Tepee Structures Research Articles

Note on the palaeoenvironmental significance of the nuccaleena formation (upper Precambrian), central Flinders Ranges, South Australia

Abstract Four distinct sedimentary facies have been recognized within the upper Precambrian Nuccaleena Formation in the central Flinders Ranges of South Australia. Three dolomite facies, containing tepee structures within either well‐laminated or stromatolitic dolomite, were deposited within supratidal and intertidal environments, whilst an interbedded shale and dolomite facies originated under very shallow submerged conditions.

Environmental setting and microstructure of subfossil lithified stromatolites associated with evaporites, Marion Lake, South Australia

ABSTRACTA variety of finely laminated, subfossil, aragonitic stromatolites and oncolites occur on a regressive marginal flat surrounding Marion Lake, South Australia. These algal forms overlie a substrate of coarse, highly porous, moldic aragonitic limestone which passes progressively towards the take centre through a zone of interstatified aragonite and gypsum and ultimately to pure crystalline gypsum. All of these facies overlie Holocene marine carbonate bank sediments which unconformably overlie at least one upper Pleistocene marine unit. Detailed petrographic and stratigraphic studies, combined with comparative studies of related nearby lakes containing a variety of living aragonitic cryptalgalaminates, provide a model for development of the Holocene sedimentary sequence. Marion Lake last became inundated by the sea around 6500 years ago during the Holocene transgression, when a protected marine environment was initiated. Lateral sediment accretion sealed marine passes into the resulting lagoon system soon after sealvel stabilized, and a variety of gypsum and gypsum‐carbonate‐algal facies evolved. Pure gypsum was deposited in waters 2–3 m deep in the central basin area concurrently with formation of seasonally alternating gypsum and aragonite layers towards basin margins. Blue‐green filamentous algae thrived in the shallower marginal areas and at least partly controlled carbonate deposition, which must have occurred during seasonal outflow of carbonate‐rich ground water from the calcareous dune aquifer over denser gypsum‐saturated waters. These systems eventually migrated towards the centre of the lake to produce the relationships preserved today. The fresher waters also leached the gypsum from the marginal gypsum‐carbonate facies. Collapse due to gypsum dissolution, along with aragonite crystallization, combined to form a lake‐marginal mega‐polygonal facies. Teepee structures formed around polygon margins, with optimum conditions for stromatolite development occurring on the teepee crests. The actual stromatolites which occur around Marion Lake are strongly indurated and involve a variety of morphologies, the most common of which are laterally linked hemispheroids. Stacked hemispheroids and oncolites are also relatively common, along with irregular forms, many of which encrust a variety of substrate irregularities. Vertical relief of the stromatolites varies from centimetres to tens of centimetres and all forms are characterized by extremely fine internal interlaminations of alternate light and dark grey laminae which typically occur several per millimetre. The microstructure comprises micritic aragonite crystals with fibrous habit associated with organic matter, and occasional zones of abundant algal filament molds which are generally oriented normal to the laminae.

Nature, origin and classification of peritidal tepee structures and related breccias

ABSTRACTDistinctive peritidal tepee antiform structures, buckled margins of saucer‐like megapolygons are common in marine vadose fenestral and pisolitic limestones and/or dolomites of carbonate platform sequences and occur in intertidal and supratidal carbonates ranging in age from Silurian to Holocene. These megapolygons commonly form and are sometimes truncated before the deposition of the next sedimentary layer. The megapolygons result from the expansion of surface sediments by as much as 15%. The expansion is caused by the following continuously repeated sequence of processes: (1) Desiccation and thermal contraction causing small fractures; (2) phases of wetting causing enlargement of fractures; (3) phases of crystallization of calcium carbonate and other minerals causing the enlargement, fill and cementation of the fractures. Precipitation is from brines and meteoric waters; (4) hydration of minerals, thermal expansion, breaking waves and faulting may add to this disruption.The development of the tepee fabric can be traced from an initially cemented subaerial fenestral crust, exhibiting expansion and compressional structures, to a completely disrupted and brecciated sediment riddled by a labyrinth of fractures and solution cavities. These spaces are filled by numerous phases of internal marine and fresh‐water cement and sediment, the latter containing penecontemporaneous or younger marine faunas.Peritidal tepees are useful tools for geologic reconstruction and provide evidence of subaerial exposure; a tropical to subtropical climate; and back‐beach or back‐barrier deposition. Proper identification of tepees is of economic importance, because they provide good early porosity and permeability for petroleum entrapment and a site for mineralization. Aesthetically, tepee rocks are a fine kaleidoscopic decorative stone.

Desiccation Features and the Reconstruction of Paleosalinities

ABSTRACT Simple experiments illustrate a marked correlation between the morphology of desiccation cracks and interstitial salinity in carbonate sediments. In a study delineating microfacies and depositional environments in the Grayburg Formation (Permian) of west Texas, a correlation was also seen between the size and spacing of mudcracks and their position in the supratidal environment. In laboratory experiments, carbonate muds of uniform composition were wetted with sodium chloride solutions of varying concentrations and allowed to desiccate under identical circumstances. With high repeatability, the less saline samples were the first to crack and, when experiments were carried to completion, possessed the largest desiccation features. Under controlled laboratory conditions, desiccation is related to vapor tension, a function of the ionic strength of pore fluids, complexing, and the activity of water. Grayburg carbonate muds from an offshore hummocky tidal flat possess deep desiccation features, profuse burrows, and exhibit a moderate array of relic bioclasts. Farther landward, coastal supratidal deposits possess shallow desiccation cracks, exhibit no burrows, and appear to have hosted only a limited or restricted indigenous fauna. The coastal sabkha-like deposits are further distinguished from their offshore exposed-flats counterparts by containing an early anhydrite cement in well-developed birdseye and teepee structures. If these faunal, structural, and mineralogical zonations are indeed indicators of syndepositional paleosalinities, then a correlation exists between salinity and the development of desiccation features in Permian sediments. Less saline offshore flats appear to b more deeply and profusely cracked than hypersaline coastal flats. A distribution of mudcracks analogous to the Permian examples has also been observed by other workers in modern carbonate sediments along the Persian Gulf. In some areas the magnitude of polygon development parallels the declining salinity profile encountered in moving seaward from hypersaline sabkha pore waters to areas characterized by more normal marine interstitial salinities.

Tepee structures and associated diagenetic features in intertidal carbonate sands (lower Jurassic, Morocco)

In southeastern Morocco, Early Jurassic inter- to supratidal carbonates can be followed for 200–300 km along the southern slopes of the High Atlas mountains. These beds form intervals of several tens to more than one hundred metres. Tepee structures, which are common in these beds, are confined to coarse-grained, early lithified beaches. Comparison with similar features in the Recent, and analysis of the diagenetic history of the sheetcrack-fills suggests that the formation of the tepees is an early, almost synsedimentary event.

Caliche Soil Horizons on Oolitic Shoals and Carbonate Mud Mounds in Carboniferous (Newman Limestone) of Eastern Kentucky: ABSTRACT

A sequence of oolitic calcareous grainstone shoals and calcareous wackestone and packstone-bank and interbank deposits is exposed near Olive Hill, Kentucky. The shoals and banks underwent periods of exposure, as they are capped by beds that represent stages of deposition or caliche weathering in the supratidal zone. The caliche weathering zones are similar to those found on Caribbean Islands and in the southwestern United States. The first type of caliche zone is analogous to the subaerial crusts of the Caribbean area. These crusts are brecciated, often silicified, laminated calcareous mudstones which formed on and within the shoals and mud mounds. They are present as beds or coat fractures which transect bedding planes. Petrographic characteristics include microbrecciation, grains coated with laminated or dense micrite, extensive micritization, pelletoid fabrics, root tubules, and micrite and yellowish-brown spar cements. The second caliche type, similar to those of the southwestern United States, is a nodular, brecciated calcareous mudstone that developed from supratidal and intertidal calcareous muds. The basal part of this caliche unit is a fossiliferous, but intensely brecciated, parent rock; this zone grades upward into a nodular sequence which may be capped by a thin-brecciated, laminated crust. Original depositional fabrics have been obliterated in the nodular section; the nodules are composed of microspar containing breccia clasts and root tubules, and are disrupted by subhorizontal fractures. Both caliche types display End_Page 785------------------------------ desiccation-expansion teepee structures. Caliche clasts are reworked into overlying intertidal-supratidal deposits, indicating that weathering occurred near sea level and contemporaneous with offshore deposition. End_of_Article - Last_Page 786------------

Triassic Carbonate Buildups of The Dolomites, Northern Italy: ABSTRACT

In the Middle Triassic of the Dolomites of northern Italy thick carbonate bodies (1,000 m) of relatively restricted extension are surrounded, and in places covered, by volcanic rocks and clastic sediments. These bodies, characterized at the periphery by inclined beds dipping basinward (15-35°), are formed by massive carbonates, mostly crystalline dolomite (Schlern Dolomite); in many places the massive facies is replaced laterally by stratified carbonate rocks. Organisms in the massive dolomite are uncommon (some corals, a few pelecypods and crinoidal plates) and poorly preserved; in the massive limestones and in the bedded carbonate rocks, calcareous algae, crinoids, mollusks, rare corals, foraminifers, and ostracods are present. The central part of the buildups lies directly on a biostromal blanket dolomite (Serla Dolomite) which extends throughout the region. A sequence of cherty micritic limestone (Livinallongo Formation), in places nodular or bituminous, End_Page 836------------------------------ lies lateral to the core between the Serla Dolomite and the Schlern Dolomite; between the buildups this sequence is covered directly by volcanics (prevalently latites) and other volcanoclastic rocks. Prevalently terrigenous sediments (Raibl Formation) ended Middle Triassic sedimentation and disconformably overlie the carbonate masses. In the literature, from the last century until the most recent papers, the carbonate buildups have been interpreted as coral reefs with internal lagoons (the bedded parts); the volcanics and the associated sediments filling the interreef basins have been regarded as contemporary with the reefs. The Dolomites became famous as a classic region for paleoreefs and spectacular lateral facies changes (heteropic contacts). The following data are from a new detailed field examination and sedimentologic analysis. The massive part of the buildups is prevalently mud-supported grainstone (pelletoid and skeletal grains) and is characterized by the presence of reef tufa (fibrous carbonate in cavities with irregular, in many places lobate, walls). the stratified part consists of thick beds of mud-supported grainstone (pelletoids). The grainstone is commonly burrowed, has crinoids and algae (Diplopora), and has thin interbeds of laminated micrites, common pisolites, intraclastic breccias, sheet cracks, and tepee structures. The boundaries between carbonate bodies and surrounding volcanoclastic rocks are abrupt, without transitional facies; the bodies interfinger with the underlying Livinallongo Formation whi h, in the transition zone, contains abundant, commonly graded breccias. In places the upper part of the buildups interfingers with muddy carbonate sediments that end the basin sequence. These data suggest a different interpretation from the classic one. On the higher parts of the basal blanket dolomite, banks of shallow-water carbonate sediments developed and grew. These carbonate banks were exposed periodically, keeping pace with subsidence, and reached thicknesses of about 1,000 m. A starved-basin sequence was deposited in progressively deeper waters to a maximum depth of about 800 m. Later the products of the submarine volcanic eruptions filled interbank depressions rather quickly, literally suffocating some buildups and fossilizing their primary morphology. At the end of the Middle Triassic the almost complete topographic leveling caused a renewal of the carbonate sedimentation which led to the development of slightly raised banks and, in places, small coral reefs. End_of_Article - Last_Page 837------------

Observations on Sandstone Cementation: ABSTRACT

Comparative study of Permian and modern caliche, laterite, bauxite, and beach rock have shown that vadose cement, regardless of composition, can be distinguished from cementation below the water table or sediment-water interface. Phreatic and connate cementation which occurs beneath the water table or sediment-water interface is more coarsely crystalline than vadose cement and is interstice-filling rather than grain-coating. Early carbonate cement precipitated from water-filled voids at shallow depths consists of sparry calcite and may push apart and replace quartz and feldspar to produce textures. Silica cement occurs as void-filling overgrowths on quartz grains. After deep burial uncemented sand grains become tightly packed, the depositional texture is modified, and late aqueous cement occupies smaller interstices. The following characteristics and stages of intensity of vadose cementation can be distinguished. 1. Initial stage: The original depositional texture is lost as precipitation of fine-grained evaporitic films of carbonate, iron oxide, or aluminum hydroxide forces the grains apart so that films separate grains at former contact points. Sedimentary structures such as beach laminae and current ripple cross-bedding still may be preserved in this stage. 2. Intermediate stage: Floating textures are produced as grains become pushed apart by evaporite cement and by replacement of quartz and feldspar by carbonate. Coalescence of evaporite films around clusters of grains forms small pisolitic concretions. Sedimentary structures become obliterated or greatly obscured. 3. Late stage: Extreme floating textures and large composite pisolites form by addition of more cement and replacement of quartz and feldspar. Brecciated caliche anticlines (also known as teepee structures) are formed by expansion resulting from the addition of large volumes of carbonate cement. Extreme caution must be employed in interpreting the genetic significance of rocks classified as sandy micrite, wackestone, and boundstone or biolithite. Transformation of quartzose sand into sandy carbonate, consisting of up to 90% fine-grained carbonate (micrite) which commonly is pisolitic (strongly resembling algal boundstone), is well documented by numerous examples. Similarly, accumulations of ooliths, clasts, skeletal fragments, pellets, and calcarenaceous material may be transformed into rocks which could be identified as oomicrite, intramicrite, biomicrite, pelmicrite, wackestone, and boundstone or biolithite (pisolitic) by vadose processes (calichefication). Many so-called oolitic iron ores represent vadose concentrations of iron oxide and hydroxide. End_of_Article - Last_Page 208------------