Bedrock Sill Research Articles

Rapid barrier estuary infill in a geologically-constrained setting: Aireys Inlet/Painkalac Creek, Victoria, Australia

The Holocene infill of an intermittently open-closed barrier estuary (Painkalac Creek/Aireys Inlet) was examined on the southern margins of Australia, through aerial LiDAR, coring, sedimentology, radiocarbon dating and pollen analysis. It is found that almost all the accommodation space within the estuary has been occupied, with a thick sequence (>9 m) of sediments being deposited rapidly soon after flooding by the sea. The rate of sedimentation was 4.8 mm/year which closely matches the rate of sea level rise (4.5 mm/year) during the 1200 year period that the estuary infilled, from around 8000–6800 years ago. The evidence of estuarine deposition above the elevation of the modern intertidal zone provides significant further evidence for the presence of the mid-Holocene highstand sea level in this relatively unstudied part of the Australian coast. The pattern of infill departs from the traditional barrier evolution models. The first phase of infill was likely tide dominated with subtidal sand shoals being found landward of a subtidal bedrock sill. Once a beach-barrier sequence formed, the estuary infilled in a more traditional manner, with lateral progradation of a flood tide delta and central mud basin infill. The barrier estuary of Painkalac Creek therefore has undergone a complex evolutionary history, characterised by a change in process dominance from classic tidal to wave-dominated form. Infill occurred in a keep-up (or fast catch-up) mode, with geological control occurring in regard to the presence of an intertidal bedrock sill at the mouth of the estuary.

Chronology and stratigraphy of the Imlay Channel in Lapeer County, Michigan, USA

The Imlay Channel in Lapeer County, Michigan, was one of two outlets for the glacial Lake Maumee phase of ancestral Lake Erie. Fifteen new radiocarbon and optical ages from within and adjacent to the Imlay Channel constrain sedimentation rates within the channel and the timing of regional deglaciation. For nearly 50 years, the deglaciation of this region of Michigan has been based on a single age from the Weaver Drain site located near the Imlay Channel, and a new radiocarbon age of 16.7–17.0 cal ka BP from 3 km east of the Imlay Channel supports this long-standing deglacial age. On average, there is a 14 m thick sediment fill within the channel. Radiocarbon and optically stimulated luminescence ages reveal that much of the alluvial fill was deposited by 14.9 ka, and alluvial fans building into the channel stabilized in the early Holocene. Cross-sections along and perpendicular to the Imlay Channel, built from geotechnical borings and water-well records, reveal a current-day bedrock sill elevation at 235 m a.s.l. that would have permitted drainage of all stages of glacial Lake Maumee in the past.

From marginal to axial tidal-strait facies in the Early Pleistocene Siderno Strait

This geological guide presents the description of locations associated with a two-day field trip arranged in relation to the 10th International Congress of Tidal Sedimentology (Tidalites), Matera, Italy. The field guide describes sedimentological features of the largest among a series of tectonically controlled tidal straits that dissected the Calabrian Arc in southern Italy during the Early Pleistocene. The WNW-ESE trending, 50x20 km- wide Siderno Strait connected the Tyrrhenian with the Ionian seas. Due to tidal phase opposition between the two basins, continuous water-mass exchanges occurred through the strait, leading to powerful, bi-directionally flowing tidal currents. Sediments filling the Siderno Strait derived from both fluvial supply from the margins and intra-basinal autochthonous carbonate-factory debris. The main objective of the two-day field trip is to guide the visitor through a cross-section of the ancient strait, starting from one of the margins, ending in the deeper axial zone. The focus during the day one is on strait-margin deltaic fluvial-dominated deposits, shed from the tectonically-controlled, northern border and reworked by tidal currents in their distal reaches (delta front). Erosively-based, 4–5 m-thick pebbly-sandstone strata intercalated with 2–3 m-thick tidally-generated cross strata stack into a ca. 170 m-thick succession, exposed in a series of outcrops progressively located down-current with respect to the inferred entry point to the north. The focus of the day two is a ca. 150–190 m-thick succession consisting of cross-stratified mixed (bioclastic-siliciclastic) deposits, forming a series of WNE-ESE-oriented, elongated ridges that accumulated in the south-eastern axial zone of the Siderno Strait. The selected stops offer panoramic views of exceptionally continuous sections and close-up observations, revealing different scales of depositional architectures and a variety of sedimentary structures and trace fossils that record the development of these tidal sand ridges during the strait lifespan. The interplay between the tectonic uplift of a central bedrock sill and a number of syn-sedimentary faults and high-frequency relative sea- level changes (induced by glacio-eustacy and active tectonics) can be deciphered from the architecture of the tidally-generated cross strata composing the main body of the ridges.

Drainage integration of the Salt and Verde rivers in a Basin and Range extensional landscape, central Arizona, USA

The Salt River and Verde River watersheds provide downstream metropolitan Phoenix, Arizona, USA with much of its water supply, and this paper explains how these rivers integrated in an extensional tectonic setting. Near the end of the Pliocene, segments of the proto-Salt and proto-Verde watersheds of central Arizona consisted of local drainage networks supplying water and sediment into internally drained basins, including depressions occupied by late Pliocene natural lakes occupying the Verde Valley and Tonto basins. A key location, the lower Verde River valley (LVRV), is where the modern-day drainages of the Salt and Verde now meet downstream of these Pliocene lakes. At the time of the Nomlaki tuff deposition ~3.3 Ma, a condition of sediment overfill existed in the LVRV, although there was no exoreic drainage and a playa was still maintained. A fanglomerate unit, named here the Rolls formation, spilled over a bedrock sill and an alluvial-fan ramp transported sediment into the Higley Basin that underlies the eastern part of metropolitan Phoenix. Lithologies of preserved remnants of this Pliocene alluvial-fan system match well cuttings of buried sediment in the Higley Basin with a cosmogenic burial isochron age of 3.90 ± 0.70 Ma. Based on cosmogenic burial isochron ages, ancestral Salt River gravels started depositing on top of this fan ramp between 2.8 and 2.2 Ma. Deposition of Salt and Verde river gravels in the Higley Basin continued for ~2 million years and eventually led to an aggradational piracy event that overtopped a bedrock ridge immediately east of Phoenix's Sky Harbor Airport. A cosmogenic burial age of 460 ± 23 ka is a rough maximum age for this river avulsion that relocated the Salt River into the Luke Basin that underlies western metropolitan Phoenix. Available chronometric data are not precise enough to determine whether the Salt River or Gila River integrated first. All of the exoreic rivers of western North America's Basin and Range Province — the lower Colorado, Gila, Rio Grande, Salt, and Verde rivers — employed lake overflow to integrate across half-graben, graben, rift and supradetachment tectonic settings.

A 50,000-year record of lake-level variations and overflow from Owens Lake, eastern California, USA

A continuous lake-level curve was constructed for Owens Lake, eastern California by integrating lake-core data and shoreline geomorphology with new wind-wave and sediment entrainment modeling of lake-core sedimentology. This effort enabled refinement of the overflow history and development of a better understanding of the effects of regional and global climate variability on lake levels of the paleo-Owens River system during the last 50,000 years. The elevations of stratigraphic sites, plus lake bottom and spillway positions were corrected for vertical tectonic deformation using a differential fault-block model to estimate the absolute hydrologic change of the watershed-lake system. New results include 14C dating of mollusk shells in shoreline deposits, plus post-IR-IRSL dating of a suite of five beach ridges and OSL dating of spillway alluvial and deltaic deposits in deep boreholes. Geotechnical data show the overflow area is an entrenched channel that had erodible sills composed of unconsolidated fluvial-deltaic and alluvial sediment at elevations of ∼1113–1165 m above mean sea level. Owens Lake spilled most of the time at or near minimum sill levels, controlled by a bedrock sill at ∼1113 m. Nine major transgressions at ∼40.0, 38.7, 23.3, 19.3, 15.6, 13.8, 12.8, 11.6, and 10.6 ka reached levels ∼10–45 m above the bedrock sill. Several major regressions at or below the bedrock sill from 36.9 to 28.5 ka, and at ∼17.8, 12.9, and 10.4–8.8 ka indicate little to no overflow during these times. The latest period of overflow occurred ∼10–20 m above the bedrock sill from ∼8.4 to 6.4 ka that was followed by closed basin conditions after ∼6.4 ka. Previous lake core age-depth models were revised by accounting for sediment compaction and using no reservoir correction for open basin conditions, thereby reducing discrepancies between Owens Lake shoreline and lake-core proxy records. The integrated analysis provides a continuous 50 ka lake-level record of hydroclimate variability along the south-central Sierra Nevada that is consistent with other shoreline and speleothem records in the southwestern U.S.

A Younger Dryas moraine ridge and fjord delta in Valldal, Norddalsfjorden, Møre og Romsdal, Norway

The fjords and valleys of Norway have been shaped during several glacial cycles of the Quaternary period. Subaquatic ridges that formed at or close to the grounding lines of glacier termini during stillstands or readvances within the last deglaciation are commonly found within the fjords. An example is a prominent moraine ridge representing a readvance of the Fennoscandian Ice Sheet during the Younger Dryas ( c. 12.9–11.5 ka BP) in Norddalsfjorden, More og Romsdal, Norway. Fjord deltas may form in the same setting at the mouths of tributary valleys or at fjord heads following deglaciation. They may grow to relatively large dimensions and have morphological features that reflect the dynamic environment that exists across the fluvial–marine transition. One such delta is located close to, and partly drapes, the proximal slope of the moraine ridge at Norddalsfjorden. Norddalsfjorden is a narrow (0.8–3.5 km wide) and relatively deep (up to about 500 m deep) fjord situated between high mountain peaks and intermittent valleys (Fig. 1). A transverse, subaquatic ridge is situated on a bedrock sill just SW of the Valldal tributary valley. High-resolution multibeam data clearly depict the ridge as a well-defined topographic high with an arcuate crest that is convex down-fjord (Fig. 1c). The crest of the ridge is c. …

A Sault-outlet-referenced mid- to late-Holocene paleohydrograph for Lake Superior constructed from strandplains of beach ridges

The most detailed Lake Superior paleohydrograph relative to the current outlet near Sault Ste. Marie, Ontario–Michigan, was constructed from four strandplains of beach ridges. This provides a history of water-level, glacial isostatic adjustment (GIA), and the active outlet prior to monitoring and regulation. Four relative paleohydrographs that are offset and subparallel owing to differences in GIA were produced from 321 basal foreshore elevations and 56 optically stimulated luminescence ages. Subtracting modeled elevations in defined millennial lake phases between relative paleohydrographs and similarity between an inferred Sault Ste. Marie (hereinafter, Sault) paleohydrograph and data near the zero isobase corroborates rates of GIA derived from water-level gauges. A change in trend in the Sault paleohydrograph is related to the final separation of Lake Superior from Lakes Michigan and Huron and is the youngest age reported at 1060 ± 100 years. A near-horizontal trend in the Sault paleohydrograph for the past millennium has an intercept that is close to the historical average for Lake Superior. A consistently linear trend from about 2 to 1 ka suggests a relatively stable outlet similar to the past millennium, but a decreasing trend from 3 to 1 ka suggests an outlet other than the Sault. Although intercept data beyond the last millennium are similar in elevation to the reported bedrock sill near Chicago (Hansel et al. 1985), we argue that the Port–Huron outlet was the active outlet during this time and the inferred paleohydrograph of Baedke and Thompson (2000) requires reevaluation.

Jökulhlaups and sediment transport in Watson River, Kangerlussuaq, West Greenland

For 3 years, during a 4-year observation period (2007–2010), jökulhlaups were observed from a lake at the northern margin of Russells Gletscher. At a gauging station located on a bedrock sill near the outlet of Watson River into Sdr Strømfjord, discharge and sediment transport was monitored during the jökulhlaups. The stage rose up to 5.3 m and a maximum discharge of 1,430 m3 s−1 was recorded. The jökulhlaups were very different, indicating varying influences of weather and englacial drainage conditions. Although the jökulhlaups caused high discharge and sediment transport rates, their share of the annual discharge and sediment transport were less than 2%.

Thunder Bay, Ontario, was not a pathway for catastrophic floods from Glacial Lake Agassiz

A series of airgun single-channel seismic-reflection profiles from Thunder Bay and the adjacent Isle Royal Trough of Lake Superior suggest that water from Glacial Lake Agassiz did not catastrophically drain into Lake Superior at this locality, as has been previously suggested. Thunder Bay is underlain by a relatively smooth bedrock surface, the double bedrock sill at its mouth is not significantly incised, and no subaqueous fans occur at the base of the escarpment into the Superior basin. The Thunder Bay area contains none of the features observed further north in Lake Superior, which have been interpreted as flood-related from earlier seismic surveys. These previously reported northern features are interpreted as resulting from a younger (post-Marquette) catastrophic release of Lake Agassiz water. The absence of such features in the Thunder Bay area suggests that Lake Agassiz did not drain catastrophically through the bay at the beginning of the Younger Dryas cold interval.

Protist diversity in a permanently ice-covered Antarctic Lake during the polar night transition

The McMurdo Dry Valleys of Antarctica harbor numerous permanently ice-covered lakes, which provide a year-round oasis for microbial life. Microbial eukaryotes in these lakes occupy a variety of trophic levels within the simple aquatic food web ranging from primary producers to tertiary predators. Here, we report the first molecular study to describe the vertical distribution of the eukaryotic community residing in the photic zone of the east lobe (ELB) and west lobe (WLB) of the chemically stratified Lake Bonney. The 18S ribosomal RNA (rRNA) libraries revealed vertically stratified populations dominated by photosynthetic protists, with a cryptophyte dominating shallow populations (ELB-6 m; WLB-10 m), a haptophyte occupying mid-depths (both lobes 13 m) and chlorophytes residing in the deepest layers (ELB-18 and 20 m; WLB-15 and 20 m) of the photic zone. A previously undetected stramenopile occurred throughout the water column of both lobes. Temporal variation in the eukaryotic populations was examined during the transition from Antarctic summer (24-h sunlight) to polar night (complete dark). Protist diversity was similar between the two lobes of Lake Bonney due to exchange between the photic zones of the two basins via a narrow bedrock sill. However, vertical and temporal variation in protist distribution occurred, indicating the influence of the unique water chemistry on the biology of the two dry valley watersheds.

Tree-ring dating of the nineteenth-century advance of Brady Glacier and the evolution of two ice-marginal lakes, Alaska

We utilized dendrochronology and precise elevation-constrained mapping to date glacially overridden and drowned trees at the margin of Brady Glacier in southeast Alaska. This technique allowed determination of the timing of the former tidewater glacier’s last advance and consequent formation and filling of two marginal lakes. The subfossil tree-ring chronology spans the interval from AD 1370 to 1861. Brady Glacier impounded Spur Lake to an elevation of 83 m a.s.l. around 1830 and 121 m a.s.l. around 1839. Soon after, Spur Lake reached 125 m a.s.l. and began to overflow a stable bedrock sill. The glacier continued to advance, thickening by at least 77 m between c. 1844 and 1859 at a site down-glacier of Spur Lake on the opposite glacier margin. Farther down-glacier, North Trick Lake began to form by 1861 and reached its highest elevation at approximately 130 m a.s.l. when Brady Glacier reached its maximum extent around 1880. Our findings add precision to the chronology of the last advance of Brady Glacier and provide insight into the evolution of glacier-dammed lakes and calving glaciers.

An 8,900-year-old forest drowned by Lake Superior: hydrological and paleoecological implications

Exposures along the lower Kaministiquia River (near Thunder Bay, Ontario, Canada) provide insight into early Holocene lake level fluctuations and paleoenvironmental conditions in the northwestern Lake Superior basin. These exposures show at least two large paleochannels which were downcut into offshore sediments, and were later filled with >2 m of sand, ~3 m of rhythmically laminated silt and clay, and ~6 m of interbedded silt and sand. Buried by the rhythmically laminated silty clay unit is a well-preserved organic deposit with abundant plant macrofossils from terrestrial and emergent taxa, including several upright tree trunks. Three AMS radiocarbon ages were obtained on wood and conifer cones from this deposit: 8,135 ± 25 (9,130–9,010 cal), 8,010 ± 25 (9,010–8,780 cal), and 7,990 ± 20 (8,990–8,770 cal) BP. This sequence records an early postglacial high-water phase, followed by the Houghton lowstand, and reflooding of the lower Kaministiquia River Valley. The drop in lake level associated with the Houghton phase forced the ancestral Kaministiquia River to downcut. By ~9,100 cal (~8,100) BP, older channels eroded into subaqueous underflow fan deposits in the Thunder Bay area near Fort William Historical Park (FWHP) were abandoned and colonized by a Picea-Abies-Larix forest. Based on stratigraphic data corrected for differential isostatic rebound, the lake was below the Sault Ste. Marie bedrock sill between at least 9,100 cal (8,100) and 8,900 cal (8,000) BP. Shortly after 8,900 cal BP, the lake quickly rose and buried in situ lowland vegetation at FWHP with varved sediments. We argue that this transgression was due to overflow from glacial Lakes Agassiz or Ojibway associated with the retreat of the Laurentide Ice Sheet from the Nakina moraine and/or the Cochrane surge margins in the Hudson Bay Lowlands. A continued rise in lake level after 6,420 ± 20 (7,400 cal) BP at FWHP may record uplift of the North Bay outlet above the Sault Ste. Marie bedrock sill and the onset of the Nipissing transgression in the Lake Superior basin.

Late Pleistocene–Holocene evolution of the northern shelf of the Sea of Marmara

Chirp sub-bottom profiling, multibeam bathymetric mapping and a combination of faunal and isotopic analysis of molluscs and foraminifera in sediment cores on the northern shelf of the Sea of Marmara (SoM) provide evidence of sea-level excursions, water exchanges between the adjacent Mediterranean and Black Seas, and oscillating salinity over the last 160 ka bp. During the marine isotope stages MIS-2, MIS-3, MIS-4 and MIS-6 the SoM disconnected from the Mediterranean Sea and evolved into a lake. During MIS-1, MIS-5 and MIS-7, the SoM reconnected and became salty once again. Sapropels formed shortly after each invasion of Mediterranean saltwater observed in our cores. Concurrent suboxic–dysoxic conditions prevailed over quite shallow substrates on the shelf. Ancient shorelines are pervasive at − 85 m on the northern shelf and in the region of Prince Islands coincident with the elevation of the modern bedrock sill in the Çanakkale (Dardanelles) Strait. At times when global (eustatic) sea level dropped below this sill, the surface of the SoM stabilized at its outlet and freshened. Thus this particular shoreline is interpreted as the edge of the most recent SoM lake that existed from about 75 ka bp to 12 ka bp. The freshening is observed in very light (− 6‰) values of δ 18O measured on freshwater molluscs and the complete absence of foraminifera. Two brief lacustrine episodes during MIS-5 suggest that the level of the Çanakkale outlet might have been as shallow as − 50 m in the past, a likelihood supported by submerged terraces along its margins bounding the modern central channel and the presence of an euryhaline biofacies in Unit L4.1 corresponding to MIS-5b. δ 18O profiles and carbon-14 dating show that salinification of the SoM and the blossoming of bioherms evolved rapidly after the latest connection with the Mediterranean at 12 ka bp. However, freshening proceeded more slowly once the connection was severed.

Postglacial evolution of a Pacific coastal fjord in British Columbia, Canada: interactions of sea-level change, crustal response, and environmental fluctuations — results from MONA core MD02-2494This article is one of a series of papers published in this Special Issue on the theme Polar Climate Stability Network.

The sedimentary record in a 40.9 m giant (Calypso) piston core (MD02-2494) raised from the inner basin within Effingham Inlet, British Columbia, Canada, during the 2002 Marges Ouest Nord Américaines (MONA) campaign, spans from 14 360 14C years BP (17 300 calibrated calendar (cal.) years BP) to about nine centuries before present. The core archives changes in sedimentation and sea level immediately following deglaciation of the Late Wisconsin Fraser Glaciation, which peaked about 15 000 14C years BP. The presence of the Mazama Ash in the core anchors a detailed chronology based on 49 radiocarbon dates and seven Pleistocene paleomagnetic secular variation correlations. Diatom assemblages identify a marine–freshwater–marine transition in the basin, which occurred 11 630 14C years BP (13 500 cal. years BP). At this time, a bedrock sill, presently at 46 m depth, was briefly exposed as sea level fell and then rose again during isostatic crustal adjustments. These data constrain a new sea-level curve for the outer coast of Vancouver Island covering the past 12 000 14C years BP (14 000 cal. years BP), providing new information on the nature of deglaciation along the west coast of Canada and informing interpretations of regional paleoceanographic records and mantle viscosity models.

Modeling ocean processes below Fimbulisen, Antarctica

Model simulations of circulation and melting beneath Fimbulisen, Antarctica, obtained using an isopycnic coordinate ocean model, are presented. Model results compare well with available observations of currents and hydrography in the open ocean to the north of Fimbulisen and suggest that Warm Deep Water exists above the level of a sub‐ice‐shelf bedrock sill, the principal pathway for warm waters to enter the sub‐ice‐shelf cavity. The model shows a southward inflow of Warm Deep Water over this sill and into the cavity, producing a mean cavity temperature close to −1.0°C. This leads to high levels of basal melting (>10 m/a) at the grounding line of Jutulstraumen and an average melting over the ice shelf base close to 1.9 m/a. The southward inflow is a compensating flow caused by the northward outflow of fresh, cold water produced by the basal melting. Results on inflow and melting are difficult to validate since no in situ measurements yet exist in the cavity. If such high melt rates are realistic, the mass balance of Fimbulisen must be significantly negative, and the ice shelves along Dronning Maud Land must contribute about 4.4 mSv of melt water to the Weddell Sea, about 15% of the total Antarctic meltwater input to the Southern Ocean.

Sea-level changes and depositional environments in the İzmit Gulf, eastern Marmara Sea, during the late glacial–Holocene period

Offshore and onshore stratigraphic studies, together with high-resolution shallow seismic reflection profiling and multibeam bathymetric mapping, were carried out in the western and central part of the İzmit Gulf. These studies indicate that the İzmit Gulf was a lacustrine environment as part of the Marmara ‘Lake’ during the late glaciation and early deglaciation until ∼12 kyr BP, when the Marmara Basin was inundated by the Mediterranean waters. Correlation of 14C-dated onshore and offshore stratigraphic units in the Western İzmit Gulf indicates that generally coarse late glacial sediments overlie a marked erosional surface formed during the low water level of the Marmara ‘Lake’. These coarse sediments are succeeded by 10.4–7 kyr BP old transgressive, and by late Holocene post-transgression mud units. The bathymetry and sub-bottom Chirp profiles clearly show that the paleoshoreline of the Gulf was located at −85 m, having been controlled by the bedrock sill depth of the Çanakkale Strait. Another paleoshoreline observed at −65 m on the northern margin of the Western İzmit and Karamürsel basins was probably formed during the Younger Dryas sea-level stillstand. The shelf areas during this time were colonized by bioherms, which were subsequently drowned and disappeared after a further rise of the sea level. The presence of a −65 m marine paleoshoreline in the Karamürsel Basin indicates that the sill restricting this basin to the west was much deeper than its present −55 m level and was located further south. The Gölcük Basin, restricted by a −38 m sill to its west, was probably not flooded by marine waters until ∼9 kyr BP.

Geoarchaeology at Gilman Falls: An Archaic quarry and manufacturing site in central Maine, U.S.A.

AbstractInterdisciplinary investigations at the Milford Reservoir, central Maine, resulted in excavation and analysis of a Middle Archaic quarry and manufacturing site at Gilman Falls, dated to between 7300 and 6300 yr B.P. Lithological analysis indicates that the majority of the artifacts came from very local outcrops, providing low‐grade metamorphic rocks. Native Americans used a specialized technique to reduce the granofels and other rocks to long rods, artifacts commonly placed in local cemeteries. The Gilman Falls site was largely abandoned once these artifacts were no longer in vogue. Therefore, access to particular bedrock outcrops seems to have played an important role in site selection. Gilman Falls and other early to middle Holocene sites are preserved where bedrock sill dams ponded water that deposited fine sand. Early site sedimentation history is paralleled by a drainage change in the headwaters of the Penobscot River. Evidence for lower mid‐Holocene lake levels and a period of higher temperatures and lower precipitation may correlate with the sedimentation history. © 2001 John Wiley & Sons, Inc.

The Composite Nature of the Provo Level of Lake Bonneville, Great Basin, Western North America

Deposits of a transgressive-phase Lake Bonneville stillstand or oscillation are found just below the elevation of the regressive-phase Provo shoreline at numerous exposures throughout the Bonneville basin. Existence of these subProvo shoreline deposits provides a new explanation for the massive size of Provo depositional and erosional landforms, which can no longer be explained by a long stillstand at the Provo shoreline. Provo coastal landforms are large because they are superimposed on subProvo landforms. Results also help to clarify divergent interpretations regarding the relative age of the Provo shoreline and the number of times it was occupied by the water plane. Occupation of approximately the same level during both the transgressive and the regressive phase of Lake Bonneville may be coincidental, or it may indicate that a bedrock sill controlled outflow at subProvo as well as Provo time. Rise to the Bonneville level could have occurred after massive slope failure plugged the outlet pass.

Dynamics of a Holocene Cliff-Top Dune along Mountain River, Northwest Territories, Canada

The dynamics of a cliff-top dune system located at the foot of the Mackenzie Mountains was reconstructed by radiocarbon dating and tree ring analysis of forest paleosols in eolian sediments. The first evidence of eolian activity dates to ca. 3000 yr B.P. when the breaching of a bedrock sill initiated the erosion of glaciofluvial deposits that resulted in the exposure of a 75-m-high cliff of erodible sediment. Sandy sediment eroded from the cliff By katabatic winds from the Mackenzie Mountains buried a mature forest growing on alluvial sediments. Despite a minor eolian event between 750 and 860 cal yr A.D., dune development began ca. 1100 cal yr A.D. and has been marked by two major sedimentation periods between ca. 1100 cal yr A.D. and 1460 A.D. and from 1865 to present. Detailed analysis of tree morbidity and mortality indicates that the dune progressed at an average rate of 78 cm/yr for the last century. Dune dynamics are directly related to slope activity that controls the episodic nature of sediment availability and to the katabatic wind regime. Comparison of the dune dynamics and paleoclimatic data suggest a possible influence of warm climate on eolian activity.

Holocene depositional history of a large glaciated estuary, Penobscot Bay, Maine

Data from seismic-reflection profiles, sidescan sonar images, and sediment samples reveal the Holocene depositional history of the large (1100 km 2) glaciated Penobscot Bay estuary of coastal Maine. Previous work has shown that the late Wisconsinan ice sheet retreated from the three main passages of the bay between 12,700 and 13,500 years ago and was accompanied by a marine transgression during which ice and sea were in contact. Isostatic recovery of the crust caused the bay to emerge during the immediate postglacial period, and relative sea level fell to at least −40 m sometime between 9000 and 11,500 years ago. During lowered sea level, the ancestral Penobscot River flowed across the subaerially exposed head of the bay and debouched into Middle Passage. Organic-matter-rich mud from the river was deposited rapidly in remnant, glacially scoured depressions in the lower reaches of Middle and West Passages behind a shallow (⩽20 m water depth) bedrock sill across the bay mouth. East Passage was isolated from the rest of the bay system and received only small amounts of locally derived fine-grained sediments. During the Holocene transgression that accompanied the eustatic rise of sea level, the locus of sedimentation shifted to the head of the bay. Here, heterogeneous fluvial deposits filled the ancestral valley of the Penobscot River as base level rose, and the migrating surf zone created a gently dipping erosional unconformity, marked by a thin (<2 m) lag deposit of coarse sand and gravel. As sea level continued to rise, a thin (⩽9 m) layer of acoustically transparent muddy sediments accumulated over a shallow platform in the eastern half of the bay head. Graded sediments within this stratum began to accumulate early in the transgression, and they record both the decrease in energy conditions and the waning influence of the Penobscot River at the head of the bay. In contrast, relatively thick (up to 25 m) silty clays accumulated within a subbottom trough in the western half of the bay head. This deposit apparently developed late in the transgression after sea level had reached −20 m and after the westward transport of fine-grained sediments from the Penobscot River had been established. During and since the late Holocene transgression of sea level, waves and currents have eroded, reworked, and redistributed Holocene sediments: (1) atop the shallow margins; (2) within constricted channels; (3) around topographic highs; and (4) over the shallow bedrock sill at the bay mouth. The variable distribution, characteristics, and thickness (0 to more than 30 m) of Holocene deposits in Penobscot Bay primarily reflect: (1) the irregular glacially eroded bedrock topography beneath the bay; (2) the paleogeography of the bay during the sea-level lowstand; (3) the postglacial location of the ancestral Penobscot River; and (4) the wave and current regime during and since the Holocene sea-level transgression.