A record of Early Pleistocene glaciation on the Plateau, northwestern I I.S. Spooner, G.D. Osborn, R.W. Barendregt, and E. Irving

Abstract: Mount Edziza is a Plio-Pleistocene volcanic complex that is located in the Stikine Terrane in northwestern British Columbia. A sequence of diamictites preserved between Formation on the northwestern blank of Mount Edziza records an Early Pleistocene regional glaciation. The lowest Ice Peak Formation flow (IP1; about 1 Ma) was probably extruded onto glacial ice because it is deformed and brecciated, it is pillowed at the base, it lies directly on hyaloclastite deposits, and there is a lack of fluvial and lacustrine sediments at the base. Fabric measurements from the underlying diamictites are consistent with lodgement processes and indicate northwest and southwest transport directions. These data, and an abundance of striated exotic cobbles, indicate that the sediment was deposited by Coast Mountain ice. Radiometric, paleomagnetic, and stratigraphic data all support the interpretation that diamictites at the section are the sedimentary record of an Early Pleistocene (about 1.1 Ma, isotope stage 32-34) regional glaciation@). The normal paleomagnetic polarity of one of the Ice Peak Formation basalts (IP2) records extrusion during the Jaramillo normal polarity subchron (1.07-0.99 Ma) and further constrains the age of the underlying diamictites.

RCsum6 : Le mont Edziza fait partie d'un complexe volcanique d'Bge Plio-PlCistocBne qui est localis6 dans le terrane de Stikine, dans le nord-ouest de la Colombie-Britannique. Une sequence de diamictites, interstatifike entre les coul6es de basalte de la Formation d'Ice Peak sur le flanc nord-ouest du mont Edziza, t6moigne d'une englaciation rtgionale au Pltistocbne prtcoce. La coulte de basalte qui apparait ii la base de la Formation d'Ice Peak (IP1; dat6e d'environ 1 Ma) a probablement fait eruption sur la glace de en raison des observations suivantes : elle est dtform6e et brtchifike, sa partie inftrieure est une lave h coussins, elle repose directement sur des d6p6ts d'hyaloclastite, et on ne trouve pas h la base de s6diments fluviaux et lacustres. Les parambtres de la fabrique des diamicitites sous- jacentes sont compatibles avec des processus typique des tills de base, et ils indiquent que les directions de transport 6taient nord-ouest et sud-ouest. Ces donntes, et l'abondance des gros cailloux exotiques striks, rtvblent que le saiment a kt6 dtpost par un glacier de la chaine CBtikre. Les r6sultats des 6tudes radiometrique, pal6omagnttique et stratigraphique plaident en faveur de l'interprktation d6crivant les diamictites expos6es dans cette coupe comme reprtsentant les archives saimentaires d'avancte(s) glaciaire(s) ayant affect6 toute la region, durant le Pltistocbne pr6coce (il y a environ, 1,l Ma, stage isotopique 32-34). La polaritt paltomagn6tique normale d'une des coultes de basalte de la Formation d'Ice Peak (IP2) enregistre 1'6ruption survenue durant le sous-chrone de le Jaramillo (1,07-0,99 Ma) de polarit6 normale, ce qui autorise une estimation plus prtcise de 1'Bge des diamictites sous-jacentes. [Traduit par la rkdaction]

Introduction northwestern British Columbia (Fin. 1) and forms art of the north - south-trending Stikine ~olc&c(Belt. Many rksearchers In this paper, the stratigraphy and paleomagnetism of a Plio- have noted that intercalated sediments and basalts are com- Pleistocene sediment section on Mount Edziza are investi- mon in the region (Dawson 1898; Johnson gated. Mount Edziza is located in the Stikine Terrane in 1926; Watson and 1944; Read and Psutka 1985). Souther and Symons (1974) identified diamictites deposited Received March 8, 1995. Accepted August 16, 1995. on the western flank of Mount Edziza as being, in part, of I.S. Spooner.' Department of Geology, Acadia University, glacial origin. Souther (1992) invoked the presence of glacial Wolfville, NS BOP 1x0, . ice on Mount Edziza to explain the morphology of specific G.D. Osborn. Department of Geology and Geophysics, The volcanic flows and their mode of emplacement. Regional University of Calgary, Calgary, AB T2N 1N4, Canada. stratigraphic associations and detailed dating of the basalt R.W. Barendregt. Department of Geography, The flows led to the suggestion that some of the sediment may be University of Lethbridge, Lethbridge, AB T1K 3M4, Plio-Pleistocene in age (Souther et al. 1984). Canada. The purpose of this paper is to describe these sediments E. Irving. Pacific Geoscience Centre, Geological Survey of in detail and to interpret them in terms of their glacial his- Canada, P.O. Box 6000, Sidney, BC V8L 4B2, Canada. tory; in particular, an attempt is made to constrain the ages ' Corresponding author (e-mail: ian. spooner@acadiau. ca) . of the sediments. In light of the paucity of terrestrial records Can. J. Earth Sci. 32: 2046-2056 (1995). Printed in' Canada 1 Imprim6 au Canada Spooner et al. of early Quaternary Cordilleran glaciations, it was felt that Fig. 1. Location of the Sezill Creek section and other these sections warranted further study. Recent research in sections referred to in this study. The Sezill Creek section is the genetic differentiation of diamictons and detailed inves- located on the north side of Sezill Creek, which incises into tigations of tills and lahars (Mills 1984, 1991) increased the the Big Raven Plateau. Fabric orientations (arrows) indicate probability that these diamictites could be reliably inter- preferred a-axes orientation in two separate Early Pleistocene preted. The Sezill Creek section was chosen for detailed diamictites DB2 qnd DB3. investigation because it offers good lateral and vertical 131" 130130' exposure of both sediments and basalt flows and is easily accessed. This latter point is critical for practicality and economy of effort, as the surrounding terrain is extremely rugged.

Regional geological setting Mount Edziza forms the northern part of the Mount Edziza Volcanic Complex and is bounded to the west by the and to the east by the subdued peaks of the and the . The is made up of overlapping basaltic shields that are overlain by composite domes of largely intermediate composition (Souther et al. 1984). Isolated outcrops of intrusive rock exist on the western flank of Mount Edziza, to the north and west of the Sezill Creek section described later in the text (Souther 1992). The section described in this paper is located on the north- western side of the Big Raven Plateau (Fig. 1) along the northern side of the Sezill Creek valley (el. 1350 m) and con- sists of diamictite, stratified and massive sediment, and inter- calated basalt flows. The sediments and intercalated basalts lie above the felsic Armadillo Formation that has been K - Ar dated at 10.2 1.4 to 6.1 f 0.1 Ma (Souther 1992). The intercalated basalt flows belong to the Ice Peak (IP) For- mation, which has not been reliably dated due to high atmospheric argon content (Souther 1992).

Stratigraphy 1 LEGEND I * Study Site 50M). Contour 2 @CacheHill In this paper all formation designations are from Souther Section I n Glacier Fabric Directions 3 Tennaya Glacier I (1992). We informally designate distinct Ice Peak Formation Section basalt flows as members within the formation to facilitate 1 RiverCreek 1 Camp Hill Section detailed section description. Sediment located between Ice Peak basalt flows (both volcanoclastic and otherwise) was considered by Souther (1992) to be part of the Ice Peak For- a unique distribution, hence the stratigraphy within the illus- mation. We refer to these sediments as informal members trated time frame may vary at any given location on the within the Ice Peak Formation, as this paper demonstrates Mount Edziza Plateau. The distributions of Ice Peak and that the sediments and the basalts are closely related. formation basalts do not overlap. informal subdivisions in this paper apply only to the Sezill Diamictite preserved below Pyramid and below the lower- Creek locality. They are required to facilitate description and most Ice Peak basalts on Mount Edziza is generally thick discussion and establish the intimate relationships between ( > 20 m) and poorly sorted and contains two or more distinct volcanic and sedimentary deposits in the area. units separated by erosional boundaries. A preliminary exam- ination of poorly exposed diamictite at Cache Hill and Camp Mount Edziza area Hill (Fig. 1) indicated that they contained exotic intrusive The basic stratigraphic and age relationships for the Mount megaclasts, complex stratigraphy, a paucity of pyroclastic Edziza Volcanic Complex (not including the ) debris, and, possibly, multiple diamictite beds. Souther (1992) from the time of deposition of the Armadillo Formation noted that at the Tennaya Glacier section (Fig. I), Pyramid (7.1 -6.1 Ma) to the deposition of the Edziza Formation Formation lacustrine tuff interfingers with a thick diamictite (0.9 + 0.3 Ma) are shown in Fig. 3. Ages for all volcano- that is overlain by Ice Peak basalt; apparently diamictite genic formations are from Souther (1992). Two sedimentary deposition and Pyramid Formation emplacement were coeval. units that contain diamictite interpreted as till are depicted in The distribution of the Pyramid and Nido basalts does not the general stratigraphic column (Fig. 3) as being bounded extend to the Sezill Creek section (Souther 1992), and hence by the Nido (about 4.4 Ma) and Ice Peak (about 1.0 Ma) the time between deposition of the basalts that overlie and formations and separated by the Pyramid Formation (about underlie the thick diamictite is greatest within this section 1.1 Ma). However, each of these basalt formations exhibited (about 5.6 Ma). 2048 Can. J. Earth Sci. Vol. 32, 1995

Fig. 2. Sezill Creek section. Ice Peak Formation basalts (Ice Peak 1, Ice Peak 2) were deposited about 1.0 Ma and the lower Armadillo Formation basalt was deposited approximately 6.0 Ma (Souther 1992). The majority of the sediments are diamictites (DA, DB1 -DB3, DC, DD). The wedge-shaped unit (CBPL sandstone) is a volcanogenic sedimentary deposit. The sediment section DB3 -DC is about 30 m thick.

Fig. 3. Composite Late Pliocene - Early Pleistocene Sezill Creek section stratigraphic section for basalt formations and sediment The Sezill Creek section consists of two thin diamictite mem- investigated in this study. Diamictites underlie both the Ice bers (DD and DA) preserved between Ice Peak Formation Peak and Pyramid Formation basalts. Figure not to scale; basalts, and a 30 m thick stratified sediment and diamictite data from Souther (1992). succession (DB1- DB3, CBPL sandstone, DC) deposited between the lowest Ice Peak and Armadillo Formation EDZIZA FM. (0.9 MU)' basalts (Figs. 2, 4).

Armadillo Formation Basalt (AFB) The AFB forms the base of the Sezill Creek section (Fig. 2). AFB consists of a single felsic flow that has been dated at about 7.1 - 6.1 Ma (Souther 1992). The top of the flow is neither polished nor striated.

PYRAMID FM. (1.1 MU)^ Diamictite member C (DC) Diarnictite C is 4-8 m thick and coarse-grained with a high percentage of gravel-sized clasts. This sediment lies strati- graphically below the CBPL volcanogenic member. The plane of contact is subhorizontal (Fig. 5). Though DC is NlDO FM. (4.5 poorly exposed, the exotic megaclasts noted are highly MU)^ weathered and friable; weathering rinds on these clasts are well developed. DC is more strongly lithified than the over- ARMADILLO FM. lying diamictites. (6.1 - 7.1 Ma) Cross-bedded and parallel-laminated (CBPL) sandstone - BASALT 1 Single K-Ar date 2 ~obate,high atmospheric member SEDIMENT argon This member has a strongly concave-up, erosional upper 3 K-Ar and fission track dates contact with DB1 and a roughly horizontal (though undulatory) 4 K-Ar dates, weighted average lower contact with underlying DC (Fig. 5). The CBPL sand- Spooner et al.

Fig. 4. Stratigraphy, age, and paleomagnetic correlations for the Sezill Creek section. The age of Ice Peak Formation basalts is interpreted as being about 1.0 Ma. Ice Peak 2 and DA are correlated with the Jaramillo subchron within the Matuyama reversed epoch. DB1 -DB3 are correlated with the Matuyama; the CBPL sandstone falls within a reversal below the Jararnillo subchron. Paleomagnetic scale after Cande and Kent (1995) and Shackleton et al. (1990); oxygen-isotope scale after Ruddiman et al. (1989). OXYGEN ISOTOPE AND PALEOMAGNETIC TIME SCALES Sl8O ICE PEAK 4 5.0 (FOI~) 4;0 , , 3.0 (warm) [LEWI) . t BRUNHES 0,78 1 %&t3 about 1.O Ma

ICE PEAK 2 (LESOS)

4 ICE PEAK 1 (LEU)4) \ \ 'c, 'c, \ \

\ \ "~bi -,Oc@ JARAMILLO \ >1.0 Ma ICE PEAK AGE Regional c 4.0 Ma Ice Advance NlDO FM. AGE . ---- 1.19 - COBB MT. 36

CBPL SAND (LES05-08) 7 N ARMADILLO FM. (LES 09) about 6.0 Ma

Ice Peak Basalt Cross-Bedded Sand Diamicton (Medium)

Armadillo Basalt 1=/ Rhythmic Beds Diamicton (Coarse)

p

stone coarsens upwards from parallel-laminated and massive samples were taken from within the parallel-laminated beds. medium to fine sandstone to tabular and trough cross-bedded coarse sandstone with occasional intercalated thin pebble Diamictite member B1 (DB1) layers. Paleocurrent measurements averaged 178". The mas- This member is about 10 m thick (Figs. 2, 5) and contains sive beds also contain isolated well-rounded pebbles. The some megaclasts that have maximum long-axis dimensions sediment that makes up this member is distinctly volcano- of about 0.5 m. Much of the sediment is fine grained genic and is strongly lithified. Individual grains are sub- (Fig. 6A). Individual parallel-laminated beds in DB1 are up rounded to angular; some exhibit vesicular internal structure. to 70 cm thick, can be traced laterally for over 40 m, and A lag is present at the top of this member where the sediment consist of alternating 1 cm thick sandy and silty laminae is in contact with overlying diamictite B1. Paleomagnetic (Fig. 6B). The laminae are often normally faulted. Deforma- Can. J. Earth Sci. Vol. 32, 1995

Fig. 5. Cross-bedded, parallel-laminated volcanogenic Fig. 6. (A) Subhorizontal erosional contact (at arrow) sandstone (CBPL sandstone). The contact with overlying DB1 between diamictites DB2 and DB1. Interspersed in the is strongly concave, erosional, and is marked by a lag of rhythmic sediment (DB1) are lenses of poorly sorted, clast- cobbles and boulders that is one clast thick. This unit is supported gravel (box, Fig. 6B). (B) Lenses shown in (A) composed primarily of felsic and mafic volcanogenic and interpreted as proglacial, lacustrine debris-flow deposits sediment. The sediment is strongly cemented, which may originating either from floating ice or a nearby ice front. The indicate that it is considerably older than the overlying, more book in the photographs is 16 cm high. friable diamictites. Person for scale at arrow.

tion is common and consists of injection structures in inter- calated gravel. At select locales the faulting crosscuts the deformation structures. The sandy laminae contain isolated, well-rounded pebbles, and more rarely, cobbles and boulders that are often striated. The laminated beds occur rhythmi- cally throughout this member and are occasionally separated by 0.1 -0.2 m thick lenses of coarse sand and pebble-rich diamictite in which both matrix and clast support is evident (Fig. 6B). These beds have an undulatory (presumably ero- sional) lower contact and a planar upper contact. Sandy silt laminae (1 cm thick) drapes are common but rarely extend laterally more than 1 m. Clasts of intrusive rock, usually phyllite or diorite, are common within the pebble beds (approx. 5%). The contact with overlying DB2 is sharp ported (60% matrix, 40% clasts), with a maximum clast diameter of 1.0 m. Megaclasts are subrounded and occasion- and erosional and is marked by an increase in the coarse sedi- ment concentration and termination of the thick rhythmically ally facetted and striated and are dominantly of igneous deposited silts (Fig. 6A). The base of DBl is concave upwards extrusive origin; however, megaclasts of intrusive (diorite, and exhibits a prominent one-megaclast-thick layer of rounded approx. 5%) and metamorphic rock are also present. The to subangular cobbles and boulders. fine-grained silty matrix is grey to tan. Lenses of parallel- laminated sand are common and tend to drape some of the Diamictite member B2 (DB2) cobbles. Isolated pods of openwork pea gravel are also Diamictite B2 is about 2.5 m thick, and contains about 75% present. Large clasts are often preferentially arranged as matrix. Megaclasts within the sediment have long axes up to undulating semihorizontal layers within this diamictite, par- 0.4 m long and are generally well rounded (Figs. 7A, 7B). ticularly at the lower boundary with DB2, where megaclasts The diamictite is light brown and exhibits faint, isolated, appear to be embedded. The upper contact of DB3 grades 1-4 cm thick, horizontal, laminated lenses (Fig. 7B). A few into a thin layer of medium to coarse, sorted volcanogenic (approx. 1 %) cobbles of intrusive and metamorphic rock sediment, which in turn grades into a layer of palagonite that were noted, and most exhibited well-developed weathering coarsens upwards into a volcanogenic breccia. rinds. The matrix sediment is dominantly silty with a fine sand component. The upper contact with DB3 is undulating Ice Peak member 1 (IPl) and is accompanied by a marked increase in megaclast con- IP1 is a thin, uniform basalt flow that has a pillowed base and centration. The lower contact is sharp, well defined, and contains yellow to buff-coloured palagonite deposits, indicat- contains a concentration of larger clasts that appear to be ing that the basalt flow was deposited in a cold, perhaps sub- embedded in the finer grained sediment of DBl. aqueous environment. The top of IPl is polished and appears striated; however, detailed measurement of striation orienta- Diamictite member B3 (DB3) tion was not possible due to the lithified nature of the contact This diamictite is 4-7 m thick (Figs. 2, 5) and matrix sup- with overlying DA. Spooner et al.

Diamictite member A (DA) Fig. 7. (A) Diamictite DB2, which contains exotic, striated Diamictite A is found between IPl basalt and IP2 basalt megaclasts. The base of DB2 contains a lag of cobble to (Figs. 2,4). This member averages about 2.5 m in thickness boulder clasts and forms a sharp erosional contact with the and contains about 50% megaclasts (pebble to cobble sized), rhythmites of underlying DB1. DB2 is interpreted as having some of which are angular and others subrounded to well formed in a glacial environment (field book is 18 cm high). rounded. A few exotic, striated igneous cobbles were noted. (B) A lens of laminated silty sand within diamictite DB2. The matrix is composed of fine sand and silt, and occasional These lenses are common but are discontinuous and tend to discontinuous laminae and convolutions were noted. The drape over large clasts. upper contact of the diamictite grades into well-sorted medium to coarse pyroclastic sediment (scoriae and lapilli) overlain by palagonite.

Ice Peak member 2 (ZP2) IP2 is similar to IP1 and also is pillowed and contains palagonite deposits at its base. However, in contrast to IPl the top of IP2 does not appear to be striated.

Discontinuous diamictite member (DD) A discontinuous diamictite lies between IP2 and IP3 basalt flows (Fig. 4). The diamictite contains both rounded and angular cobbles, which account for about 40% of the sedi- ment volume. The matrix is medium grained, the main con- stituent being fine sand and silt; no internal structures were evident. The megaclasts are almost solely of extrusive ori- gin; megaclasts of intrusive rock accounted for less than 1 % of the clast volume. Palagonite deposits and isolated pods of graded medium-grained pyroclastic material (scoriae, ash, and lapilli) form the upper contact with the overlying basalt.

Ice Peak 3 and 4 (IP3, ZP4) IP3 is a discontinuous basalt flow that has a pillowed base which is also highly fractured and contains abundant palagon- ite; it was probably deposited in an aqueous environment. The top of this basalt flow does not appear to be striated. IP4 has neither a pillowed base nor a polished top surface; it does not appear to have been deposited in contact with water or ice. Poorly exposed, discontinuous volcanogenic sediment lies between these two flows.

Exotic clast lithology and provenance The bulk of the megaclasts in the diamictite members are of extrusive origin and presumably have a local source; how- ever, DC, DB l - DB3, and DA contain a significant number dip of the long axis in both members is relatively shallow, of exotic (intrusive) clasts. Well-rounded and angular diorite however, the mean directions are offset by about 90". These pebble- to boulder-sized clasts are most common (approx. strong fabrics suggest that the megaclasts in both sample 5 %) and likely originated from Jurassic and Triassic plutons groups are preferentially aligned; the cluster tendency is an that circumscribe the Edziza Plateau. Phyllite pebbles and indication of the strength of the directional force. Limited cobbles were also noted (approx. < 2 %) and were most com- exposure and (or) a paucity of rod-shaped megaclasts pre- monly found as isolated, striated, and facetted angular clasts cluded fabric measurements in DD, DB1, DA, and DC. in DB1. These clasts likely originated from the Palaeozoic- aged rocks in the of the Coast Mountains Interpretation of stratigraphy and located to the west of the Sezill Creek site (Fig. 1). sedimentology

Fabric data It is essential to determine the genesis and provenance of the diamictites. Three possibilities exist: (i) local volcanogenic At the Sezill Creek section, fabric measurements were taken origin, (ii) local (Mount Edziza) glacial origin, or (iii) regional on rod-shaped cobble-sized clasts in DB2 and DB3. Both glacial origin. diamictites exhibit a preferred orientation of rod-shaped Till described by Souther and Symons (1974) was classi- clasts and a well-developed cluster tendency (Fig. 8). Two fied as such due to the presence of well-rounded, exotic fabric directions were noted. N07"E for the upper diamictite clasts. They surmised that these clasts could only have been DB3, and S78"E for the middle diamictite DB2 (Fig. 8). The transported to the Edziza Plateau by regional glacial pro- Can. J. Earth Sci. Vol. 32, 1995

Fig. 8. Fabric orientations for diamictites DB3 and DB2 ments, though in the latter they are primarily the result of (lower hemisphere projections) indicate that the direction of debris-flow processes (Scott 1986). transport for these diamictites was most likely from the south The distinction between regionally and locally derived and west. Arrows indicate a-axis orientation. Paleocurrent tills must also be established. A large ice cap exists on Mount directions in the underlying CBPL sandstone are from the Edziza at present (Fig. 1). Though the moraines left behind northeast. Both diamictites exhibit strong fabrics in which the by Holocene advances extend only 2 km from the present ice cluster tendency is greater than the girdle tendency. margin, it is possible that a larger ice cap may have existed Diamictite DB3 in the past on Mount Edziza. The glacial sediments preserved N between basalts then may be a record of Mount Edziza centred rather than regional ice advance. DC is sporadically exposed and does not contain features diagnostic of either volcanogenic or glaciogenic deposition, although the presence of exotic megaclasts may indicate that glaciation, at the very least, preceded its deposition. The CBPL sandstone is composed primarily of fluvially derived volcanogenic sediment. The sediment may have been deposited " as outwash in a meltwater stream similar to those that now Peak Position: 194.9°/14.80 bisect the eastern portion of the Big Raven Plateau. Both the CBPL sandstone and underlying DC are well cemented, indicating that they both may predate deposition of DB1 by a considerable amount of time. DB1 contains sedimentological characteristics more con- sistent with subaqueous deposition. The fine-grained beds are rhythmic and contain isolated phyllitic and dioritic peb- bles and cobbles, interpreted as dropstones, that are striated, angular, and deform underlying sediment. The poorly sorted coarser sand and gravel beds are deposited directly on the laminated sediments, which, at that contact, exhibit deforma- tion, indicating that the coarser sediment was probably "dumped," with little potential for sorting, on the rhythmic sediments. DB3 and DB2 exhibit strong, unimodal fabrics, suggest- ing that the sediment was deposited by lodgement or engla- cial processes (Fig. 8). The fabrics compare favourably with other basal ice debris fabrics described by Mills (1991) and fall well outside the envelope for debris-flow deposits. Many studies have summarized the general characteristics of fabrics formed in subglacial environments (Levson and Rutter 1988; Mark 1973; Mills 1977, 1991; Shaw 1985), and these sedi- ments are generally characterized by strong fabrics in which the a axes of rod-shaped stones in basal tills are oriented near parallel to ice flow and lie in planes that vary from subhori- zontal to dipping up-glacier. The fabric data also suggest transport directions (from the north and the west) that are incongruous with volcanically initiated debris flows that, if they occurred, must have originated to the east of the Sezill Creek locality. The pillowed basalts and abundant palagonite at the con- tact of IPl and underlying DB3 may be further evidence of the presence of ice at the section. Both of these volcanogenic features are indicative of basalt deposition in saturated condi- tions. As there is no indication that fluvial or lacustrine con- ditions existed immediately prior to deposition of IPl, the presence of ice is inferred. The diamictite succession (DB1 -DB3) is interpreted as being indicative of deposition in an ice-proximal, subaqueous cesses. However, in the absence of additional indicators of (DBl),and subglacial environment (DB3 and DB2). The glaciogenic transport, there exists the possibility that the truncation of the underlying CBPL sandstone and the deposi- exotic megaclasts were incorporated during reworking of tion of a boulder lag may reflect fluvial erosion at the onset previously deposited sediment. As well, diamictons can be of regional glaciation. An increase in the availability of melt- produced in both glaciogenic and volcanogenic environ- water and blockage of drainage routes during advance may Spooner et al.

Table 1. Paleomagnetic site results. No.of No.of

Site Polaritya Lat. (N) Long. (W) cores specimens M(AIm) D (O) I (") k cr,, (") Material

LESOl R 57'50' 130°50' 4 6 5.570 158.7 -38.9 232 4.4 IP4 basalt LESO2 R 57"50f 130°50' 4 6 0.834 209.2 -57.3 18.5 16 IP3 basalt LES03 N 57"50f 130°50' 5 8 5.682 5.2 58.2 393 3.2 IP2 basalt LESM R 57"50f 130°50' 8 15 3.589 183.4 -69.3 113 4.1 IP1 basalt LESO5 - LES08 R 57'50' 130°50' 15 17 0.058 146.2 -69.8 15.8 9.2 CBPL sandstone LES09 N 57'50' 130°50' 8 15 2.090 333.7 66.4 245 2.5 Armadillo basalt Notes: Sites are located by latitude, longitude, and map reference. M, mean intensity of magnetization of the specimens; D and I, declination and inclination of the mean direction relative to the present horizontal; k, Fisher's estimate of precision; radius of the circle of confidence. "N, normal; R, reversed. have resulted in the formation of an advance-phase proglacial unit and stepwise thermal demagnetization was completed lake and the deposition of rhythmically bedded silts and clays using a Schonstedt TDS-1 furnace. (DB1 sediment). The presence of phyllitic clasts is an indica- Normal-polarity magnetization was observed in IP2 basalts tion that Coast Mountain Ice had inundated the Edziza (LES03; Fig. 9) and Armadillo Formation basalts (LES09). Plateau at this time. Diamictites DB2 and DB3 indicate The decay upon AF demagnetization is smooth and a sub- regional glacial ice cover conditions. The normal faults that stantial part remains after treatment at 100 mT. Intensities crosscut both the poorly sorted coarse sediment and rhythmic fall by almost an order of magnitude during demagnetization. sediment in DB1 may be indicative of postdepositional defor- Orthogonal plots (Fig. 9) illustrate that small magnetizations mation by overriding ice. Different transport directions imposed by the present field were present and were removed suggested by the fabrics in DB2 and DB3 may reflect an at low fields, at which there was linear decay to the origin. adjustment in regional ice flow direction as glaciation pro- There is no evidence to suggest that the normal magnetiza- gressed. tions at these sites record anything other than the ambient Striated exotic clasts and the lack of stratification and sort- field at the time of basalt deposition. ing suggest that DA may have been deposited by glaciogenic The principle magnetization was reversed for IP4 (site processes. The polished surface of the underlying basalt LESOl), IP3 (site LES02), and the IP1 (site LES04) basalts flow (Ice Peak 1) indicates that glaciation may have occurred (Fig. 9). Orthogonal plots are sometimes "hooked" (a direc- after extrusion of the basalt. The base of the overlying basalt tional migration), and the first part of the plot may be show- (Ice Peak 2) is pillowed and contains palagonite, indicating ing the removal of magnetizations directed along the present aqueous deposition, perhaps by extrusion onto ice or into field, but the remainder of the plot shows linear decay to the meltwater. Combined, these features indicate that glacial ice origin. The reversed magnetizations at these sites also very cover may have occurred after deposition of Ice Peak 1 and probably record the reversed ambient field at the time of prior to extrusion of Ice Peak 2. deposition. DD does not contain sedirnentological features that con- For IP3 basalt (site LESO2), the initial magnetization is clusively indicate deposition by either glaciogenic or vol- normal, which, upon treatment, gives way to a reversed canic processes. However, exotic clasts within the member polarity magnetization (Fig. 9). Decay occurs rapidly during indicate that at some time prior to or synchronous with the demagnetization, which corresponds to a normally magne- deposition of the diamictite, regional glacial processes must tized overprint of low wercivity. At high fields, the mag- have been active at the section. netization tends to be scattered, but is clearly of reversed polarity. Paleomagnetism Paleomagnetic samples at the CBPL sandstone site (LESO5 -LES08) do not exhibit smooth decay upon thermal Paleomagnetic data demagnetization. Orthogonal plots exhibit a pronounced The Sezill Creek section contains five basalt flows, all of hooked form and begin to decay in a linear fashion after the which were sampled. The only fine-grained, undisturbed third treatment level (200°C). No sense of direction can be sediments suitable for paleomagnetic sampling were found in derived for sites exhibiting this type of magnetization (Fig. lo), the CBPL sandstone; all diamictites were far too coarse for however there is no evidence that the reversed magnetization paleomagnetic work. The interpreted stratigraphy and paleo- is anything other than a record of the ambient field at the time magnetics of the Sezill Creek locality are shown in Fig. 4; of sediment deposition. paleomagnetic data for each sampling site are summarized in Table 1. Paleomagnetic conclusions All samples were collected using a gas-powered diamond IP2 basalt has normal polarity. The age of the Ice Peak For- drill, and cores were oriented by solar and (or) topographic mation cannot be radiometrically determined, due to argon bearings. Remanence measurements were made on a Schon- contamination (Souther 1992); however, the age of the stedt SSM-1 spinner magnetometer, which was controlled by underlying Pyramid Formation is about 1.1 Ma and the age an IBM personal computer. Stepwise alternating-field (AF) of the overlying Edziza Formation is about 0.9 Ma. Hence demagnetization was carried out using a Schonstedt GSC-5 the Ice Peak Formation is about 1 million years old. It is 2054 Can. J. Earth Sci. Vol. 32, 1995

Fig. 9. Site magnetizations, normal and reversed. At most sites the dirm:ion of magnetization can be determined. Polatity can be determined for all sites. Ice Peak 3 basalt (site LES023A) exhibits reversed magnetization but has a normal overprint.

Sits ICE PEAK 5 Slte: ICE PEAK 1 Site ICE PEAK 2 Sam e LESMSA Sample: LESodbA lo Sample -1. 6 lo\ NR$= 1.18 A!rn NRM = 2.8 A/m \ NRM = 5.22 A/m z k! z a 5 YI

0.0 00.1oh40bO -p 100 0 20 40 60 i-bo OD b ?u 40 60 h 160 STEP STEP STEP

Reversed

, Nonnal Polarity /

\ Reversed Polarity \,., 1, \ \--1-'

NIN NIN I

Scale - 500 mA/m

SCALE = lMKI mA/m

therefore likely that IP2 basalt correlates with either the Jaramillo normal polarity subchron (1.070 -0.990 Ma; Cande and Kent 1995) or the Cobb Mountain subchron (1.19 \Ye attribute diamictites Dl32 and DB3 to deposition during Ma; Shackleton et al. 1990). Correlation with the Jaramillo Coast Mountain ice advance rather than by Mount Edziza event is more probable, as the radiometric age of the lower c:entred ice on the basis of fabric orientation and exotic mega- bounding Pyramid Formation is 1.1 Ma. IP basalts 1, 3, and clast lithology. Interpretations of late Fraser (Late Wiscon- 4 have reversed polarity, indicating that they were deposited sinan) ice flow in the region provide an effective analogue. during the Matuyama Epoch. We infer that IP3 and IP4 In the valley east of (Fig. I), basalts were deposited after the Jaramillo event and are Glacial Lake Stikine developed when advancing Coast therefore younger than 0.990 Ma (the upper age limit for the lvlountain ice blocked westward drainage of the Stik-ine River Jaramillo event, Cande and Kent 1995). We infer that IP1 (Ryder and Maynard 1991; Spooner 1994). East-flowing ice basalt was deposited before the Jaramillo event and therefore c:ventually inundated the Edziza Plateau depositing exotic was probably deposited between 1.10 and 1.070 Ma. Coast Mountain erratics in the suficial till (Spoonec 1994). The reversed polarity of the volcanogenic sediment (CBPL The ages of DB2 and DB3 are difficult to constrain, sandstone) does not aid in the resolution of the age of the t~causeneither the underlying CBPL sandstone (reverse overlying strata, as at least 13 reversals occur in the period polarity) nor the lower bounding Armadillo Formation following the deposition of the underlying Armadillo For- (about 6 Ma) provide closely limiting ages. However, the mation (7.1-6.1 Ma) and prior to the deposition of the Ice following stratigraphic indicators suggest that these diamictites Peak Formation (about 1.0 Ma) (Cox 1982). However, the date from the same time as the IF1 basalt. In addition to the low k values and nonlinear decay to origin may be the conse- palagonite described earlier. pillow structures and breccia- quence of complex overprinting indicative of a prolonged tion characterize the base of PI; it is most likely that TP1 period of deposition for both the CBPL sandstone and the was extruded onto ice, the same ice that deposited the underlying DC. diamictites immediately underneath the basalt. These con- Spooner et al.

Fig. 10. Paleomagnetic site ChRM mean directions and polarities. At sites LES03, LES04, and LES09, site means approximate the paleopole position. Site LESOl shows a significant displacement to the east which may reflect the influence of the nondipole field. Sites LES02 and LESOS-LES08 show high scatter and, though directions are not determinable, specimens polarities are clearly reversed. REVERSED N N

E . LESIM , o '... Ice Peak 3 Basalt

s

REVERSED N LES04 t!b$ Ice Peak 1 Basalt

\ 1 LESOl \ t Ice Peak 4 Basalt \

siderations would place the age of DB2 and DB3 between 1.1 lacustrine tuff interfingers with the diamictite; he further and 1.070 (Fig. 2). believed that much of the Pyramid Formation he observed in There is evidence from elsewhere on the Edziza Plateau outcrop was probably deposited during a period of ice cover. that is complementary to this conclusion. Similar sediment These relationships suggest the diamictites there are coeval successions to that at Sezill Creek were observed at a recon- with the Pyramid Formation, with an age of 1.1 Ma. We sus- naissance level at Camp Hill and Cache Hill, and Souther pect, but cannot demonstrate, that the diamictite sequences (1992) described similar successions at the Tennaya Glacier from Sezill Creek and Tennaya Glacier, and possibly the section (Fig. 1). Multiple diamictites, striated megaclasts of other sections as well, are records of a single, or closely intrusive origin, and a lack of volcanogenic debris within related, glaciation. these sections indicate that they too are probably the sedi- DA was most likely deposited by glacial processes as mentary record of glacial conditions. Souther (1992) noted well. As we were unable to collect fabric data for DA, we that at the Tennaya Glacier section, the Pyramid Formation were unable to establish whether the glaciation responsible Can. J. Earth Sci. Vol. 32, 1995 was either regional or local. However, Souther (1992) con- of glaciogenic deposits. Edited by R.P. Goldthwait and sidered that Ice Peak basalts were deposited close in time, C.L. Matsch. A.A. Balkema, Rotterdam, pp. 117- 140. and IP1 basalt (which underlies DA) has a polished top sur- Mark, D.M. 1973. Analysis of axial orientation data, including till face and a pillowed and fractured base, indicating extrusion fabrics. Bulletin of the Geological Society of America, 84: below ice. It is possible, therefore, that both DA and 1369- 1374. Mills, H.H. 1977. Textural characteristics of drift from some repre- DB2-DB3 were deposited during the same glacial event. sentative Cordilleran . Geological Society of America Bulletin, 88: 1135- 1143. Conclusions Mills, H.H. 1984. Clast orientation in Mount St. Helens debris flow deposits, North Fork Toutle River, Washington. Journal of Fabric data, regional stratigraphy, and sediment and basalt Sedimentary Petrology, 54: 626 - 634. morphology at the Sezill Creek section indicate that ice Mills, H.H. 1991. Three-dimensional clast orientation in glacial advance in the Coast Mountains inundated the Big Raven and mass-movement sediments: a compilation and preliminary Plateau between 1.1 and 1.070 (isotope stage 32 - 34; Fig. 4). analysis. United States Geological Survey, Open-file Report Other diarnictites may record separate glacial events, the 90-128. ages of which are difficult to constrain. Read, P.B., and Psutka, J. 1985. Quaternary and Late Tertiary The Sezill Creek section also provides an Early to Middle volcanism, Stikine River projects area, British Columbia. An Pleistocene paleomagnetic record. The paleomagnetic stra- internal report to B.C. Hydro and Power Authority. Geotex tigraphy has helped constrain the age of the Ice Peak Forma- Consultants, 1200-100 West Pender St., Vancouver. Ruddiman, W.F., Raymo, M.E., Martinson, D.G., Clement, tion basalts, which, due to contamination, do not provide B.M., and Backrnan, S. 1989. Pleistocene evolution: Northern reliable ages. Hemisphere ice sheets and North Atlantic Ocean. Paleoceanog- raphy, 4: 353-412. Acknowledgments Ryder, J.M., and Maynard, D. 1991. The Cordilleran Ice Sheet in northern British Columbia. GBographie physique et Quaternarie, We would like to thank the Natural Science and Engineering 45: 355-363. Research Council of Canada (operating grant 9026), the Scott, K.M. 1986. Lahars in the Toutle-Cowlitz River system, Arctic Institute of North America (Northern Science Train- Mount St. Helens, Washington-origin, behaviour and sedi- ing Program), the Canadian Circumpolar Institute, the Geo- mentology . United States Geological Survey, Open-file Report logical Society of America, the Geological Survey of Canada 86-0500. (Pacific Geoscience Centre), and The University of Calgary Shackleton, N.J., Berger, A., and Peltier, W.R. 1990. An alterna- for support of this research. The original manuscript bene- tive astronomical calibration of the lower Pleistocene timescale fited from reviews by R. Fulton and A. Plouffe. based on ODP Site 677. Transactions of the Royal Society of Edinburgh: Earth Sciences, 81: 251 -261. This paper is dedicated to the late Ronald Janzen, owner - Shaw, J. 1985. Subglacial and ice marginal environments. In operator of Tel-Air Flight Services, Telegraph Creek, B.C. Glacial sedimentary environments. Edited by G.M. Ashley, Ron provided accommodation and logistical support for J. Shaw , and N.D. Smith. Society of Economic Paleontologists many researchers in northern British Columbia during the and Mineralogists, Short Course No. 16, pp. 7-84. last decade. He was killed while flying in September 1994. Souther, J.G. 1992. The Late Cenozoic Mount Edziza Volcanic He will be missed by many. Complex, British Columbia. Geological Survey of Canada Memoir 420. References Souther, J.G., and Symons, D.T.A. 1974. Stratigraphy and palaeo- magnetism of Mount Edziza Volcanic Complex, Northwestern Cande, S.C., and Kent, D.V. 1995. Revised calibration of the British Columbia. Geological Survey of Canada, Paper 73-32. geomagnetic polarity timescale for the Late Cretaceous and Souther, J.G., Armstrong, R.L., and Harakal, J. 1984. Chronology Cenozoic. Journal of Geophysical Research, lOO(B4): 6093 - of the peralkaline, late Cenozoic Mount Edziza Volcanic Com- 6095. plex, northern British Columbia, Canada. Geological Society of Dawson, G.M. 1898. Report on an exploration in the America Bulletin, 95: 337 -349. District, N.W.T., and adjacent northern portion of British Spooner, I.S. 1994. A record of Quaternary environmental change Columbia. Geological Survey of Canada Paper, Queen's on the Stikine Plateau, northwestern British Columbia. Ph.D. Printer, Ottawa. thesis, The University of Calgary, Calgary. Johnson, W.A. 1926. Gold placers of the area. Canada Watson, K.DeP., and Mathews, W.H. 1944. The Tuya-Teslin Department of Mines, Geological Survey Summary Report, area, northern British Columbia. British Columbia Department 1925, Part A, No. 2113, pp. 33-75. of Mines, Bulletin 19. Levson, V.M.,and Rutter, N.W. 1988. A lithofacies analysis and interpretation of depositional environments of montane glacial diamictons, Jasper, , Canada. In Genetic classification