate substrates beneath well developed crusts (Schneider 1977).2.Another type of coated grains originates from encrusted macrophytes, which produce CaCO₃ crusts solely by themselves and/or together with epiphytic microphytes (Passarge 1904, pp. 108/109; Wetzel 1960, p. 232; Allen 1971, p. 124; Schäfer 1973; Allanson 1973; Schröder 1982).

According to Peryt’s (1981) definition oncoids contain nuclei of any petrographical composition which are irregularly coated by cyanobacterial envelopes of the Porostroma (with visible algal filaments) or Spongiostroma type (without visible filaments) (Monty 1981). Grains and rock fragments which are coated only by coralline algae are termed “rhodoids” by Peryt (1981) and “rhodolites” by Bosellini and Ginsburg (1971). Difficulties in nomenclature arise if the grains are coated with both blue-green and red algae, as will be described in this paper. The authors of this paper are therefore inclined to use the broader definition of oncoids given by Flügel (1978), which includes any algal, foraminiferal algal, serpulid algal and micritic oncoids.

An oncolite (sic) is a stromatolite with centrifugal growth vectors, and encapsulating laminae formed around an intermittently mobile nucleus (Hofmann 1969).

The stratigraphy of the Campos Basin (Fig. 1 A, B) was controlled by the following sequence of tectonic stages characteristic of rifted continental margins (Ponte et al. 1980); pre-rift arching: denudation of Paleozoic cover (Permian-Jurassic); intracratonic rift valley: N-S collapse trough enlarged to rift valley system (late Jurassic — early Cretaceous) filled by Lagoa Feia fluvio-deltaic to lacustrine clastics; proto-oceanic gulf: restricted embayment (late Aptian) with deposition of Lagoa Feia evaporites; open marine continental margin: subsidence and seaward tilting, generation of carbonate platform consisting of offshore bars of oncolitic packstones separated by mudstones (Lower Macaé, Albian); further tilting, subsidence of platform, transgression of argillaceous pelagic mudstones and intercalated turbidites (Upper Macaé, Cenomanian) followed by basinal shales and turbidites (Carapebus Member, Campos Formation, Senonian-Eocene); renewed tilting of continental margin and rejuvenation of source areas with intense deltaic progradation (Ubatuba Member, Campos Formation (Oligocene — Pliocene).

Phosphate-rich macrooncoids occur in the Albian condensed limestones in the Tatra Mts (Poland) and they are especially well developed in the Turnia Ratusz succession (Fig. 1). These condensed limestones are built of several alternating layers (5 – 40 cm thick) of calcisphaeral and foraminiferal wackestones and echinoderm wackestones. The macrooncoids, like other cryptalgal structures (mostly stromatolites) and the majority of authigenic non-carbonate mineralizations, are associated with pelagic horizons only (Fig. 1).

A specific type of oncoids, mostly found in association with stromatolitic domes in decimetric cycles, is quite common in red nodular limestones (Rosso Ammonitico) of the Trento Plateau area (Southern Alps). This was a pelagic horst block of the southern continental margin of the Tethys (Fig. 1) which evolved from a strongly subsiding Liassic carbonate platform. Thin Bajocian sequences of clean-washed Bositra-bearing coquinas and crinoidal sands (the “Posidonia alpina” beds) commonly underlie — and in places interfinger with the lowermost part of — the “Rosso Ammonitico” pelagic limestones (Sturani 1971). Theseform condensed and discontinuous sequences in the Trento Plateau area, and are usually represented by two members. The lower member (Rosso Ammonitico inferiore, Upper Bajocian to Middle Callovian) is often dominated by an oncolitic-stromatolitic facies and is more massive; moreover, in places it includes sparse interbeds of clean-washed and rippled microcoquinas of thin-shelled pelecypods. The upper member (Rosso Ammonitico superiore, Upper Oxfordian to Tithonian) grades upwards into Majolica limestones and shows a lower frequency of cryptalgal structures as well as the presence of sparse thin shaly or marly interbeds; it is most commonly characterized by a flaser-nodular facies in which evidence of exposure of the nodules at the depositional interface is often lacking. The two members in addition are locally separated by some meters of cherty limestones.

In eastern Sardinia a Middle to Upper Jurassic marine sequence more than 1000 m thick rests unconformably on the Palaeozoic basement and consists in the coastal area of three main lithostratigraphic units: the Dorgali Formation, consisting for the most part of dolostones (Bathonian), the ammonite-bearing Mt. Tului Limestone (Lower Callovian to Uppermost Kimmeridgian) and the Mt. Bardia Limestone (Portlandian to Berriasian). The second unit its gradually replaced westwards by oolitic and oncolitic dolostones, and is therefore incorporated in the first one (Fig. 1 A, C). A transgressive trend can be recognized in the lower part of the Jurassic sequence, from Bathonian to Upper Kimmeridgian. During this time span asymmetric carbonate shelf cycles mostly consisting of upward shoaling oolite sequences were formed. They indicate the existence of a wide shallow shelf with a system of tidal oolitic bars periodically prograding seawards and separating an internal shelf lagoon largely open to marine influences from an outer shelf area. The latter during the Early Callovian-Late Kimmeridgian was the deposi-tional setting of the Mt. Tului unit which in its lower part is represented by an oncolitic facies, mainly occurring in the more condensed and pelagic sequences.

Oncolites occur abundantly in the Upper Jurassic limestones of French Jura mountains, Paris Basin, Aquitain basin and also in the Spanish province of Valencia. On the basis of preliminary observations of oncolites of these regions, seven representative and well exposed oncoid-bearing carbonate sequences from the French Jura were studied in detail for stratigraphical and petrological observations (Fig. 1).

Oncolites are very characteristic in the Carnian (Middle Triassic) of the North Alpine facies. In the Northern Calcareous Alps oncolites are very abundant in the Raibl Bed. But they only occur south of the line between the localities Lech am Arlberg — Heiterwand — Wetterstein (foreland) — Guffert — Kaisergebirge — Rauschberg — Staufen — Bad Ischl — Steyerling — Windischgarsten (see also Jerz 1966, Schuler 1968, Grottenthaler 1978). As in the Drau Range oncoids occur at the beginning of the Raibl sequence. The so-called “Grenzoolithbank” or “Raibler Grenzlager” is regarded by Schneider (1953) to be the top of the Wetter-stein Formation (which is also Carnian: Cordevolian in age), but it is the very base of the Raibl Member according to the author. Oncolites often occur at the transition from carbonate facies into a clastic facies. The most characteristic “Sphaerocodium Limestone” of the Northern Calcareous Alps is within the first shale layer unlike the Drau Range where this layer is missing (Jerz 1966, p. 53; Schuler 1968, p. 33). Further oncolites appear at the top of the first shale bed. There, in the Wimbach-Valley the oncolites are 7 m thick (Schuler 1968). Far to the south-east the thickness rapidly decreases.

Several types of coated grains occur within the Middle to Upper Triassic carbonate rocks of the Dolomite Alps. These structures have been mentioned by a number of authors and have been interpreted in various ways. Leonardi (1967) discussed laminated, spherical nodules, which he called Sferocodi, that were presumably formed by encrustations of the alga Sphaerocodium. These nodules are common in biostromal accumulations near the crests of many carbonate buildups in the Dolomites (Leonardi 1967, p. 217). Bosellini and Rossi (1974) provided several excellent illustrations of vadose pisoids from the Dolomites. In their interpretation, the vadoids occur in thin soil horizons and in areas that were subjected to Triassic subaerial exposure.

Recently an attempt has been made to modify the traditional concepts of the bathymetric and energy significance of oncoids in paleofacies interpretations (Peryt 1981). It has been pointed out that oncoids are important indicators of breaks and low rates of sedimentations as well as of positive tectonic lineaments and nearshore facies (Weiss 1969, Toomey 1974, Peryt 1977, 1981). It seems that the study of the oncoids occurring in the Ladinian-Carnian sediments of Central Balkanides may throw some light on the problem.

Algal nodule limestones form a distinctive lithotype in platform carbonate successions from the Upper Carboniferous of the Cantabrian Mts., N.W. Spain (Rácz 1964, de Meijer 1971, van de Graff 1971, Bowman 1979). This paper presents an analysis of nodule bearing rocks from the Late Namurian-Westphalian A San Emiliano Formation in its type area in northern Leon (Fig. 1). It represents the first detailed synthesis of algal nodule limestones from the Spanish Carboniferous, discussing their genesis and palaeogeographic significance. It also discusses the relative importance of allogenic and autogenic processes in the growth and development of the nodules and their microfabrics.

Oncoids are ubiquitous in the peritidal facies of the Lower Carboniferous of Britain. A study has been made of the variety of oncoids which occur in one of these peritidal units, the Llanelly Formation in South Wales, with the aim of elucidating the factors controlling oncoid morphology.

Vadoid is a coated grain which originated in a vadose environment. To vadoids belong grains which have been (and still are) variously termed, e.g., cave pearls, fluvial pisoids, caliche pisoids, and vadose pisoids. Although commonly of pisoid size, vadoids are often of microid size and these cases were earlier called “diagenetical ooids” (Siesser 1973), “calcrete ooids” (Read 1974), “vadose ooids” (Harrison 1977) or simply “ooids” (Braithwaite 1975, Elloy and Thomas 1981, Richter, this vol.).

Holocene-Pleistocene (?) age travertine deposits occur throughout southeastern Idaho, U.S.A., associated with the volcanic strata along the Snake River. Warm springs at some of these sites are still actively forming travertine deposits. Lithi-fied travertine accumulations at two locations, one along Fall Creek in eastern Bonneville County, and the other along Fritz Creek in northwestern Clark County (Meredith 1980), have been investigated in detail (Fig. 1).

Caliche with pisolitic horizons have been recognized in the soils in the Mediterranean coast area of Spain from Almeria to Barcelona and Lleida. In the southern area the pisolitic horizons develop on old calchified aluvial fans as well as on volcanic rocks. In the middle and north of the Mediterranean coast they appear overlaying carbonate and metamorphic rocks.

The “type locality” for travertine is at Tivoli, about 25 km east of Roma. In fact, the name “travertine” descends from the Latin, “Lapis Tiburtinus”, referring to the stone quarried at Tibur (the ancient name for Tivoli). Quarries in the area around Tivoli have been active for two millenia, and the stone has been used for most of the famous monuments of Rome ranging from the Coliseum through St. Peter’s to the modern University. Today there are a great many active quarries, and travertine from Tivoli is exported worldwide.

Pisoids, concentrically laminated grains between 2 mm and 10 mm in diameter, originate in a variety of environments and the word pisoid is now seldom used without a qualifying genetic adjective (Dunham 1969, Kendall 1969, Scholle and Kinsman 1974, Esteban 1976, Jones and Wilkinson 1978). Attempts have been made to define the characteristics of specific pisolites (Esteban 1976, Chafetz and Butler 1980), but conflicting points still exist. This paper describes the characteristics and processes of formation of Quaternary ferruginous caliche pisovadolite from the Lau group, Fiji, and the results are compared with recent and fossil caliche pisovadolite described from elsewhere.

The abundant pisolite of the back-reef facies of the Capitan Limestone of the Guadalupe Mountain, New Mexico and Texas has long been of geologic interest. Its interpretation is important for better understanding the famous Permian Reef Complex and for interpreting pisolite, pisoids and associated facies elsewhere in the world. But major changes in prevailing concepts of the Guadalupe Mountains pisolite have occurred in the past 20 years. Consensus has not yet been reached. The purpose of this article is to summarize our current data and interpretations, emphasizing field-observable features.

The Sinian Tongying Formation is widespread in Southwest China, attaining a thickness of about one thousand meters (Fig. 1).

The rocks of the McArthur Group, which includes the Amos Formation (Fig. 1), are mainly carbonates which have many features typical of deposition under shallow water, frequently hypersaline, and emergent conditions. A sabkha model has been proposed for the Mallapunyah Formation and parts of the Amelia Dolomite (Muir 1979a), and a model based on the ephemeral lakes of the South Australian Coorong has been suggested for the Yalco Formation (Muir et al. 1980). Further evidence for shallow water and emergent conditions has been described for most formations in the measured sections summarised by Jackson et al. (1978) and Plumb (1979). The persistence of such uniform depositional conditions for most of the time required to form nearly 6 km of sedimentary rocks is remarkable. The rocks of the McArthur Group extend from the Queensland border to the northern coast of Arnhem Land. The sediments can be interpreted with certainty as shallow water and emergent at least as far north as Roper River. Poor preservation in Arnhem Land makes environmental interpretations more difficult there.

Coated grains constitute an ubiquitous component of the bioclastic carbonate sediments of the reef tract throughout the Great Barrier Reef Province (Fig. 1). They include ooids, rhodoids and a variety of encrusted grains which display a range of sizes and shapes depending upon the form of the object being coated by crustose coralline algae. The latter may contribute between 15% and 60% of the bulk of the reef top sediments.

Coated grains are very frequent in the Mesozoic carbonate rocks of the Outer Dinarides. This chapter describes the coated grain facies in the Lower Cretaceous of the Adriatic region (Fig. 1) the aim is to characterize briefly the main types of coated grains and to evaluate their environmental significance.

The paleoenvironmental conditions in the Northern Apennines area during the Norian and Lower Liassic were highly different: the Norian climate was hot-arid whereas the Liassic one was hot-humid; in the Norian a large amount of sulphate evaporites precipitated, whereas no evaporites have been found in the Liassic sequences; the Norian platforms were bordered by low-energy, shallow seas, whereas the Liassic ones were built up in a pene-oceanic situation (Fig. 1).

From the Zechstein Limestone, the carbonate member of the lowermost Zech-stein evaporite cycle, the Werra, of different parts of the basin ooids, oncoids, rhodoids and vadoids have been described by various authors (e.g., Kerkmann 1966, Kostecka 1966, Füchtbauer 1968, Piekarska and Kwiatkowski 1975, Peryt 1977, 1978, 1981a, Peryt and Piatkowski 1977, Belka 1979, Peryt and Wagner 1981, Smith, in press). Quite often the same type of coated grains occurring in the Zechstein Limestone rocks has been variously termed, depending on the general conviction of a given author as to the significance of processes leading to the origin of that type. On the other hand, usually the fact that in any given rock the grains which originated in different times in quite different environments can co-occur, has been ignored and the common practice was that the features observed in a part of coated grains occurring in a given Zechstein rock and characteristic of a certain type of coated grains, led to generalization that other grains of somewhat different features are in fact variations of that type. All these caused the picture of conditions of occurrence and formation of particular types of coated grains in the Zechstein deposits to become obscured.

An unusual type of coated-grain nodule is common throughout reef and fore-reef facies of the Capitan Limestone (Guadalupian) in New Mexico (Fig. 1). These nodules, which may attain a maximum diameter of 3.5 cm, consist internally of concentric, irregularly laminated cortices, surrounding skeletal fragment nuclei, of alternating Archaeolithoporella (?red alga) and fibroradial calcite replacive of marine, acicular aragonite. The occurrence within these nodules of crustiform red algae suggests their comparison to rhodoids. However, such nodules are distinct from classic modern and ancient rhodoids in terms of the absence of characteristic algal growth forms (Bosellini and Ginsburg 1971), the interpreted dual accretionary mode of cortical laminite formation (viz algal encrustation and carbonate precipitation), and mineralogy (aragonite vs high-magnesian calcite). Alternatively, the characteristic laminated cortices and radial crystalline microfabrics of these nodules suggests a certain similarity to both vadoids and coniatoids, albeit algal encrusted and solely of sub tidal marine origin. This contribution describes the geologic occurrence, morphologies, internal structures, microfabrics, and origin of these peculiar coated grains.

In the northeastern Armorican Massif the Pont-aux-Bouchers Formation (formation d in Fig. 1), which is of Middle to Upper Siegenian age, is truncated by an erosion surface. This formation, although mainly composed of terrigenous detritus (fine sands, shales) includes a locally developed reefal horizon (reefal horizon of Baubigny; Poncet 1968). A study of the facies of this horizon has enabled the distinction of two different zones of sedimentation resulting from the development of bioherms (Poncet 1972, 1976); a zone of sedimentation lateral to the Fig. 1Location map and stratigraphy.Stippled: Lower Devonian. a St. Germain-sur-Ay formation,b La Haye-du-Puits formation, c Néhou formation, d Pont-aux-Bouchers formation. 1 Shale, 2 sandstone, 3 linestone, 4 stromatolites bioherms on the one hand and a zone of back-reef sedimentation on the other. A lithologic study showed that in both zones numerous and diversified coated grains occur.

Since the pioneering work of Hall (1918) spheroidal or “oolitic” rocks have been described by a number of authors from the rocks of the Swaziland Supergroup (3.3 – 3.6 b.y.). Accretionary lapilli were first recognized in them by Reimer (1967). An unusually conspicuous band between the Onverwacht and Fig Tree Group, together with several similar occurrences in the Onverwacht Group, was described as accretionary lapilli by Lowe and Knauth (1978). These and other spheroidal rocks are described in this paper, and the implications of their occurrence are discussed. While the larger occurrences are described in some detail, information on the smaller ones is compiled in Table 1.