Abstract

The seismic stratigraphy and architecture of the Beaufort Sea shelf and slope are investigated using a comprehensive grid of high-resolution two-dimensional seismic reflection data. Three cross-shelf troughs, representing locations of former ice streams draining a 1000-km-long section of the Laurentide Ice Sheet (LIS) are examined: the Mackenzie, Amundsen Gulf, and M’Clure Strait systems. Dynamics of these paleo-ice streams influenced ice-sheet configuration and may have forced abrupt climatic change through delivery of ice and freshwater to the Arctic Ocean. A comprehensive understanding of their geometry and dynamics is crucial for constraining numerical models of the LIS. Evidence for two Quaternary ice advances to the shelf break is interpreted from the Mackenzie Trough. By contrast, seismic stratigraphy of the Amundsen Gulf Trough, 400 km east of the Mackenzie Trough, records at least nine Quaternary ice advances. Here, the outer shelf consists of stacked till sheets, extending to the shelf break and forming a trough-mouth fan. The contrasting glacial histories of these neighboring ice streams are explained by their positions within the LIS; the Mackenzie Trough ice stream was situated at the extreme northwest ice-sheet margin, whereas the Amundsen Gulf ice stream had a more central location and larger drainage basin, supplying significant quantities of ice and sediment to the Arctic Ocean through much of the Quaternary. The M’Clure Strait Trough probably possesses a similar architecture to the Amundsen Gulf Trough, and an even larger trough-mouth fan.

INTRODUCTION

The bathymetry of the Beaufort Sea shelf on the Canadian margin of the Arctic Ocean is dominated by three cross-shelf depressions: the partially infilled Mackenzie Trough, and the well-defined Amundsen Gulf and M’Clure Strait Troughs (Fig. 1A). These troughs represent the locations of ice streams which drained the northwest Laurentide Ice Sheet (LIS) margin during the last, Late Wisconsinan glacial maximum (Fig. 1B) (Dyke et al., 2002; Stokes et al., 2006) and a hitherto unknown number of earlier glacial periods. The paleo–ice streams of the Beaufort Sea margin were pathways for ice drainage and delivery of sediment to the Arctic Ocean (Stokes et al., 2005) and are suggested to have fed ice shelves in the Beaufort Sea during a number of Quaternary glaciations (Jakobsson et al., 2010). The Mackenzie and Amundsen Gulf troughs are incised into fluvio-deltaic sediments of the Pliocene-Pleistocene Iperk Sequence (Dixon and Dietrich, 1990).

The LIS occupied the Mackenzie Delta (Fig. 1A) during at least one Quaternary glaciation, including the Toker Point Stade (Rampton, 1988). Although previously assigned to the Early Wisconsinan (Vincent and Prest, 1987), recent evidence suggests that the Toker Point limit was reached during Late Wisconsinan glaciation (Murton et al., 1997; Dyke et al., 2002; see Figure DR1 in the GSA Data Repository1). The offshore extent of this advance is poorly constrained, and little is known about the timing or extent of pre-Late Wisconsinan glaciations. Farther east, the stratigraphy of Banks Island (Fig. 1) has been suggested to record evidence of multiple glaciations, including the earliest and most extensive advance, the Banks Glaciation, which occurred >780 k.y. ago (Vincent, 1983). This interpretation has been challenged by a revision to Banks Island’s stratigraphy, which categorizes all tills previously assigned to older glaciations to the Late Wisconsinan (England et al., 2009). Analyses of landform assemblages in the Canadian Arctic support this notion of extensive Late Wisconsinan ice cover (Stokes et al., 2005, 2006).

This study uses high-resolution two-dimensonal (2-D) seismic reflection data collected by ION Geophysical Corporation as part of the BeaufortSPAN East survey (http://www.iongeo.com/Data_Library/Arctic/BeaufortSPAN_East), supplemented by older seismic data, to examine the seismic stratigraphy and architecture of a 1000-km-long section of the Beaufort Sea margin. Interpretations were made on two-way time (TWT) seismic profiles, and depth converted using a 3-D velocity model based on seismic velocities tied to wells where available. Due to restrictions on the use of industrial data, details of the acquisition parameters and streamer arrays are not included. Comprehensive grids of data covering ∼30,000 km2 and 200,000 km2 were analyzed from the Mackenzie and Amundsen Gulf Troughs, respectively, together with ∼800 line kilometers of seismic data from M’Clure Strait (Fig. 1A). Here we constrain the number of ice-stream advances through the Mackenzie and Amundsen Gulf Troughs and discuss their possible timing. We examine the impact of these ice advances on the architecture of the Beaufort Sea margin. Observations from the three systems are considered in relation to LIS dynamics and Beaufort Sea glacial history.

MACKENZIE TROUGH

The seismic stratigraphy of Mackenzie Trough has been examined previously by Blasco et al. (1990), who suggested that the trough was excavated by an ice stream during the Early Wisconsinan. Blasco et al. (1990) identified five seismic units within the trough, including a 70-m-thick basal unit, interpreted as till, overlain by seismically semi-transparent sediment, interpreted as sand. A second ice advance into the trough may have occurred during the Late Wisconsinan (Blasco et al., 1990; Murton, 2009).

The present study identifies eight seismic facies and five sequences, categorized into two mega-sequences, derived from newly available seismic lines from the Mackenzie Trough. These two mega-sequences provide evidence for two Quaternary ice advances to the shelf break. Sequence 1 is composed predominantly of two chaotic facies (facies Ci and Cii; Figs. 2 and 3) and corresponds with Unit 5 of Blasco et al. (1990). The chaotic seismic character of facies Ci and Cii, together with their position above an erosional unconformity (Fig. 3), suggest that they are subglacial till deposited by an ice stream. The seaward transition from high- to low-amplitude chaotic facies (Fig. 3D) probably results from differences in sedimentological properties of the tills. Facies Siii (Fig. 2) occurs as an asymmetric wedge (Fig. 3D) of maximum thickness 100 m and length 14 km (Fig. DR1). Wedge dimensions and geometry suggest it is a grounding-zone wedge (GZW) produced during an ice-margin still-stand (Dowdeswell and Fugelli, 2012).

Sequence 2 is composed of facies Si (Fig. 2) and is interpreted as glaciomarine sediment deposited during deglaciation. Sequence 3 consists of facies T (Figs. 2 and 3) and corresponds with Unit 4 of Blasco et al. (1990), who interpreted it as sand. The distribution, thickness and seismically semi-transparent character of this sequence (Fig. 3) are, however, suggestive of subglacial till. Sequence 3 is therefore interpreted as subglacial till deposited by a second ice advance through the Mackenzie Trough.

Two trough-parallel ridges of facies M (Fig. 2) overlie sequence 3 in the landward region of the trough (Fig. DR1). Each ridge has a length of ∼60 km, a width of ∼5 km, and a maximum thickness of 90 m. These features probably represent lateral moraines produced during a stillstand of the ice margin. Sequences 4 and 5 are composed of facies Si and Sii, respectively (Figs. 2 and 3), and are interpreted as glaciomarine to open-marine sediment.

The slope beyond the Mackenzie Trough is characterized by incised canyons. Less-disturbed areas of the slope are capped by a 60-m-thick deposit of facies L (Fig. 2), probably representing glaciogenic debris flows produced by the delivery of deformable till. Although it is possible that evidence of older glaciations has been removed, our interpretation of only two ice advances through the trough is supported by the lack of a major glacial-sedimentary depocenter on the slope.

AMUNDSEN GULF AND M’CLURE STRAIT TROUGHS

Our seismic investigations reveal that the inner shelf of Amundsen Gulf is composed of bedrock, overlain by a thin (<20 m) veneer of sediment. Accumulations of sediment >150 m thick occur within several isolated depressions. Material is interpreted to have been removed from the inner shelf by the Amundsen Gulf ice stream.

The base of the Amundsen Gulf Trough appears as an erosional unconformity on seismic records (Figs. 4A and 4B) and is located up to 600 m below the outer-shelf seafloor (Fig. 4C). Eight seismic sequences, categorized into four mega-sequences, are identified within the trough, and are composed entirely of facies T (Figs. 2 and 4). The eight sequences are interpreted as stacked till sheets, extending to the shelf break and forming a trough-mouth fan (Fig. 4D). Boundaries between till sheets are identified as high-amplitude erosional unconformities. A total of nine till sheets is identified from the stratigraphy, providing evidence for at least nine Quaternary ice advances to the shelf break. Several of these till sheets have been removed from the shelf by subsequent ice advances but are preserved on the slope (Fig. 4B).

A near-surface asymmetric wedge of facies T (Fig. 2), of length 50 km and maximum thickness 200 m, occurs at the northern trough margin. This feature is interpreted as a GZW produced at an ice-stream lateral margin. A buried asymmetric landform, also interpreted as a GZW, occurs in the middle-shelf region of the trough, recording a former ice margin stillstand (sequence 2, Fig. 4B).

Our limited seismic data from M’Clure Strait, 350 km northeast of Amundsen Gulf (Fig. 1A), demonstrate that the inner shelf is characterized by a thin sediment veneer overlying bedrock. An asymmetric wedge of facies T (Fig. 2), of length 40 km and maximum thickness 150 m, occurs at the trough’s southern margin and is interpreted as a GZW. The presence of a trough-mouth fan on the slope beyond M’Clure Strait has been inferred previously from bathymetric maps (Stokes et al., 2005) and sub-bottom echo-sounder profiles (Niessen et al., 2010).

The M’Clure Strait Trough is interpreted to possess similar large-scale architecture to the Amundsen Gulf Trough. Both depressions were probably occupied by ice streams during a number of Quaternary glaciations, leading to excavation of sediment from the inner shelf, formation of defined outer-shelf lateral margins, and development of trough-mouth fans on the slope (Fig. 1C).

DISCUSSION: ICE-SHEET AND ARCHITECTURAL VARIATIONS ALONG THE BEAUFORT SEA MARGIN

Extensive seismic data from the Mackenzie and Amundsen Gulf margins of the Arctic Ocean (Fig. 1A) show significant contrasts in the number of times Quaternary ice advanced to the shelf break (Figs. 3A–3D, 4A, and 4B). Sequences of chaotic to semi-transparent facies are interpreted as till sheets, representing ice advances across the shelf. The number of cycles of ice-sheet growth and decay varies markedly between the Mackenzie Trough, with only two recorded events, and the Amundsen Gulf Trough, with at least nine.

The variable history of Quaternary ice-sheet advance across the Beaufort Sea margin produced significant contrasts in the large-scale architecture of the neighboring Mackenzie, Amundsen Gulf, and M’Clure Strait systems (Fig. 1C). There is seismic evidence of considerable progradation across the Amundsen Gulf margin (Fig. 4), in the form of a major trough-mouth fan containing a minimum of 10,000 km3 of glacier-derived debris. Bathymetric data from the M’Clure Strait margin suggest that an even larger fan, with a volume of ∼60,000 km3, is present beyond the M’Clure Strait Trough (Stokes et al., 2005; Niessen et al., 2010). These fan volumes are of the same order as other Arctic and Antarctic fan systems, such as the Scoresby Sund Fan in the Greenland Sea (15,000 km3), and the Belgica Fan in West Antarctica (60,000 km3) (Dowdeswell et al., 2008).

By contrast, there is little seismic or morphological evidence suggesting a major fan beyond the more westerly Mackenzie Trough (Fig. 1A). Given that buildup of high-latitude trough-mouth fans is related to rapid delivery of glacial sediments over successive glaciations (Vorren et al., 1998), observation of only two ice advances through the Mackenzie Trough is likely to be a factor in this lack of a major glacial-sedimentary depocenter. When ice did occupy the Mackenzie Trough, the ice stream was positioned at the extreme northwest LIS margin (Fig. 1B), whereas the Amundsen Gulf and M’Clure Strait ice streams had a more central location, with larger paleo-drainage basins (∼500,000 km2 each, compared with <250,000 km2 for the Mackenzie Trough ice stream).

Although the timing of each ice advance cannot be constrained, the onshore record suggests that glaciation of the Mackenzie Delta became progressively more extensive through the Quaternary (Murton et al., 1997; Dyke et al., 2002). A Late Wisconsinan ice stream has been inferred to have extended across Richards Island (Fig. 1A) sometime between 22 and 16 k.y. ago (Murton, 2009). This event is interpreted to correspond with the younger of the two ice advances inferred from the Mackenzie Trough stratigraphy (Fig. 3). The older ice advance, which excavated the trough, probably occurred during the penultimate (Illinoian) or Early Wisconsinan glaciation.

By contrast, seismic stratigraphy of the Amundsen Gulf Trough (Fig. 4) records evidence of ice advances earlier in the Quaternary, suggesting that the onset of cross-shelf glaciation on the eastern Beaufort Sea margin occurred significantly prior to the initial glaciation of the Mackenzie Trough to the west (Fig. 1A). Previous work has indicated that ice-rafted debris (IRD) and detrital-carbonate maxima in sediment cores from the Arctic Ocean reflect IRD input from the Banks Island area during LIS disintegration at the end of several Quaternary glaciations, including marine oxygen isotope stages 16, 12, 10 and 8 (Stein et al., 2010), as well as the Late Wisconsinan (Stokes et al., 2005). In addition to delivering significant quantities of ice and sediment to the Arctic Ocean through much of the Quaternary, ice streams in the Amundsen Gulf and M’Clure Strait probably nourished ice shelves beyond their grounding lines during some Quaternary glaciations, including the Illinoian, in which a particularly extensive ice shelf is suggested to have developed around the margins of the Arctic Ocean (Jakobsson et al., 2010).

CONCLUSIONS

The number of Quaternary ice advances across the Beaufort Sea shelf varies markedly between the Mackenzie Trough (two) and the Amundsen Gulf Trough (at least nine). The Mackenzie Trough was probably occupied by an ice stream during the Late Wisconsinan and either the Illinoian or Early Wisconsinan glaciation. The Amundsen Gulf ice stream was initiated earlier in the Quaternary.

Whereas the slope beyond the Mackenzie Trough lacks a significant glacial-sedimentary depocenter, major trough-mouth fans (of volumes of ∼10,000 km3 and 60,000 km3) are present beyond the Amundsen Gulf and M’Clure Strait, respectively. Ice streams in the Amundsen Gulf and M’Clure Strait delivered large quantities of ice and sediment to the Arctic Ocean through much of the Quaternary. Their dynamics influenced past ice-sheet configuration and may have forced abrupt climatic change through transport of ice and freshwater to the Arctic Ocean (e.g., Stokes et al., 2005).

We thank ION Geophysical Corporation for permission to reproduce seismic data from the Beaufort Sea margin, and Liz Jolley for her support. The project was funded by BP America and a UK Natural Environment Research Council studentship to Batchelor.

1GSA Data Repository item 2013032, stratigraphic table and seismic reflection profiles, is available online at www.geosociety.org/pubs/ft2013.htm, or on request from editing@geosociety.org or Documents Secretary, GSA, P.O. Box 9140, Boulder, CO 80301, USA.