Stromatolite Biostromes Research Articles

Sponge-microbial build-ups from the lowermost Triassic Chanakhchi section in southern Armenia: Microfacies and stable carbon isotopes

The end-Permian mass extinction was the most severe biotic crisis in Earth's history. In its direct aftermath microbial communities colonized some of the space left vacant after the severe decline of skeletal metazoans. The Permian-Triassic boundary microbialites were peculiarly abundant on low-latitude shallow-marine carbonate shelves of central Tethyan continents. Armenia features particularly well preserved and diverse basal Triassic sponge-microbial build-ups (BTSMBs), which were not studied in detail to date. Here, the Chanakhchi section in southern Armenia is described petrographically and by means of stable isotope analyses. The Armenian BTSMBs formed in a distally open marine setting on a pelagic carbonate ramp in the course of two phases of microbial growth during the Induan (Lower Triassic). The BTSMBs are represented by predominantly thrombolitic but also dendrolitic and digitate stromatolite biostromes and mounds that vary in height between 5cm to 12m. The digitate stromatolites are associated with calcium carbonate crystal fans (CCFs). Microfacies analyses revealed that the BTSMBs exhibit a number of different growth forms and internal fabrics. The formation of CCFs was apparently not devoid of biological influence and took place above the sediment surface. The abundance of sponges in the BTSMBs reveals that ecologically complex metazoan-microbial reefs have been present already early after the end-Permian mass extinction. However, the formation of biostromes and mounds did not depend on sponges or other metazoans. BTSMBs that formed during the second microbial growth phase revealed similar δ13C-values like the surrounding sediment. In contrast, the δ13Cmicrobialite and δ13Csediment values from the BTSMBs and CCFs of the first growth phase show a difference of up to +2.3‰, suggesting a significant influence of photoautotrophy during microbially induced carbonate precipitation.

Ediacaran stromatolites and intertidal phosphorite of the Salitre Formation, Brazil: Phosphogenesis during the Neoproterozoic Oxygenation Event

The Ediacaran Nova America and Gabriel members of the Salitre Formation are composed of limestone and economic phosphorite that accumulated on an unrimmed epeiric ramp along the margins of the Irecê Basin, Brazil. Deposition occurred during a marine transgression punctuated by higher-order fluctuations in relative sea-level that produced m-scale, shallowing-upward peritidal cycles. Cycles consist of six lithofacies rich in microbial sedimentary structures including subtidal, cross-stratified grainstones and hemispheroidal columnar stromatolite reefs overlain by intertidal flat sediments indicative of decreasing accommodation. Phosphorite is restricted to the paleocoast where digitate stromatolite biostromes colonized tidal flats.Phosphorite accumulation is interpreted to have been associated with biostromes because photosynthetic oxygen production created a redox gradient beneath the seafloor that phosphogenic chemosynthetic bacteria exploited. The concentration of francolite or sedimentary apatite in microbial laminae suggests these bacteria actively stored, released, and concentrated phosphate to promote in situ precipitation. The sealing effect of interbedded, fine-grained tidal deposits was also critical for maintaining the high levels of pore water phosphate required. The absence of francolite in subtidal columnar stromatolite reefs implies phosphogenesis was prevented in deeper, more energetic environments because wave pumping of oxygenated seawater through reefs surrounded by constantly moving grainy sediment promoted the recycling of P directly back to the water column.The Salitre Formation has a complex paragenesis, including hydrothermal alteration that produced Mississippi Valley-type Pb-Zn mineralization. δ18O values of Nova America member dolomites range from −10.2‰ to −0.5‰ (mean=−3.9‰) and δ13C ranges from −9.2‰ to +10.0‰ (mean=+2.8‰). Samples contain varying proportions of low-Mg calcite and saddle dolomite. δ18O values of hydrothermal veins range from −4.7‰ to −3.0‰ (mean=−4.2‰) reflecting equilibration with temperatures >80°C. δ13C values are between −7.0‰ and +5.6‰ (mean=−1.8‰,). Late lateritic weathering produced calcretes with δ18O values between −3.3‰ and −1.3‰, and δ13C values from −9.2‰ to −8.0‰ (mean values are −1.8‰ and −8.7‰, respectively). Petrographic analysis, generally low δ18O, and high δ13C values suggest hydrothermal dolomitization and remobilization of P led to secondary phosphate mineralization of intertidal stromatolite biostromes to produce economic phosphorite.Collectively, these results suggest that the benthic P-cycle in the Neoproterozoic was more complex than previously surmised and emphasize the multifaceted significance of microbial, paleoenvironmental, and diagenetic processes that allowed phosphorite to accumulate on the São Franciscan craton. Such information further elucidates attributes of the onset of Earth's second major phosphogenic episode, which is roughly coincident with the Neoproterozoic Oxygenation Event (NOE) and the evolution of multicellular animals.

Milankovitch-driven cycles in the Precambrian of China: The Wumishan Formation

Carbonate strata of the Mesoproterozoic Wumishan Formation in the Jixian area near Tianjin are ~3300m thick and were deposited over some 100million years (from ~1310±20 Ma to ~1207±10 Ma). Metre-scale cycles (parasequences) dominate the succession. They are generally of the peritidal carbonate type, and mostly show an approximately symmetrical lithofacies succession with thick stromatolite biostromes and small thromboliteoncolite bioherms constituting the central part and tidal-flat dolomites forming the upper and lower parts. Lagoonal-supratidal dolomitic shales with palaeosol caps make up the topmost layers. The boundaries of the Wumishan cycles are typically exposure surfaces, and there is abundant evidence for fresh-water diagenesis.Widespread 1:4 stacking patterns indicate that the individual Wumishan cycles are sixth-order parasequences, with 4 parasequences constituting one fifth-order parasequence set. Locally, 5–8 beds or couplets, can be discerned in some of the cycles. The regular vertical stacking pattern of beds within the sixth-order parasequences, forming the fifth-order parasequence sets, are interpreted as the result of environmental fluctuations controlled by Milankovitch rhythms, namely the superimposition of precession, and short and long-eccentricity. The widespread 1:4 stacking pattern in the cyclic succession, as well as the local 1:5–8 stacking patterns of the beds within the cycles, suggest that the Milankovitch rhythms had similar ratios in the Mesoproterozoic as in the Phanerozoic. Based on the cycle stacking patterns, 26 third-order sequences can be distinguished and these group into 6 second-order, transgressive-regressive megasequences (or sequence sets), all reflecting a composite, hierarchical succession of relative sea-level changes.

Lacustrine Facies In Response To Millennial-Century-Scale Climate Changes (Lake Hayk, Northern Ethiopia)

Abstract The different response of depositional systems to changes in accommodation causes lacustrine shorelines to develop a marked along-strike variability, which is well known in modern lakes but poorly documented in the fossil record. The late Holocene shoreline deposits of Lake Hayk (Southern Wollo, Ethiopia) provide an outstanding example of this variability. The succession is time-calibrated through numerous radiocarbon dates and provides a detailed record of climate-driven, lacustrine oscillations that occurred during the past 3500 yr. These oscillations are also recorded in coeval East Africa lakes and promoted three main highstand phases, which occurred at about 3250–3000, 2600–950, and 650–160 Cal yr BP. Oscillations in lake levels are documented in fluvio-deltaic systems and colluvial fan deltas with stromatolite biostromes along low- and high-relief lake margins, respectively. Along low-relief margins, most rises in lake level were recorded by aggradation of fluvio-deltaic or marsh successions, which range in thickness from 2 to 6 m. Along the high-relief margins the same lake-level rises are documented by 0.2 to 4.5 cm thick, wave-winnowed, sandy lag deposits. Development soils, which stabilized the steep slopes and prevented their erosion, allowed the occurrence of nondepositional transgressions. Isotope data show that stromatolite biostromes formed during phases of intense evaporation and lacustrine contraction, which are also recorded by fluvial erosion in adjacent areas. Changes in accommodation can be differently documented in nearshore lacustrine successions as a consequence of a marked along-strike variability of shoreline depositional dynamics.

Evidence for reversal of basin polarity during carbonate ramp development in the Mesoproterozoic Borden Basin, Baffin Island

Distribution of facies in the lower half of the Bylot Supergroup suggests overall westward deepening of the Mesoproterozoic Borden Basin. In marked contrast, the upper half of the succession records a reversal in the overall bathymetric trend, such that the eastern portion underwent relative deepening as the west experienced relative shallowing. Strata deposited during this reversal belong to the Victor Bay Formation, a ramp composed predominantly of limestone. Karsting of carbonate strata and development of an angular unconformity in the west contrast with back-stepping and drowning of the ramp in the east, followed by mantling by deep-water limestone, carbonaceous carbonate, and turbidites. Increased accommodation space during this time, via both tectonic subsidence and eustatic sea-level rise, led to a profusion of stromatolite pinnacle reefs and large biostromes. The reversal of basin polarity is best reconciled with development of a distal foreland basin superimposed on the Borden aulacogen. Crustal rethickening and uplift occurred along reactivated basement faults during an eastward-directed compressional event and could be related to thrusting of similar age and vergence in the Coppermine River Group of northwestern Canada.

Basic Lithofacies‐Succession Model for the Wumishan Cyclothems: Their Markov Chain Analysis and Regularly Vertical Stacking Patterns in the Third‐Order Sequences

Abstract The Mesoproterozoic Wumishan Formation in the Jixian section of Tianjin is a succession of 3300‐m‐thick carbonate strata formed in a period of about 100 Ma (1310 ± 20 Ma‐1207 ± 10 Ma). In this succession of strata, the carbonate metre‐scale cyclic sequences belonging to peritidal type with an approximately symmetrical lithofacies‐succession are best developed. The wide development of 1:4 stacking patterns shows that these metre‐scale cyclic sequences are genetically related to the short‐eccentricity cycles, which are called the Wumishan cyclothems that could truly represent sedimentary cycles. Generally, massive and thick‐bedded calcareous dolomites and dolomitic limestones of stromatolite biostromes and thrombolite bioherms constitute the central part of the Wumishan cyclothems. The lower and upper parts consist of tidal flat dolostones, sandy‐muddy dolostone and the top part is composed of lagoonal facies dolomitic shales with a paleosol cap. Therefore, an approximately symmetrical lithofacies‐succession is formed. Many features such as the clear deepening and shoaling vectors of cyclothems, and all kinds of marks of fresh‐water diagenesis indicate that the Wumishan cyclothems are the product of autocyclic sedimentation governed by allocyclic high‐frequency sea‐level changes. The results of a Markov chain analysis reaffirm the existence of the lithofacies‐succession model of the Wumishan cyclothems. The boundaries of the Wumishan cyclothems are marked by the instantaneous exposed punctuated surface, which leads to the discrepancy between the cyclothems and the parasequences of the sequence stratigraphy terminology system. It is difficult to form a judgment that the time span of the Milankovitch cycles in the Precambrian is certainly equal to that of the Phanerozoic, but the regularly vertical stacking patterns of the seventh‐order rhythms, sixth‐order cyclothems and fifth‐order parasequence sets still indicate their consistency with the duration of the Milankovitch cycles in the Phanerozoic.

A tempestite-stromatolite-evaporite association (late Vendian, East Greenland): a shoreface-lagoon model

Sections of the Canyon Formation (310 m) and overlying Spiral Creek Formation (25 m) have been studied on Ella Ø (Ella Island), central East Greenland. These strata cap Vendian glacial deposits and are overlain disconformably by probable basal Cambrian sandstones. The Canyon Formation is informally divided from bottom to top into mudrock (250 m), siltstone (15 m), flakestone (25 m) and stromatolitic (27 m) members. Overlying a thin basal sandstone interpreted as littoral facies, the main bulk of the mudrock member consists of variably dolomitic silt-shales (offshore deposits below wave base). Bituminous films in the upper half of the member are thought to represent microbial mats. The overlying units represent a major shallowing-upwards sequence. The siltstone and flakestone members consist largely of sharp-based coarse dolomitic siltstones (0.05-0.6 m thick) containing combinations of: gutter casts, parallel and undulatory lamination, wave-ripple lamination and flexed dolostone intraclasts. A tempestite origin is indicated. Dolostone clasts in the flakestone member are analogous to shell fragments in Phanerozoic tempestites and are derived by reworking of thin dolomite crusts within the background sediments of the flakestone member. There is a transitional boundary to the overlying stromatolitic member which consists dominantly of finely crystalline dolostones with thin (<0.05 m) storm layers and stromatolite biostromes and bioherms. The lower half of the member lacks evidence of desiccation, but does contain subaqueous shrinkage cracks. The upper half contains a number of scoured and desiccated dolostone surfaces, abundant rigid intraclasts and several thin oolitic beds. Stromatolite morphology is characteristically broad (metre-scale), fairly smooth domes, whose synoptic relief tends to decrease upwards in individual beds and overall in the member. Scattered anhydrite crystal and nodule pseudomorphs are common, varying in abundance at different horizons. The Spiral Creek Formation contains a basal cross-stratified sandstone, with desiccated dolostone layers, interpreted as a tidal sandflat deposit. The central part of the Spiral Creek Formation consists of centimetre-scale alternations of dolomitic sandstones with short-wavelength wave ripples, and mudrocks with desiccation cracks and halite pseudomorphs interpreted as playa lake deposits. The overall facies assemblage above the mudrock member is interpreted as representing the progradation of a mudrock-dominated evaporative lagoon-sandflat-mudflat-playa assemblage over a shoreface dominated by storm action. The lagoon may have been partly barred by stromatolites, but is seen to be transitional with the shoreface. This requires a new facies model in which muddy coastal and lagoonal carbonate facies are not bounded seaward by a continuous reef or sandbody. This is indicative of low wave energy, except during infrequent storms, and a low tidal range, so that strong tidal currents are only locally developed.

Is cratonic sedimentation consistent with available models? An example from the Upper Proterozoic of the West African craton

The Atar Group, part of the Upper Proterozoic sequence covering the West African craton, stable since 2000 Ma, is characterized by an alternation of extensive carbonate beds and mixed siliciclastic and carbonate facies. The carbonate beds comprise essentially columnar stromatolite biostromes and bioherms which reflect sublittoral environments. The mixed facies contain a variety of laterally discontinuous facies which imply more variable environmental conditions. The settings of the mixed facies are not always clear but they do not contain thick sequences of high-energy facies. Few obvious facies sequences are discernable; those that are present are considered to be punctuated aggradational cycles (PACs) and they always start with biostromes of columnar stromatolites with very few sediments. Composite sequences are interpreted as due to shallowing upward or increasing energy environments that may be laterally contiguous, despite the fact that the contacts are not gradational. However, much of the stratigraphic sequence cannot be subdivided into cycles and seems to consist of unrelated individual facies, bound by sharp boundaries. The basin analysis reveals that biostromes of columnar stromatolites start after an instantaneous geological event corresponding to a sea-level rise. Consequently, their appearance can be considered as a time-line. We describe, in the Atar Group and its equivalents, three sedimentation trends, all of which are interpreted to be of shallowing upward character. The Atar Group appears to have been deposited in an epeiric sea (i.e. an extremely flat ramp). There are two contrasting styles of sedimentation: (1) after the submergence of the whole area, columnar stromatolites built extensive biostromes; (2) during the stable phase, sediments are deposited in a mosaic of laterally-discontinuous facies. Tidal influence cannot be recognized in the sequence, neither can a salinity increase toward the land; both common features in published epeiric sea models. A cratonic sedimentation area such as this is characterized by its size and flatness. Only during the stable phase of the cycle does small-scale topographic relief lead to deposition of a mosaic of facies. The sedimentation is storm- and wave-dominated.

Palaeogeographic significance of stromatolitic buildups on late proterozoic platforms: The example of the West Congo basin

The West Congo Basin, equatorial Africa, is filled with a Late Proterozoic sedimentary succession. Its middle part is made up of the “Schisto-calcaire” (shale-limestone) Group. A study of the lower part of this group has resulted in a model of carbonate sedimentation. A stromatolite buildup, very similar to the present barrier reefs and cut by tidal and/or swell channels, separates an intertidal and supratidal inner platform from a subtidal outer one. The inner platform is characterized by deposition of dolomitic limestones containing gypsum crystals epigenetically replaced by calcite and chalcedony. The outer platform is made up of stromatolite biostromes arranged in elongate domes parallel to marine currents alternating with limestones with thin shale stringers. In Proterozoic basins, stromatolitic barrier reefs mark transitions either between inner and outer platforms, or between platform and deep basin. Subtidal stromatolite biostromes, with an elongation of ten to hundreds of meters, locally encroached upon some of the slowly subsiding, flat-bottomed epicontinental basins.

Supralittoral ponded algal stromatolites of the late Precambrian Annijokka Member of the Båtsfjord formation, Varanger peninsula, north Norway

Abstract Non-columnar stromatolites occur in the late Precambrian Annijokka Member of the Batsfjord Formation of the Varanger peninsula, north Norway. The stromatolites form biostromes up to 0.5 m thick embedded within peritidal, shallowing-up, siliciclastic-dolomite cycles. The stromatolite biostromes tend to occur in the upper, dolomite-rich portions of the cycles and are mostly calcite-dominated. Interpretation of the origin of the host sediments, combined with the location of the biostromes in the depositional sequences and with the contrasting mineralogy between the latter and the stromatolites, suggests that the majority of the stromatolite biostromes originated in freshwater and schizohaline supralittoral ponds.

Late Precambrian tidal-flat deposits and algal stromatolites in the Båtsfjord formation, East Finnmark, North Norway

The Annijokka Member of the Late Precambrian Båtsfjord Formation consists of about 300 m of siliciclastic and carbonate deposits which accumulated on tidal flats, and includes several horizons of stromatolite biostromes. Seven main lithofacies are arranged in fining-up cycles a few metres thick. The cycles are terrigenous in the lower part and carbonate-rich higher up and are interpreted as reflecting shallowing-up conditions of deposition. Lithofacies distribution in the member as a whole also shows an upward decrease in the terrigenous component and grain size and increase in carbonates, thus suggesting that the Annijokka Member is regressive. The stromatolite biostromes of the member contain domal forms composed of calcite while the non-stromatolitic, though possibly in part algal-laminated, carbonate-rich beds of the member are dolomitic. This contrasting mineralogy suggests (1) penecontemporaneous supratidal dolomitization, and (2) a possibly freshwater-influenced origin of the domal stromatolites.

Stromatolite biostromes and associated facies in the late precambrian porsanger dolomite formation of Finnmark, Arctic Norway

The Late Precambrian Porsanger Dolomite Formation, occurring beneath the Varanger tillite in Arctic Norway, consists of various dolomitic lithofacies of subtidal, intertidal and supratidal environments. The lithofacies belong to three facies associations, A, B and C, which are repeated several times in the sequence. Facies association A comprises cryptalgal laminites, dolomicrites and thin-bedded grainstones and flakestones. The environment represented by this facies is broadly intertidal (locally supratidal) flat, with the interbedded carbonate sands representing storm deposits. Facies association B, of shallow subtidal to low intertidal origin, comprises cross-bedded carbonate sands (flakestones, grainstones and oolites) forming units up to 10 m thick. Small stromatolite bioherms (5 m wide, 2 m high) are locally developed within these “high-energy” deposits. Facies association C formed in a subtidal environment consists of laterally extensive (over 20 km) uniformly developed stromatolite biostromes, up to 16 m thick. The biostromes, locally divided by channels filled with grainstones and intraformational conglomerates, are composed of cylindrical and turbinate columnar (SH-V and SH-C) and digitate stromatolites ( Gymnosolen, Inseria and Tungussia) in their lower parts. Larger, bulbous (SH-C and LLH-C) and conical ( Conophyton) stromatolites occur in the upper parts, as well as the branching conophyte, Jacutophyton. All of the biostromes are always developed above cross-bedded carbonate sands (facies association B). A broadly symmetrical cyclic pattern, A B C B A, of tidal flat deposits (facies association A) passing up into carbonate sands (B), into biostrome (C), overlain by carbonate sands (B) and then tidal flat deposits (A), is repeated four times in the Porsanger Dolomite sequence. The pattern is interpreted in terms of two controls on sedimentation: (1) a slow transgressive phase followed by (2) depositional regression. The former (1) took place either through eustatic sea-level rise or more likely through accelerated subsidence because of tectonic instability and compaction of underlying sediments. This resulted in the sequence: tidal flat sediments, low intertidal/shallow subtidal carbonate sands, subtidal biostrome (A, B, C). Depositional regression through prograding tidal flats, generated the shoaling upward part of the cycle: biostrome, carbonate sands, tidal flat sediments (C, B, A).