Antarctic Peninsula

Most of the outcrops in the Antarctic Peninsula have been relatively well investigated in recent years. They form two main types: outcrops dominated by a major stratovolcano (James Ross Island Volcanic Group) and outcrops formed by multiple monogenetic volcanic fields and small isolated centres (Bellingshausen Sea Volcanic Group; Smellie, 1999). James Ross Island, situated at the northern end of the Antarctic Peninsula, comprises a single large stratovolcano 1.5 km high and about 60 km in diameter, together with numerous small satellite centres. About 50 individual eruptive episodes have been identified, with most retaining information on environmental conditions between 6.25 Ma and today. All but a few show features diagnostic of glacial conditions (Smellie, 2006). The associated ice sheets had a provenance

Table 10.2: Characteristics of volcanic outcrops of Neogene Age (15-2 Ma) in West Antarctica.

Outcrop

Age (Ma) Dating Environmental method characteristics3

Comments

Principal sources

Antarctic Peninsula

Merrick Mountains 6

James Ross Island <6.5 region

Ellsworth Land Snow Nunataks

Undated

Jones Mountains 10-7?

(imprecise) Hudson Mountains 8.5-3.7

Marie Byrd Land Mount Flint

Ames Range

14.1

K-Ar Glacial (1 and 2)b; subaerial

K-Ar Subaerial0; glacial?

40/39Ar Glacial (1) and glaciomarine (interglacial)

Glacial (1)

K-Ar Subaerial, subaqueous and/or glacial

K-Ar Unknown

K-Ar Unknown

Multiple monogenetic centres

Tiny degraded outcrop remnants

Dissected stratovolcano and multiple satellite centres

Multiple monogenetic centres

Multiple small centres

Stratovolcano

Three overlapping stratovolcanoes

Smellie and Hole (1997), Smellie (1999) Smellie (1999)

Nelson (1975), Skilling (1994)

LeMasurier and Thomson (1990)

LeMasurier and Thomson (1990)

LeMasurier and

Thomson (1990) LeMasurier and Thomson (1990)

Whitney Peak, 13.7 K-Ar Subaerial?c Executive Committee Range

Mount Hampton, 11.7-8.6 K-Ar Subaerial?0 Executive Committee Range

Mount Hartigan, 8.5-6.4; 7.86- K-Ar Subaerial?0

Executive 7.57 Committee Range

Mount Cumming, 10.4-10.0; 3.0 K-Ar Subaerial?c Executive Committee Range

Mount Sidley, 5.7-4.2 40/39Ar Subaerial0 and snow

Executive contact Committee Range

Mount Bursey 10.4-8.54; 6.4 K-Ar Unknown

Toney Mountain 10.1-9.1 K-Ar Unknown

Hobbs Coast 9.56-3.19 K-Ar Glacial

Mount Murphy 9.39-8.22 40/39Ar Glacial (1 and

(mainly) 2) and subaerial0

Undissected stratovolcano

LeMasurier and Thomson (1988)

Undissected stratovolcano

Two poorly exposed volcanoes: Lavris Peak and Tusing Peak Undissected stratovolcano

Three coalesced stratovolcanoes

Two overlapping undissected stratovolcanoes Plateau lava basal succession Numerous small dissected centres Dissected stratovolcano

LeMasurier and Thomson (1989)

LeMasurier and Thomson (1990)

LeMasurier and Thomson (1990)

LeMasurier and Thomson (1990)

LeMasurier and

Thomson (1990) LeMasurier and

Thomson (1990) Smellie (2000a,b), LeMasurier (2002), Wilch and Mcintosh (2002)

Table 10.2: (Continued).

Outcrop

Age (Ma)

Dating method

Environmental characteristics3

Comments

Principal sources

Mt. Murphy

6.8-5.95

40/39Ar

Glacial (1)

Small monogenetic

Smellie (2001 a,b),

satellite centres

centres

Wilch and Mcintosh (2002)

Crary Mountains

9.13-6.82; 8.46-5.74; 4.25-1.81

40/39Ar

Glacial (2); subaerial0

Three volcanoes: Mt Rees, Mt Frakes and Mt Steere

Wilch and Mcintosh (2002)

Mount Moulton

4.9-4.7

K-Ar

Unknown

Two overlapping undissected stratovolcanoes

LeMasurier and Thomson (1990)

Mount Berlin

2.58-2.32

K-Ar

Subaerial0 and glacial?

Two coalesced undissected stratovolcanoes

LeMasurier and Thomson (1990)

Fosdick Mountains

4.5-3.4

K-Ar

Unknown

LeMasurier and Thomson (1990)

aIgnores late-stage subaerial parasitic cones (Marie Byrd Land), often very much younger (by m.y.) than the main stratocone successions, which they overlie. bGlacial 1: "thick ice" conditions (few hundred metres); Glacial 2:"thin ice" conditions (<150m); after Smellie (2000a,b). cEnvironmentally undiagnostic since subaerial volcanics may be supraglacial; evidence for (lower elevation) glacial conditions not exposed.

aIgnores late-stage subaerial parasitic cones (Marie Byrd Land), often very much younger (by m.y.) than the main stratocone successions, which they overlie. bGlacial 1: "thick ice" conditions (few hundred metres); Glacial 2:"thin ice" conditions (<150m); after Smellie (2000a,b). cEnvironmentally undiagnostic since subaerial volcanics may be supraglacial; evidence for (lower elevation) glacial conditions not exposed.

rooted both in the Antarctic Peninsula and in a local ice cap centred on the volcano itself (Hambrey and Smellie, 2006). The ice was wet based and erosive throughout, although there are few thermal regime data for units younger than 2 Ma. Conversely, a few of the volcanic units showed evidence for eruptions in marine conditions, and marine fossils in interbedded sedimentary deposits also suggest that warmer ice-poor conditions (interglacials) occurred during at least three broad periods: 6.5-5.9, 5.03-4.22 and < 0.88 Ma (Smellie et al., 2006a,b). The James Ross Island Volcanic Group is now known to contain numerous sedimentary interbeds, mainly varieties of diamicts with a glacial association that are only beginning to be investigated fully (Hambrey and Smellie, 2006; Hambrey et al., 2008).

By contrast, the Bellingshausen Sea Volcanic Group contains multiple small volcanic centres with ages ranging between 7.5 and 2.5 Ma (Smellie, 1999). The glacial environment was wet-based, temperate or subpolar, and varied between thin and thick ice conditions (<70 and >500m, respectively), although not all outcrops yielded the full range of information (Table 10.2; Smellie et al., 1993; Smellie and Hole, 1997). There also exists a small outcrop of 2.5 Ma tephra (Hornpipe Heights Formation, northern Alexander Island) erupted under essentially dry subaerial conditions, presumably when any associated glacial cover was below the present outcrop (i.e. below 750 m a.s.l.). Scant evidence from elsewhere on Alexander Island suggests that glacial conditions may have been present at 2.5 Ma, although few Pliocene samples are dated and the age resolution is poor (Smellie, 1999).

10.3.2. Ellsworth Land

Three outcrops occur in this region: Snow Nunataks, Jones Mountains and Hudson Mountains (Table 10.2). The Hudson Mountains are particularly poorly known. Conversely, Snow Nunataks comprises several small undated nunataks that have been divided into the Mount Benkert and Mount McCann formations, formed of subaqueous stratified hyalotuff and subaerial lava, respectively (Smellie, 1999). The formations are cogenetic and resemble basaltic glaciovolcanic centres known as tuyas (Smellie, 2000a,b) erupted probably under ice thicknesses that locally exceeded 350 m. However, a marine environment, while probably less likely, cannot be excluded since so little is known about the area. A 700m-thick Cenozoic volcanic succession resting on a subhorizontal polished and striated glacial unconformity crops out at Jones Mountains (Rutford et al., 1972; Hole et al.. 1994). Diamictite lenses interpreted as tillite overlie the unconformity and the overlying sequence has been divided into at least two units of pillow lava, hyaloclastite breccia and hyalotuff. Striated and facetted erratic boulders are also scattered throughout the lower volcanic unit. The descriptions most closely resemble volcanic sequences formed under a thin glacial cover. However, since individual ''thin ice sequences'' are theoretically limited to thicknesses <150m (Smellie, 2000a,b, 2001a,b), the much greater total thickness of volcanic rock present suggests that more than two volcanic sequences are present. This is also suggested by the presence of numerous reworked hyalotuff lenses dispersed within the sequence, and possibly by the wide range in the published isotopic ages (mainly 7-10 Ma; Table 10.2), which might suggest an outcrop constructed from multiple eruptions that occurred over a long period.

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