Stromatolites of the lower Missoula Group (Middle Proterozoic), Belt Supergroup,Glacier National Park,Montana

The mode of formation and environmental setting of stromatolites from the lower Missoula Group (ca. 1.1·10⁹ years old) in Glacier National Park, Montana, have been determined. The stromatolite-bearing interval in the lower Missoula Group was deposited in a shallow, intermittently exposed setting of very low relief, the stromatolites forming during periods of submergence. In situ carbonate precipitation was the dominant process involved in the formation of encrusting stromatolitic laminae. This precipitate was deposited within, and probably beneath, algal mats, most likely as a result of the photosynthetic removal of carbon dioxide by the mat-building microscopic algae. Calcite also was precipitated in several types of open-space structures occurring within these stromatolites. Other laminae were produced by the organic stabilization of detrital particles; by the solely physical accumulation of terrigenous material; and probably, by bacterially induced precipitation of iron sulfide which was later oxidized to form hematite layers.Three forms of filamentous microfossils, two of which appear to be oscillatoriacean cyanophytes and the third of which is probably either a cyanophyte or filamentous bacterium, have been detected in these structures. In addition, hematitic pillar-shaped microstructures, interpreted to have been produced by filamentous bacteria, are abundant locally.In gross morphology, the lower Missoula Group stromatolites are simple, unbranched, domal structures ranging from several millimeters to several decimeters in both height and diameter. Physical conditions played a major role in determining the macrostructure of these stromatolites. Of particular importance were the shape of the positive sediment-surface irregularities upon which the stromatolites initially formed, the rate of sedimentation between stromatolite colonies, and the deposition of layers of terrigenous material on stromatolite growth surfaces. The effect of biological factors on stromatolite structure is clearly seen in those portions of stromatolites relatively free of terrigenous material; biological activity was apparently greatest on positive irregularities of the growth surface, resulting in preferential enhancement of such irregularities and development of second- and higher-order hemispheroidal structures.     

Sedimentary geology and stromatolites of the Middle Proterozoic Belt Supergroup, Glacier National Park, Montana

The Belt Supergroup is a thick, dominantly fine-grained sequence of Middle Proterozoic strata occurring in western Montana, northern Idaho, and parts of Washington state, Alberta, and British Columbia. The sequence in Glacier National Park is located along the northeastern part of present exposures of the Belt Supergroup; it is 2.9 km thick, extremely well exposed, and for the most part structurally simple. Although it was subjected to lowermost greenschist-facies metamorphism, primary sedimentary structures are exceptionally well preserved.Subtidal, intertidal, alluvial and possibly deltaic depositional environments appear to be represented in the Belt sequence in Glacier National Park. The lowermost unit, the Altyn Limestone, is not entirely exposed in the park. A partial section, 150 m thick, consists of impure dolostones deposited largely in shallow subtidal and intertidal settings. This carbonate unit is overlain by terrigenous strata of the Appekunny and Grinnell Argillites. The Appekunny Argillite is 700 m thick, consists largely of green-colored, fine-grained terrigenous material and appears to have been deposited predominantly in offshore and/or deltaic settings. The overlying Grinnell Argillite is 605 m thick and consists of red-colored terrigenous material deposited largely on an alluvial plain. The overlying Siyeh Limestone is 780 m thick and consists largely of impure dolostones and dolomitic limestones deposited in shallow subtidal and intertidal settings. Overlying the Siyeh Limestone is the 385 m thick Snowslip Formation, which consists of slightly dolomitic, predominantly fine-grained terrigenous strata deposited largely in intertidal settings. The overlying Shepard Formation is not exposed in its entirety in the central part of Glacier National Park. A 270 m thick section, which excludes the uppermost part of the formation, consists of impure dolostones and argillites, and appears to have been deposited in subtidal and intertidal settings.Stromatolites are abundant, diverse and well preserved in Glacier National Park, with mound-shaped forms and columnar forms of the group Baicalia occurring in the Altyn Limestone and Siyeh Limestone, and mound-shaped stromatolite-like structures occurring in the Snowslip and Shepard Formations. Particularly prominent is a 24–32 m thick stromatolite unit in the upper Siyeh Limestone, which contains Baicalia and Conophyton and appears to represent a prograding stromatolite reef, with Baicalia originating in a moderate-energy reef-front setting, and Conophyton originating in a lower energy back-reef setting. Individual units in these cycles can be correlated for 90 km. Many of the Conophyton in these cycles are inclined, probably as a result of gentle wave action, and the direction of inclination is relatively constant for 90 km, with the axes trending SW-SSW and plunging 30–60° SW.     

Silicification in the Belt Supergroup (Mesoproterozoic), Glacier National Park, Montana, USA

Nodular cherts can provide a window on the original sediment composition, diagenetic history and biota of their host rock because of their low susceptibility to further diagenetic alteration. The majority of Phanerozoic cherts formed by the intraformational redistribution of biogenic silica, particularly siliceous sponge spicules, radiolarian tests and diatom frustules. In the absence of a biogenic silica source, Precambrian cherts necessarily had to have had a different origin than Phanerozoic cherts. The Mesoproterozoic Belt Supergroup in Glacier National Park contains a variety of chert types, including silicified oolites and stromatolites, which have similar microtextures and paragenesis to Phanerozoic cherts, despite their different origins. Much of the silicification in the Belt Supergroup occurred after the onset of intergranular compaction, but before the main episode of dolomitization. The Belt Supergroup cherts probably had an opal-CT precursor, in the same manner as many Phanerozoic cherts. Although it is likely that Precambrian seas had higher silica concentrations than at present because of the absence of silica-secreting organisms, no evidence was observed that would suggest that high dissolved silica concentrations in the Belt sea had a significant widespread effect on silicification. The rarity of microfossils in Belt Supergroup cherts indicates that early silicification, if it occurred, was exceptional and restricted to localized environments. The similarity of microtextures in cherts of different ages is evidence that the silicification process is largely controlled by host carbonate composition and dissolved silica concentration during diagenesis, regardless of the source of silica.     

Impressions of algal mats from the Middle Proterozoic Belt Supergroup, northwestern Montana, U.S.A.

Irregular, low relief bedding-plane markings which probably represent impressions of thin, wrinkled algal mats have recently been detected in mudstones from the 1100 m.y. old Snowslip Formation, Belt Supergroup, Glacier National Park, Montana. Algal-mat impressions are rarely reported from ancient strata, yet algal mats are common in certain modern environments. Originating primarily in moist settings and in relatively quiescent shallow-water settings, such structures may be of some value in interpreting ancient depositional environments.     

Calcitic, aragonitic and mixed calcitic-aragonitic ooids from the mid-Proterozoic Belt Supergroup, Montana

A great variety of ooid types occurs within the Siyeh and Snowslip Formations of the mid-Proterozoic Belt Supergroup, Montana. Cortical layers are inferred to have been composed either of calcite in a radial-concentric or radial-with-dark-rays fabric or, aragonite in a radial or concentric fabric. The calcitic cortical layers record their original fabrics but the originally aragonitic cortical layers have been replaced by calcite in a range of textures and by quartz and dolomite. Some formerly aragonitic cortical layers are replaced by calcite spar which contains relics of the original cortical structure. Others consist of calcite spar without inclusions, or columnar calcite which grew radially from the nucleus, commonly a calcitic ooid. Some ooids were wholly composed of calcite, others were of aragonite, but two phase ooids were common, mostly consisting of an inner calcitic part and an outer aragonitic part. Probable microdolomite inclusions suggest a high Mg content of the calcitic cortical layers. The depositional environment of these oolites was probably analogous to Baffin Bay, Texas, where a similar range of ooid types is forming today.     

Facies-controlled shrinkage-crack assemblages in Middle Proterozoic mudstones from Montana, USA

Shrinkage-crack morphotypes in the Libby Formation (upper Belt Supergroup) are confined to distinct environmental facies. The lower facies is characterized by flat rip-up clasts, stromatolites, oolites, small-scale symmetrical ripples, and fenestral fabric. These rocks were deposited above fair-weather wave base on a periodically exposed mudflat. Shrinkage cracks in this facies are predominantly branching, incompletely connected features in plan view, except for local examples of completely connected polygonal cracks on purple argillite bed tops and rare, long spindle-shaped cracks on bed tops of dark grey argillite. The upper facies was deposited below fair-weather wave base and contains mainly unconnected, short spindle-shaped shrinkage cracks, and rare slightly branching cracks. Restriction of some crack types to certain facies better constrains interpretation of the origin of these shrinkage cracks. The cracks in the upper facies were strongly influenced by sediment loading, and may have formed by compaction-induced expulsion of water from pore space, resulting in synaeresis cracks. In the underlying shallower facies, polygonal cracks formed by desiccation. Elsewhere in this facies, incomplete, partially connected cracks and long spindle-shaped cracks on the same bedding plane are interpreted as having formed by desiccation. Shrinkage cracks are an under-used source of environmental information, but confusion as to their origin sometimes restricts their potential. More intensive analysis of properties of host sediment and crack fills may further our understanding of depositional and diagenetic influence on crack morphology. Crack cross-sections, which are often more commonly exposed than bedding plane cracks, may provide critical additional information on crack genesis. Better understanding of crack genesis will strengthen our ability to interpret unfossiliferous muddy sequences common in Precambrian and lacustrine settings.     

Paleomagnetism of Middle Proterozoic mafic intrusions and Upper Proterozoic (Nankoweap) red beds from the Lower Grand Canyon Supergroup, Arizona

Paleomagnetic data from lavas and dikes of the Unkar igneous suite (16 sites) and sedimentary rocks of the Nankoweap Formation (7 sites), Grand Canyon Supergroup (GCSG), Arizona, provide two primary paleomagnetic poles for Laurentia for the latest Middle Proterozoic (ca. 1090 Ma) at 32°N, 185°E (dp=6.8°, DM=9.3°) and early Late Proterozoic (ca. 850–900 Ma) at 10°S, 163°E (dp=3.5°, DM=7.0°). A new ⁴⁰Ar/³⁹Ar age determination from an Unkar dike gives an interpreted intrusion age of about 1090 Ma, similar to previously reported geochronologic data for the Cardenas Basalts and associated intrusions. The paleomagnetic data show no evidence of any younger, middle Late Proterozoic tectonothermal event such as has been revealed in previous geochronologic studies of the Unkar igneous suite. The pole position for the Unkar Group Cardenas Basalts and related intrusions is in good agreement with other ca. 1100 Ma paleomagnetic poles from the Keweenawan midcontinent rift deposits and other SW Laurentia diabase intrusions. The close agreement in age and position of the Unkar intrusion (UI) pole with poles derived from rift related rocks from elsewhere in Laurentia indicates that mafic magmatism was essentially synchronous and widespread throughout Laurentia at ca. 1100 Ma, suggesting a large-scale continental magmatic event. The pole position for the Nankoweap Formation, which plots south of the Unkar mafic rocks, is consistent with a younger age of deposition, at about 900 to 850 Ma, than had previously been proposed. Consequently, the inferred 200 Ma difference in age between the Cardenas Basalts and overlying Nankoweap Formation provides evidence for a third major unconformity within the Grand Canyon sequence.     

Geomorphic and climatic change over the past 12,900 yr at Swiftcurrent Lake, Glacier National Park, Montana, USA

Glaciated alpine landscapes are sensitive to changes in climate. Shifts in temperature and precipitation can cause significant changes to glacier size and terminus position, the production and delivery of organic mass, and in the hydrologic energy related to the transport of water and sediment through proglacial environments. A sediment core representing a 12,900-yr record collected from Swiftcurrent Lake, located on the eastern side of Glacier National Park, Montana, was analyzed to assess variability in Holocene and latest Pleistocene environment. The spectral signature of total organic carbon content (%TOC) since ~ 7.6 ka matches that of solar forcing over 70-500 yr timescales. Periodic inputs of dolomite to the lake reflect an increased footprint of Grinnell Glacier, and occur during periods when sediment sinks are reduced, glacial erosion is increased, and hydrologic energy is increased. Grain size, carbon/nitrogen (C/N) ratios, and %TOC broadly define the termination of the Younger Dryas chronozone at Swiftcurrent Lake, as well as major Holocene climate transitions. Variability in core parameters is linked to other records of temperature and aridity in the northern Rocky Mountains over the late Pleistocene and Holocene.     

Tectonic Setting of the Late Proterozoic Lavalleja Group (Dom Feliciano Belt), Uruguay

The Lavalleja Group exposed along the Dom Feliciano orogenic belt is located in the southeast of Uruguay. This group consists of volcano-sedimentary rocks, developed during the Neoproterozoic Brasiliano cycle. The geochemical signature of the igneous rocks of the Lavalleja Group, mainly metagabbros and basic and acidic metavolcanic rocks, indicates a back-arc basin tectonic setting. The metamorphic grade increases to the southeast, from very low grade, lower green-schist facies, in the Minas Formation, to a medium grade, amphibolite facies, in Fuente del Puma and Zanja del Tigre Formations. The metamorphic mineral assemblages correspond to a low-pressure regional metamorphism associated with a high thermal gradient. A compressive deformational event that probably corresponds to the closure of the Lavalleja basin during a continental collision, was recognized. The petrology, geochemistry, metamorphic grade, and tectonic setting are consistent with a back-arc basin setting for the Lavalleja Group.     

Syndepositional volcanism in the Rietgat and arenaceous Bothaville Formations,Ventersdorp Supergroup (late Archaean—early Proterozoic), in South Africa

The volcanic-sedimentary succession of the Ventersdorp Supergroup which is virtually undisturbed tectonically and of low-grade (greenschist facies) metamorphism, affords a unique opportunity for studying the interplay between volcanic and sedimentary processes. The transitional sequence between the Rietgat and Bothaville Formations consists of a number of lithofacies. These are a basal breccia representing pyroclastic and laharic deposits, an overlying breccia—arenite—conglomerate (BAC) which formed by debris flow and fluvial processes, an arenite deposited offshore during a transgression, and an upper conglomerate laid down on a beach. In the volcaniclastic BAC and arenite lithofacies the presence of thin tuff beds, deformed acid lava fragments (bombs?) and glass shards in the arenaceous matrix suggest syndepositional volcanism.Sedimentation took place along the flanks of an asymmetrical, actively volcanic, domal structure which consisted partly of unstable pyroclastic deposits in the east. Resedimentation of the pyroclastic debris by subaerial debris flows and braided streams built a volcaniclastic fan lobe at the foot of the domal structure. As volcanic activity subsided, sands derived from a granitic terrain, mixed with minor air-fall debris to subsequently cover the fan lobe during a regional transgression.     

Large-scale moraine deformation at the Athabasca Glacier,Jasper National Park,Alberta, Canada

In this paper the development of a large-scale gravitational deformation involving the eastern lateral moraine of the Athabasca Glacier in Jasper National Park, Alberta, Canada, is described. Interpretation and analysis of sequential aerial photographs indicates that a 540-m-wide segment of the eastern lateral moraine began to deform in the early 1950s; however, significant movement only began in the late 1960s. Since then, the moraine has undergone progressive gravitational deformation leading to a network of fractures, bulging, and the development of a large gap in the moraine crest. Geographic information system analysis of topographic changes between 1967 and 2006 indicates that the displaced volume of the moraine is approximately 9.0 × 10⁵ m³. In the last 39 years, the moraine crest has displaced 55 m (1.4 m yr⁻¹) down towards the glacier. The development of slope instability is linked to a combination of debuttressing from recent glacier recession, deformation of the moraine, as well as the movement of a large, mobile, debris-mantled slope impinging the upslope margin of the lateral moraine. This case study illustrates the importance of glacial conditioning and local geomorphological factors in creating conditions for large-scale moraine instability in recently deglacierized alpine basins.     

Possible indicators of microbial mat deposits in shales and sandstones: examples from the Mid-Proterozoic Belt Supergroup, Montana, U.S.A.

It has been suspected for some time that microbial mats probably colonized sediment surfaces in many terrigenous clastic sedimentary environments during the Proterozoic. However, domination of mat morphology by depositional processes, post-depositional compaction, and poor potential for cellular preservation of mat-building organisms make their positive identification a formidable challenge. Within terrigenous clastics of the Mid-Proterozoic Belt Supergroup, a variety of sedimentary structures and textural features have been observed that can be interpreted as the result of microbial colonization of sediment surfaces. Among these are: (a) domal buildups resembling stromatolites in carbonates; (b) cohesive behaviour of laminae during soft-sediment deformation, erosion, and transport; (c) wavy–crinkly character of laminae; (d) bed surfaces with pustular–wrinkled appearance; (e) rippled patches on otherwise smooth surfaces; (f) laminae with mica enrichment and/or randomly oriented micas; (g) irregular, curved–wrinkled impressions on bedding planes; (h) uparched laminae near mud-cracks resembling growth ridges of polygonal stromatolites; and (i) lamina-specific distribution of certain early diagenetic minerals (dolomite, ferroan carbonates, pyrite). Although in none of the described examples can it irrefutably be proven that they are microbial mat deposits, the observed features are consistent with such an interpretation and should be considered indicators of possible microbial mat presence in other Proterozoic sequences.     

Stratigraphy, sedimentology and bulk organic geochemistry of black shales from the Proterozoic Vindhyan Supergroup (central India)

Four organic-rich shale units of the Proterozoic Vindhyan sedimentary succession have been scanned to reveal their origin and hydrocarbon potential. The wavy-crinkly nature of the carbonaceous laminae is suggestive of a microbial mat origin of the shales. These shales are thus different from Phanerozoic black shales which typically exhibit planar laminae. The hydrocarbon potential of the black shale units has been evaluated by Rock-Eval pyrolysis. Total organic carbon content of many of the shales exceeds 1%. The meanT _max for the black shales translate to a vitrinite reflectance range of 2.05-2.40% Rm based on standard conversion techniques. These shales have reached the catagenetic stage near the beginning of anthracite formation.     

Hazard assessment of the Tidal Inlet landslide and potential subsequent tsunami, Glacier Bay National Park, Alaska

An unstable rock slump, estimated at 5 to 10 × 10⁶ m³, lies perched above the northern shore of Tidal Inlet in Glacier Bay National Park, Alaska. This landslide mass has the potential to rapidly move into Tidal Inlet and generate large, long-period-impulse tsunami waves. Field and photographic examination revealed that the landslide moved between 1892 and 1919 after the retreat of the Little Ice Age glaciers from Tidal Inlet in 1890. Global positioning system measurements over a 2-year period show that the perched mass is presently moving at 3–4 cm annually indicating the landslide remains unstable. Numerical simulations of landslide-generated waves suggest that in the western arm of Glacier Bay, wave amplitudes would be greatest near the mouth of Tidal Inlet and slightly decrease with water depth according to Green’s law. As a function of time, wave amplitude would be greatest within approximately 40 min of the landslide entering water, with significant wave activity continuing for potentially several hours.     

Hydrocarbon source potential of the Proterozoic Sirban Limestone Formation,NW Himalaya,Jammu

The Proterozoic Sirban Limestone Formation (SLFm) crops out as detached allochthons in the northwest Himalaya (Jammu region, India) and has its coeval equivalents laterally disposed in the west in Salt Range, in the northwest in Abbotabad (Pakistan) and in southeast in Himachal Pradesh (India). The oil and gas occurrences have been reported from the Proterozoic successions globally and the hydrocarbon potential of the SLFm cannot be ruled out.The interbedded shales and algal laminated dolostones within the SLFm have yielded microflora comparable to those reported in the North African Neoproterozoic sandstones and the Late Proterozoic carbonates of the giant oil and gas fields of the Siberian Platform. The SLFm contains a rich and diverse biota comprising ~ 10% of the rock volume in thin section. The rich organic assemblage justified a hydrocarbon source potential analysis of the SLFm, tested in this study by Rock Eval (RE) pyrolysis.RE pyrolysis yielded a total organic carbon (TOC) content of 0.02 to 1 wt. % with very low Hydrogen Index (HI) values for the shales and TOC content averaging 0.02 wt. % for the dolostones. The organically lean shales and dolostones exhibit T_max values indicative of immature to post mature stage. But, since these values are for the samples with complex thermal and tectonic history the results may be unreliable. The highly altered organic matter and kerogen present in the SLFm had the potential to generate hydrocarbons and presently indicates no significant source potential. This study is important for understanding the hydrocarbon occurrences in the SLFm particularly in light of the recent oil and gas discoveries from the coeval Proterozoic successions.     

Contourites associated with pelagic mudrocks and distal delta-fed turbidites in the Lower Proterozoic Timeball Hill Formation epeiric basin (Transvaal Supergroup), South Africa

Five genetic facies associations/architectural elements are recognised for the epeiric sea deposits preserved in the Early Proterozoic Timeball Hill Formation, South Africa. Basal carbonaceous mudrocks, interpreted as anoxic suspension deposits, grade up into sheet-like, laminated, graded mudrocks and succeeding sheets of laminated and cross-laminated siltstones and fine-grained sandstones. The latter two architectural elements are compatible with the Te, Td and Tc subdivisions of low-density turbidity current systems. Thin interbeds of stromatolitic carbonate within these first three facies associations support photic water depths up to about 100 m. Laterally extensive sheets of mature, cross-bedded sandstone disconformably overlie the turbidite deposits, and are ascribed to lower tidal flat processes. Interbedded lenticular, immature sandstones and mudrocks comprise the fifth architectural element, and are interpreted as medial to upper tidal flat sediments. Small lenses of coarse siltstone–very fine-grained sandstone, analogous to modern continental rise contourite deposits, occur within the suspension and distal turbidite sediments, and also form local wedges of inferred contourites at the transition from suspension to lowermost turbidite deposits. Blanketing and progressive shallowing of the floor of the Timeball Hill basin by basal suspension deposits greatly reduced wave action, thereby promoting preservation of low-density turbidity current deposits across the basin under stillstand or highstand conditions. A lowstand tidal flat facies tract laid down widespread sandy deposits of the medial Klapperkop Member within the formation. Salinity gradients and contemporaneous cold periglacial water masses were probably responsible for formation of the inferred contourites. The combination of the depositional systems interpreted for the Timeball Hill Formation may provide a provisional model for Early Proterozoic epeiric basin settings.     

A Duplex Model for the Eastern Cape Fold Belt? Evidence from the Palaeozoic Witteberg and Bokkeveld Groups (Cape Supergroup), Near Steytlerville, South Africa

The eastern part of the Cape Fold Belt, near Steytlerville, South Africa, reveals a typical pattern of numerous, north-verging thrust faults and associated folds, interpreted as part of a large duplex structure that formed along the southern margin of Gondwana during the Late Palaeozoic. Steeply-dipping fore- and backthrusts occur in the Bokkeveld Group (middle Cape Supergroup), where strata are composed of predominantly argillaceous rocks, whereas in the more arenaceous Witteberg Group (upper Cape Supergroup) there are fewer recognizable and less closely-spaced thrusts. Open style folds characterize areas in which the Bokkeveld Group crops out, but in areas of Witteberg outcrop, folds, especially those adjacent to thrusts, are often overturned.In spite of a general absence of marker horizons, a displacement of at least 500 metres can be inferred for one prominent thrust, the Jackalsbos thrust. This fault, the northernmost in the area investigated, is probably the sole thrust in the duplex structure, linked through southward-dipping imbricates to a projected roof thrust (the Baviaanskloof thrust) cropping out immediately south of the study area.Displacements on imbricates within the duplex are difficult if not impossible to measure, but the net effect is certainly accumulative and incremental. Truncation by a roof thrust and subsequent erosional processes may explain why so few of the many thrusts so far identified in the eastern part of the fold belt can be successfully mapped, and their displacements measured. Normal and strike-slip faults, less common than thrust faults, formed during extensional tectonism related to the breakup of Gondwana, during the Mesozoic.     

Comparative study of two relatives,MISS and Stromatolites: example from the Proterozoic Kunihar Formation,Simla Group,Lesser Himalaya

Comparison of microbially induced sedimentary structures (MISS) and stromatolitic bearing horizons from the Proterozoic Kunihar Formation, Simla Group, Lesser Himalaya, has been scrutinised to understand the formative processes and controls on MISS and stromatolites in the context of sedimentary facies and response to sea level fluctuations. MISS structures recorded are wrinkle structures, Kinneyia ripples, load casts, domal structures, sand chips, palimpsest and patchy ripples with limited desiccation cracks. Stromatolitic morphotypes recorded are solitary, branching, wavy and domal forms of stromatolites associated with ooids, peloids and fenestral laminae. MISS structures flourished within tidal flats to shallow intertidal while stromatolites mushroomed in environments ranging from tidal to deep subtidal. MISS structures were favoured by resistant substratum, low energy conditions, consistent water supply and low terrigenous input. Stromatolites boomed when the volume of carbonate accumulation exceeded siliciclastic deposition. Fluctuating environmental conditions and sediment budget controlled morphology of stromatolites. Owing to limited siliciclastic input during deposition of dolomudstones (characterizes transgressive systems tract), microbial growth was enhanced. Calcareous shales were deposited over dolomudstones which marks the maximum flooding surface (MFS) indicating the culmination of transgression. Deposition of storm-dominated sandstone-siltstone (FA1), wave-rippled sandstones (FA2), tide-dominated sandstones (FA3), heteroliths (FA4), wackestone-packestone (FA6), boundstone (FA7) and ooid-peloid grainstone (FA8) on top of the MFS reflects initiation of highstand systems tract (HST) which is mainly characterized by stromatolitic horizons, alternation of carbonates and siliciclastics with flourishing microbial activity. Eventually, increased sedimentation in upper part of Kunihar Formation marks late stage of regression.     

Tree-ring dates on two pre-Little Ice Age advances in Glacier Bay National Park and Preserve, Alaska, USA

Two interstadial tree ring-width chronologies from Geikie Inlet, Glacier Bay Southeast, Alaska were built from 40 logs. One of these chronologies has been calendar dated to AD 224–999 (775 yr) crossdating with a living ring-width chronology from Prince William Sound, Alaska. Trees in this chronology were likely killed through inundation by sediments and meltwater from the advancing Geikie Glacier and its tributaries ca. AD 850. The earlier tree-ring chronology spans 545 yr and is a floating ring-width series tied to radiocarbon ages of about 3000 cal yr BP. This tree-ring work indicates two intervals of glacial expansion by the Geikie Glacier system toward the main trunk glacier in Glacier Bay between 3400 and 3000 cal yr BP and again about AD 850. The timing of both expansions is consistent with patterns of ice advance at tidewater glaciers in other parts of Alaska and British Columbia about the same time, and with a relative sea-level history from just outside Glacier Bay in Icy Strait. This emerging tree-ring dated history builds on previous radiocarbon-based glacial histories and is the first study to use tree-ring dating to assign calendar dates to glacial activity for Glacier Bay.