Heiner Josenhans

Drowned forests and archaeology on the continental shelf of British Columbia, Canada

We have used high-resolution digital terrain imaging and sea-floor sampling to reveal drowned late glacial to early postglacial terrestrial landscapes at water depths as great as 150 m. In situ tree stumps and shellfish-rich paleobeaches are present on these drowned landscapes. A stone tool encrusted with barnacles and bryozoa was recovered from a drowned delta flood plain now 53 m below mean sea level. This is the first tangible evidence that the formerly subaerial broad banks of the western North American Continental Shelf may have been occupied by humans in earliest Holocene and possibly late-glacial time. Analyses ( 14 C) of the drowned terrestrial and intertidal deposits were used to refine the local sea-level curve, which shows very rapid change within this glacio-isostatically dynamic region.

Post glacial sea levels on the Western Canadian continental shelf: evidence for rapid change, extensive subaerial exposure, and early human habitation

Grounded piedmont type glaciers inundated and isostatically loaded the deep troughs which indent the Western Canadian continental shelf, as far west as the shelf edge. Glaciers do not appear to have covered the offshore banks east of the Queen Charlotte Islands (Haida Gwaii). Ice retreated from the shelf at approximately 13,500 14C yr B.P. Rapid emergence of the crust followed the ice retreat and resulted in a relative fall of sea level. At 10,400 14C yr B.P. relative sea level on the continental shelf was more than 100 m below that of today and large areas of the Queen Charlotte Basin were subaerially exposed. Eustatic sea-level rise, coupled with subsidence of a glacioisostatic forebulge, allowed sea levels to rise very rapidly, and reach the present shoreline on the Queen Charlotte Islands (Haida Gwaii) by about 9100 14C yr B.P. Dated shoreline deposits (shells), submerged wood remnants, and barnacles on stone tools at elevations between −110 m and +14 m suggest a sea-level rise of 6.3 cm per year between 12,200 and 11,000 calender years. Our reconstructions of the paleogeography and paleoenvironments suggest a hospitable environment for human habitation existed in areas that are now submerged. Stone tools excavated from intertidal deposits support this interpretation. Significant local variations in the depth of synchronous shorelines are described and attributed to localized differences in isostatic load. The documented rates of crustal adjustment are much greater than those used in conventional geophysical (forebulge) models. Regional high-resolution seismic reflection data (3400 line km) shows little evidence for post-glacial faulting and suggest that most crustal adjustments appear to have been isostatically rather than tectonically driven. Subaerial exposure and subsequent sea-level transgression were the dominant post-glacial processes that determined the morphology, texture and paleoenvironment of the Western Canadian continental shelf.

Submerged late quaternary terrestrial deposits and paleoenvironment of northern Hecate Strait, British Columbia continental shelf, Canada

Abstract Stratigraphic and paleoenvironmental analyses of sediments recovered from cores on the western Canadian continental shelf indicate that areas of the north-central shelf were subaerial and supported terrestrial vegetation during the regional Late Wisconsinan deglaciation. Paleoecological analyses of now submerged shelf sediments reveal a Late Quaternary treeless environment locally covered by wet sedge tundra. Samples from a coeval onshore section on eastern Graham Island indicate that a dwarf shrub tundra with mosses and willows also occupied lowland areas on the Queen Charlotte Islands between 13 and 14 ka. Ice-free subaerial conditions persisted for a period of at least 3200 years on some parts of the emergent shelf, between 13.2 and 10 ka. During the same time period clay-rich glaciomarine sediment with ice-rafted debris was accumulating in the troughs of the shelf. Low relative sea levels during deglaciation on the continental shelf adjacent to the eastern Queen Charlotte Islands suggest that Late Wisconsinan ice was thin and limited in extent in this area. The presence of a habitable lowland environment during this time interval strengthens the view that a coastal migration of humans from Beringia was possible along portions of the exposed continental shelf.

The Seismic Record of Glaciation in Nachvak Fiord, Northern Labrador

Nachvak Fiord is a 45 km long glacial trough in the Torngat Mountains of northern Labrador, Canada (Fig. 1). The fiord is 2 to 4 km wide, increasing gradually eastward to Nachvak Bay, which opens to the Labrador Sea. The sidewalls are generally steep, rising in places 1000 m vertically from sea level. Bathymetry reveals a succession of basins, four of which occur between Tasiuyak Arm and Nachvak Bay (Fig. 1). Maximum water depths in the four basins are 90, 160, 170, and 210 m from west to east. The four basins are separated by shallow barriers between 10 and 180 m below sea level. Two of these, at Kogarsok and Tinutyarvik, have many of the characteristics of riegeln or glacial steps. The fiord threshold at the entrance to the fiord is very shallow with an average depth of <50 m and numerous bedrock-cored shoals.

Glacial Sole markings on Bedrock and Till in Hudson Bay, Canada

Huntec DTSTM seismic reflection profiles and sidescan sonograms are interpreted to indicate subglacial and ice marginal sole markings [Josenhans and Zevenhuizen, 1990]. The profiles reveal a thin (<5 metres) veneer of undulating, acoustically unstratified sediment in central Hudson Bay, interpreted as till overlying a smooth bedrock unconformity. The sidescan sonogram covers an area of seafloor 1.2 × 5.5 km where the surface of the till is moulded into parallel flutes which trend 340 degrees. They are thought to have been formed at the base of a moving ice mass flowing in a northwesterly direction. The direction of these flute marks is regionally variable with flow trending toward the deep basins. The regional variations in flow direction, can be used to infer the pattern of ice break up and the location of ice domes. Comparative flute marks from terrestrial areas northwest of Hudson Bay are illustrated by the aerial photograph. Note that on a local scale, the (marine) features are mostly parallel throughout and change only slightly along track, presumably in response to an eroding ice base. We have visually observed similar changes in iceberg scour marks from submersibles where abrasion of the ice keel results in modified scour morphology along track. Similar glacial sole marks have been reported from the northern Barents Sea [Solheim et al., 1989].

Glacially Overdeepened Troughs on the Labrador Shelf, Canada

The continental shelf off Labrador is underlain by a clastic wedge of Early Paleozoic to Cretaceous-Tertiary sandstones, siltstones and limestones which onlap the Precambrian metasediments of the mainland. Regional seismostratigrphic mapping [Josenhans et al., 1986] shows that the contact between Precambrian basement and the clastic wedge is faulted in places. Buried valley fill remnants are recognised on the middle shelf together with faulted sequences at the base of the fluvial sequences. Fluvial down-cutting along fault-weakened zones is suggested to have occured in late Tertiary-Pliocene time [Grant, 1966]. Intense glacial erosion, particularly on the inner shelf near the Precambrian/clastic wedge contact, has down cut the clastic sediments and shaped a marginal trough which connects to a transverse channel (Hopedale Saddle) with depths of up to 800m. The intensity of glacial erosion appears most pronounced at the Precambrian contact and diminishes toward the outer shelf edge as demonstrated by the presence of remnant valleyfill and early glacial progradational deposits. Water depth on the outer shelf of Hopedale saddle/trough is only about 300m suggesting up to 500m of glacial overdeepening on the inner shelf. Note that the upper till lies directly on the bedrock unconformity on the inner shelf although older glacial(?) deposits are preserved on the outer shelf. This is interpreted to result from increased glacial erosion on the inner shelf.

Glacially Overdeepened Troughs and Ice Retreat ‘Till Tongue’ Deposits in Queen Charlotte Sound, British Columbia, Canada

The continental shelf of Queen Charlotte Sound, British Columbia, was inundated by piedmont glaciers which extended to the shelf edge by way of the shelf transverse troughs [Josenhans et al., 1992; Luternauer et al., 1989]. Seismic reflection profiles indicate that glacial erosion overdeepened these troughs and developed a smooth glacial unconformity on bedrock. Glacial and deglacial sediments comprising basal till, multiple ice-contact sequences, and stratified glaciomarine sediments overlie the glacial unconformity and typically attain 50m in thickness. The offshore banks, which separate the troughs, appear to be devoid of glacial deposits and may have remained ice free during the last glaciation. Regional seismic profiles from the trough axes reveal multiple generations of channel fill deposits in channels cut into the Plio-Pleistocene bedrock on the inner shelf. The upper surface of the channel deposits is truncated by the glacial erosional unconformity and overlain by till. These channel deposits have not been sampled and may represent a preglacial subaerial drainage system or possibly subglacial channel sequences that were deposited prior to the last glacial advance.

Simultaneous use of Multiple Seismic Reflection Systems for High Resolution and Deep Penetration

It is possible to resolve the facies architecture of unconsolidated bottom deposits and deeper structures within underlying bedrock by using a combination of seismic reflection profiling systems. Data from Hudson Bay Canada, was concurrently collected using a Huntec DTSTM high resolution subbottom profiler, a.6 litre (40 cubic inch) airgun with single channel seismic eel, and a 16 kilojoule sparker with multichannel seismic array. No detrimental “cross talk” was recorded due to carefull adjustment of the trigger pulses and descriminate filtering for each system. Individual seismic sequences can readily be correlated between the various systems. The finest resolution (30 cm) is obtained with the Huntec DTSTM system (with a centre frequency between 3–4 kHz) [Parrott, et al 1980]. This data reveals detailed structure within the surficial sediments as well as within bedrock which is helpful in interpreting the depositional setting. The low angle bedding and regional consistency of the bedrock seismic horizons within Hudson Bay allows detailed structure revealed by the Huntec DTSTM high resolution system, to be extrapolated to depths where they are visible on the multichannel data. This approach has led to the recognition in Hudson Bay, of sedimentological structures such as fluvial point bars, reefal structures, microfaults and salt collapse structures as well as the facies architecture of the overburden above bedrock.

Subglacial Channels in Hudson Bay, Canada

Huntec DTSTM seismic reflection profiles and sidescan sonograms from Hudson Bay Canada, show the presence of large anastomosing channels eroded into sediments interpreted to represent till.