Tide- and wave-dominated inshore and shoreline sequences from the late Precambrian, Finnmark, North Norway

The Late Precambrian Ekkerøy Formation of Finnmark, northern Norway represents a 40 m thick coarsening upward, prograding deltaic sequence. The upper 6 m of this formation consists of two small-scale fining upward sequences representing two contrasting shoreline environments. These two sequences have been the focus of a detailed facies analysis based on grain size, bed thickness and sedimentary structures. The study area comprises a series of horizontally bedded rocks which are continuously exposed for 3 km enabling individual facies and sedimentary structures be to traced-out laterally. The lower sequence exhibits a wide variety of tidal features including directional bimodality of both large-scale and small-scale cross-stratification, herringbone cross-stratification, various sandstone-mudstone alternations including lenticular and wavy bedding, and a general fining upward tendency. A detailed field analysis of the internal structures within the large-scale cross-stratified unit is presented and is interpreted as a sub-tidal channel complex. The associated sandstone-mudstone facies represents intertidal flats built up by tidal and storm-generated currents. The upper sequence displays a wide variety of wave-formed structures including wave-ripples and swash-lamination. Palaeocurrent data indicates a dominance of shoreward migrating megaripples and wave-ripple crests oriented approximately parallel to the inferred shoreline with a distinct longshore component. The sequence is interpreted as a beach-shoreface environment. In terms of sedimentary structures, palaeocurrent patterns and facies distributions, the sequence bears a remarkable similarity to the high-energy, non-barred shoreline of the modern Oregon coast U.S.A., described by Clifton, Hunter & Phillips (1971).     

Sedimentation and water escape structures in some late Precambrian shallow marine sandstones from Finnmark, North Norway

Water escape structures are abundant in the Grønnes Formation, a tectonically undeformed, late Precambrian shallow marine sandstone deposit in North Norway. Trough cross-bedded sandstones of the current-dominated shallow marine environment were frequently liquefied, presumably due to recurring seismic shocks. Subsequent dewatering resulted in deformation of the cross-bedding and the formation of convolute lamination. A three-fold upward vertical sequence developed where liquefaction occurred below the sediment-water interface: convoluted bed → passively deformed bed → undeformed bed. The passively deformed bed resulted from differential subsidence of a relatively plastic bed above a liquefied bed. It is characterized by anticlinal ridges and sand volcanoes at the sites of vertical sediment extrusion, and synclinal troughs at the sites of lateral sediment movement. Liquefaction may have been induced by either tectonic (earthquake shocks) or non-tectonic (storm-induced microseisms) trigger mechanisms, or a combination of both. The restriction of such a high frequency of water escape structures to deposits immediately above a gentle regional unconformity lends support for a tectonic trigger mechanism.     

A late Precambrian tidal shelf deposit, the Lower Sandfjord Formation, Finnmark, North Norway

The Lower Sandfjord Formation is a 1.5 km thick late Precambrian sandstone. It is a remarkably homogeneous unit consisting largely (98%) of cross-bedded, texturally and mineralogically mature, coarse or medium sandstone, and is interpreted as a shallow marine deposit. This interpretation is based on the maturity, the exclusively tabular bed geometry, occasional sets of herring-bone cross-bedding and most importantly, the abundance of sheet-like pebble layers only 1–5 grain diameters thick and sometimes overlain by thin siltstone drapes. Various different types of compound cross-bedding, all of which show evidence of reversing currents, are interpreted as sub-tidal sandwaves. These sets range in thickness from 0.5 to 14 m, and in conjunction with the overall abundance of cross-bedding probably indicate strong tidal currents. A tide-dominated current regime is also considered essential to explain the derivation of such large quantities of sand from the contemporary coasts. It is suggested that sand transport offshore took place during the erosional transgression of abandoned delta lobes. However, the predominance of a single, easterly, mode in the palaeocurrent patterns suggests that the tidal currents were reinforced by some other current system. The predominantly unimodal palaeocurrent patterns and the coarse, sand-rich nature of the succession, taken together with the thickness do not superficially seem likely characteristics for a shallow marine sequence. Nevertheless this study appears to demonstrate that such deposits were formed on tidal shelves in at least late Precambrian time.     

Superposition of plane strain on an initial sedimentary fabric: an example from Laksefjord, North Norway

Shape fabric determinations in deformed grits, microconglomerates and conglomerates in the Landersfjord region of the Laksefjord Nappe, Finnmark, North Norway, have demonstrated the existence of prolate fabric ellipsoids. The correlation between shape fabric symmetry and the orientation of the principal fabric axes suggests that the fabrics are the result of a tectonic plane strain superimposed roughly coaxially on an initially oblate sedimentary fabric. A plot of the fabric ellipsoids on a three axis diagram allows the effect of the initial fabric to be removed, and the amount of strain to be determined. A contour map of the strain shows it to increase to the north and west. Models are proposed for the origin of the strain.     

Petrographic variations within thick turbidite sequences: an example from the late Palaeozoic of eastern Australia

The subdivision of thick sequences of turbidite sediments has been problematical because of the monotonous nature of the units. One method, of using detailed detrital petrography for a large number of specimens, has delineated variations with a sequence of Late Palaeozoic age in eastern Australia. The rocks occur within a single structural block and are all members of one sedimentary petrographic province. They have been subdivided into three stratigraphic units (Moombil Beds, Brooklana Beds and Coramba Beds) and greywackes from these units are quartz-poor to quartz-intermediate, feldspathic or volcanolithic types. Dacitic volcanism has provided most of the detritus and the contribution from non-volcanic sources is small. The Coramba Beds are further subdivided into four petrographic units which are parallel to the stratigraphic boundaries. These lithostratigraphic units are based on the presence or absence of detrital hornblende, and the relative ratio of volcanic lithic fragments to feldspar. Vertical petrographic variations within the entire sequence indicate that although the acid volcanic source was predominant throughout the time of deposition, there is a noticeable increase in the contribution from intermediate-volcanic, acid-plutonic, low-grade metamorphic and sedimentary sources towards the top of the sequence. Detrital hornblende is also present in the upper parts of the sequence.     

Significance of late Precambrian turbidite sequences bordering the East Antarctic Shield

A late Precambrian turbidite succession is described from North Victoria Land, Antarctica, in its sedimentological, biological and tectonic aspects. Similar sequences occurring across the whole continent along the present Transantarctic Mountains are discussed. The geotectonic significance of this flysch belt is analysed in its relation to the East Antarctic Shield, the Gondwana craton and a proto-Pacific.

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Zusammenfassung Aus dem Nord-Viktoria-Land, Antarktis, wird eine spätkambrische Turbiditfolge mit ihren sedimentologischen, biologischen und tektonischen Kriterien beschrieben. Ähnliche Sequenzen, die quer durch den ganzen Kontinent im Zuge des Transantarktischen Gebirges vorkommen, werden diskutiert. Die geotektonische Bedeutung dieses Flysch-Gürtels wird analysiert in seiner Beziehung zum ostantarktischen Schild, zum Gondwana Kraton und zu einem Proto-Pazifik.

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Résumé L'auteur décrit, avec ses critères sédimentologiques, biologiques et tectoniques, une série de turbidites du Précambrien supérieur dans le Nord de la Terre Victoria de l'Antarctique. Il discute des séries analogues se présentant dans tout le continent suivant la présente Chaîne transantarctique. Il analyse la signification géotectonique de cette ceinture de flysch dans sa relation avec le bouclier de l'Antarctique oriental, du craton du Gondwana et d'un Proto-Pacifique.

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前寒武纪海侵砂岩中的微生物砂质碎片：以北京南口虎峪剖面大红峪组为例

碎屑岩中微生物形成的原生沉积构造,由于缺乏直接的微生物证据,其识别常常比较困难。位于北京西北郊的南口虎峪一带,古元古代晚期的大红峪组(1650～1600Ma),海侵体系域砂岩地层与高水位体系域砂泥质白云岩和叠层石白云岩组成了较为有规律的沉积相序特征,成为将大红峪组划分为三个三级层序的主要标志。在大洪峪组的海侵体系域砂岩层中,一种类似于碳酸盐岩中的“内碎屑”的奇特的砂质碎片,代表了前寒武纪海侵砂岩沉积面上曾经发育过微生物席;该砂质碎片与变余波痕和皱饰构造组成系列席底构造,特殊的形态和产出特点成为窥视前寒武纪砂岩中微生物席发育的一种典型标志。因此,南口虎峪剖面大红峪组海侵砂岩中的微生物砂质碎片及其相关的席底构造的发现,有益于今后对类似沉积构造的阐释与研究,特别是那些常常被厘定为前寒武纪古老的后生动物遗迹化石的沉积构造;同时也表明,在前寒武纪地层之中,除了要关注叠层石之类的微生物沉积构造以外,碎屑岩中也存在微生物活动的若干证据即席底构造(第五类原生沉积构造),这些特别的沉积构造对前寒武纪沉积环境重塑具有重要意义。     

Glacial retreat sedimentation in the Smalfjord Formation, Late Precambrian, North Norway

Tillites, conglomerates and sandstones occurring in the basal part of the Smalfjord Formation along the Varangerfjord, East Finnmark, North Norway are believed to have formed during the retreat of a glacier. At Kvalnes, on the south side of the fjord, the following sequence, up to 20 m thick, is found: (1) massive monomict tillite interpreted as a subglacial till, (2) massive polymict tillite with lenticular intercalations of stratified sandstone and tillite, interpreted as supraglacial/proglacial drift, (3) polymict conglomerate interstratified with laminated sandstones, interpreted as braided stream deposits. The last named interfingers laterally and is overlain by marine sandstones. At Bigganjargga, near the head of the fjord, a lens of tillite about 3 m thick rests on a striated pavement and is overlain by sandstones and shales. Part of the tillite, containing irregular patches of slightly winnowed tillite, is interpreted as a melt-out till, while a marginal part consisting of inclined tillite beds is interpreted as a series of flow till deposits. The lens is believed to be an oblique section through what was originally an ice-cored moraine ridge. During a subsequent transgression, the moraine was partially eroded, a lag conglomerate was formed, and overlying marine sediments were deposited. Bedded flow tills formed in a supraglacial/proglacial environment may be preserved where the extent of current reworking is very low (such as an isolated end moraine). Stratified conglomerate and sandstone, intimately intercalated with tillite, is to be expected at a glacier margin where glacial meltwater is locally and occasionally abundant, and glacier ablation permits downslope flowage of mobilized supraglacial fluid till.     

Multi-textured garnets from a single growth event: an example from northern Norway

Abstract Textural relationships between porphyroblasts of biotite and garnet in metasediments in the Nordkinn Peninsula area of the Finnmarkian Caledonides of North Norway are apparently complex. There is evidence for two textural zones in both mineral phases and superficially the development of these appears to have overlapped, at least in part, in time and space. This apparently complex porphyroblast growth history can be considerably simplified if only one period of garnet growth occurred and if different inclusion fabrics developed where garnet replaced biotite porphyroblasts and where it overgrew the matrix foliation. The possibility that porphyroblasts with textural evidence for multiphase growth histories actually grew during a single crystallization event is of importance in the interpretation and elucidation of tectonometamorphic relationships.     

Linnaeite from the Precambrian Raipas group of Finnmark,Norway

The previously described minerals of the linnaeite series occuring in the Borras and Raipas deposits near Alta, Finnmark, have been re-examined, mainly with the help of the electron microprobe. — As an introduction, a survey is made of the chemistry of the linnaeite series from published data and a new diagram is presented to show the relative variations of the four possible metallic elements. — The analyses confirm the respective chemical natures of the linnaeites from the two deposits and show a rather even chemistry within each deposit. Individual grains show inhomogeneities in element contents from outsides to centres and the forms of the inhomogeneities appear to vary depending on the relative abundance of the element concerned. The most abundant element in each case is relatively depleted in a very narrow outer zone or rim, whereas the minor elements show a more gradually diminishing (often irregular) content from grain boundary to centre. The intermediate elements show varying forms of distribution. The rim zones in the case of the major elements may be the result of diffusion out into the surrounding minerals, whereas the minor element distribution is possibly a primary one related to the growth of the linnaeite grains. — Results from one large, fractured linnaeite grain in the Raipas ore indicate considerable replacement by bornite along the internal fractures, though not at the original grain boundaries.

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Zusammenfassung Die früher beschriebenen Mineralien der Linneit-Serie, die in den Lagerstätten Borras und Raipas in der Nähe von Alta, Finnmark, vorkommen, wurden erneut untersucht, besonders mit der Elektronen-Mikrosonde. — Zunächst wird eine Übersicht der Daten über die Chemie der Linneit-Serie gegeben und ein neues Diagramm konstruiert, um die Variationsbreiten der vier metallischen Elemente von Linneit zu zeigen. — Die Analysen bestätigen die chemische Zusammensetzung der Linneite von den zwei Vorkommen, und zeigen eine weitgehende Übereinstimmung innerhalb jedes Vorkommens. Die Einzelkörner weisen Inhomogenitäten in der Elementverteilung vom Rand zur Mitte auf und ihre Form scheint mit der relativen Häufigkeit der betreffenden Elemente zu variieren. Das häufigste Element ist in jedem Fall in einer schmalen äußeren Zone relativ verarmt, während die untergeordneten Elemente eine mehr stufenweise Abnahme von Korngrenze bis Zentrum zeigen. Die intermediären Elemente zeigen wechselnde Verteilungsformen. Im Fall der Hauptelemente mag die verarmte äußere Zone ein Ergebnis von Diffusion in die umgebenden Mineralien sein, wogegen die Verteilung der untergeordneten Elemente möglicherweise primär, also an das Wachstum der Linneitkörner geknüpft ist. — Untersuchungen an einem großen aufgespaltenen Linneit-Korn aus dem Raipas-Erz deuten auf beträchtliche Verdrängungen durch Bornit entlang den inneren Spaltrissen, nicht aber entlang den ursprünglichen Korngrenzen.

     

Evidence for currents associated with waves in Late Precambrian shelf deposits from Finnmark, North Norway

Abundant wave ripples with wavelengths of up to 0.3 m, top-surface granule lags and tabular sand-stone beds suggest that the interbedded sandstones and siltstones of the Skaergårdnes Formation are shallow marine deposits. A simple orthogonal relationship between crestal trends of wave ripples and trough axes/cross-bed dip directions indicates that bottom currents and surface waves were closely related, suggesting that wind-drift currents were responsible for deposition. Interbedding of the tabular sandstones with siltstone is more probably due to the patchy nature of sand cover on the sea floor than to episodic sand derivation from the coast. The top-surface granule lags indicate local, temporary excess of sand removal over supply, and were probably formed by both wave-winnowing and non-depositional megaripple migration. The well developed lags in this and similar ancient sediments suggest that winnowing, leading to bed armouring and sediment bypassing may be an important shallow marine process.     

Silicified Precambrian evaporite nodules from Northern Norway: A preliminary report

Nodular silica concentrations reminiscent of evaporite bodies have been observed in the Late Precambrian (Riphean) Porsangerfjord Group in western Porsangerfjord, northern Norway. The nodules are individual or coalescent kidney-shaped bodies measuring a few centimetres across, and consist predominantly of megaquartz and of length-slow chalcedony. Length-fast chalcedony occurs only as a red layer present in some large nodules. Relics of a fibrous, felted or radiating texture are preserved in numerous crystals of megaquartz and quartzine.The quartz nodules are interpreted as silica-replaced, early diagenetic evaporite (anhydrite) bodies. The paleoclimatic importance of these Precambrian evaporites is briefly discussed and the significance of length-slow chalcedony as an indicator of evaporite environments is confirmed.     

Transitional submarine fan deposits from the late Precambrian Kongsfjord Formats submarine fan, NE Finnmark, N. Norway

ABSTRACT Three transitional submarine fan environments are recognized in the late Precambrian, 3-2 km thick Kongsfjord Formation in NE Finnmark, North Norway, namely: (1) middle to outer fan; (2) fan lateral margin, and (3) fan to upper basin-slope deposits. Middle to outer fan deposits have a high proportion of sandstones, typically showing Bouma T _bede with T _a in the thicker beds. Deposition was mainly from sheet flows with rare shallow channels. Middle to outer fan deposits are an association of sandstone packets less than 10 m thick but commonly only a few metres thick, interpreted as channels or lobes. Interchannel and fan fringe deposits occur as discrete packets of beds between the thicker bedded and coarser grained channel or lobe deposits. Fan lateral margin deposits are recognized on the basis of their stratigraphic position adjacent to inner/middle fan deposits. They are characterized by: (a) a relatively high proportion of fine-grained sandstone/siltstone turbidites compared to other major fan environments; (b) relatively small channels oriented at various angles to the regional basin slope; (c) lobes associated with channels, and (d) abundant clastic dykes and other soft-sediment deformation. Fan lateral margin deposits are distinguished from the outer fan/basin plain successions on account of the very high proportion of siltstone turbidites comparable with middle fan inter-channel deposits. Fan to upper basin-slope deposits occur at the top of the formation as an alternation of sandstone turbidites, most of which are laterally discontinuous, and very thin-bedded upper basin-slope siltstones with slide deposits.     

Crustal contaminants in the Permian Oslo Rift, South Norway: constraints from Precambrian geochemistry

During the late Paleozoic Oslo rifting event, the SW part of the Baltic Shield was penetrated by mantle-derived magmas from a depleted lithospheric or sublithospheric source. Along the way to their final emplacement, these magmas may have interacted with a heterogeneous continental crust, consisting of a mosaic of continental terranes, each with its unique composition and internal crustal history. Information on radiogenic isotope ratios and trace element distributions in the Precambrian terranes surrounding the rift can be used to define characteristic crustal components. These components may be used as endmembers in petrogenetic modelling of the Oslo Rift magmatic system. Based on available data, six endmember components can be identified, and (semi) quantitatively characterized in terms of Sr, Nd and Pb isotopes and selected trace elements. Data on the distribution of rock-types along the rift flanks allow estimates to be made of the relative importance of the components in different parts of the rift. Combining these data with petrological information may allow a realistic understanding of crust–magma interaction in the Oslo Rift magmatic system.     

Evolution of microstructures in Precambrian shear zones: An example from eastern India

Shear zones are areas of intense deformation in localized zones which can be used as natural laboratories for studying deformation characteristics. Metre to-micro scale structures that develop in response to a progressive simple shear in a shear zone are characterized by a protracted history of deformation and are immensely useful in delineating the history of progressive deformation. To decipher these localized zones of deformation and to establish the continuous non-coaxial character of deformation, detail microstructural studies are very useful. Singhbhum shear zone (SSZ), a regional Precambrian tectonic dislocation zone in eastern India, depicting a top-to-south thrust movement of the hanging wall provides a scope for studying microstructural characteristics developed in response to a progressive shear at mid-crustal level. SSZ is characterized by intense stretching lineation, isoclinal folds, shear planes, superposed schistosity and deformed quartz veins. Quasi-plastic (QP) deformation mechanisms were predominantly active in the SSZ. The overprinting relationship between the earlier and later schistosity with a consistent sense of shear indicates that earlier schistosity is transposed to later schistosity through the intermediate stages of crenulation cleavage during a progressive non-coaxial deformation. The recrystallization of quartz in mylonitic quartzite suggests protracted history of deformation. The analysis of the character of quartz grains of both the porphyroclasts and recrystallized grains suggests that strain was partitioned between the most intensely deformed central part of the shear zone and the shear-related deformation zone outside the central part of the shear zone.     

Granulite xenoliths from western Saudi Arabia: the lower crust of the late Precambrian Arbian-Nubian Shield

Mafic and intermediate granulite xenoliths, collected from Cenozoic alkali basalts, provide samples of the lower crust in western Saudi Arabia. The xenoliths are metaigneous two-pyroxene and garnet granulites. Mineral and whole rock compositions are inconsistent with origin from Red Sea rift-related basalts, and are compatible with origin from island arc calc-alkaline and low-potassium tholeiitic basalts. Most of the samples are either cumulates from mafic magmas or are restites remaining after partial melting of intermediate rocks and extraction of a felsic liquid. Initial⁸⁷Sr/⁸⁶Sr ratios are less than 0.7032, except for two samples at 0.7049. The Sm-Nd data yield T_DM model ages of 0.64 to 1.02 Ga, similar to typical Arabian-Nubian Shield upper continental crust. The isotopic data indicate that the granulites formed from mantle-derived magmas with little or no contamination by older continent crust. Calculated temperatures and pressures of last reequilibration of the xenoliths show that they are derived from the lower crust. Calculated depths of origin and calculated seismic velocities for the xenoliths are in excellent agreement with the crustal structure model of Gettings et al. (1986) based on geophysical data from western Saudi Arabia. Estimation of mean lower crustal composition, using the granulite xenoliths and the Gettings et al. (1986) crustal model, suggests a remarkably homogeneous mafic lower crust, and an andesite or basaltic andesite bulk composition for Pan-African juvenile continental crust.     

Recognition of ancient tidal inlet sequences: an example from the Upper Silurian Keyser Limestone in Virginia

The Upper Silurian Keyser Limestone is a relatively thin (< 85 m) unit of lagoonal, barrier, and shallow offshore sediments that crops out in the central Appalachians. Lithologies include massive micritic limestones to calcarenites, calcisiltites, and calcareous quartz arenites. The barrier lithofacies is preserved predominantly as tidal inlet channel-fill. Its presence is supported by two lines of evidence: (1) the sequence of sedimentary textures and structures resembles that observed in modern inlets, and (2) the sequence occupies a position immediately above a disconformity, and is accompanied by an abrupt vertical change in faunal diversity, which is interpreted as representing the transgression of open marine over back-barrier environments The inlet channel sequence comprises fine- to medium-grained, well-sorted quartz arenites that disconformably overlie sediments deposited on carbonate tidal flats (laminated, mudcracked pelmicrites). The sandstone displays a fining-upward texture, and contains a broken and abraded mixed fauna. Cross-bedding is bipolar, with major modes oriented obliquely to depositional strike. Decimetre-scale sets of planar and trough cross-beds grade upward to centimetre-scale sets of ripple cross-lamination, washed-out ripples, and plane beds. This sequence represents the change from deep to shallow channel environments, and is attributed to lateral inlet migration. The inlet sequence was preferentially preserved during marine transgression because of its relative thickness and lower stratigraphic position with respect to overlying and adjacent barrier-beach sediments. The vertical relationships of this inlet-lagoon complex emphasize that care must be taken in interpreting shallow-water transgressive sequences. Vertical ‘jumps’ in faunal diversity accompanied by scour surfaces could be misconstrued as major unconformities. Instead, such sequences may represent the shoreface erosion normally associated with the transgressive migration of barrier islands. Whether or not the faunal jump is accompanied by a barrier lithosome is greatly dependent on the geometry, frequency, and migration rate of tidal inlets.     

Preboreal deglaciation chronology and marine limits of the Lyngen-Storfjord area, Troms, North Norway

Scattered marginal moraines in the Lyngen-Storfjord area proximally to the Tromsø-Lyngen moraine were formed by the Scandinavian ice-sheet during its retreat in the Preboreal. They correspond to ice-front positions in the main fjords and fjord-valleys where between three and four major and, in places, some minor ice-front accumulations occur. These have been correlated using the marine limits related to synchronous shorelines. Dates for the shorelines and moraines have been derived from a shoreline emergence curve based on ₁₄C dated shore levels from North Norway. Two major, and probably at least one minor, climatically induced, glacial events are indicated: the Ørnes event c. 9800–9900±150 B.P., the Skibotn event 95–9600±150 B. P., and a younger event c. 9400±250 B. P. The inner fjord-valleys were probably deglaciated by c. 9100 B. P. Final deglaciation of the innerplateau during late Preboreal or early Boreal was characterized by downwasting.     

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.