East Arm Research Articles

The Romanesque East Arm and Crypt of Winchester Cathedral

In common with most other English romanesque cathedrals, the east arm at Winchester was completely replaced above ground level, in successive campaigns between the thirteenth and the sixteenth centuries. The eleventh-century crypt survives, however, and provides the original plan of this part of the cathedral. The first crossing tower collapsed in 1107 and was rebuilt, together with the adjoining transept piers; this also affected the crypt, which was shortened by 1 m at the western end. The earliest entrances for which evidence survives date from the early twelfth century. The plan of the crypt may be regarded as a fusion of continental romanesque deriving from Upper Normandy (notably from Rouen Cathedral) and the developing Anglo-Norman traditions first found at St Augustine's, Canterbury. The high water-table caused the crypt to be abandoned by at least the 1320s (if not earlier) and its entrances were remodelled at that time, and again c. 1513 and c. 1820.The evidence for the position of the choir in the romanesque cathedral has hitherto received little attention. It may be shown that the choir extended into the nave by one bay, where there may have been lateral entrances aligned to the western aisles of the transepts. There was the usual western entrance through a pulpitum situated in the second nave bay. There is evidence for a later rood-screen in the fourth bay.The plan of the east arm is unusual because of the square-ended chapels aligned to the aisles, and is perhaps best regarded as an experimental form which has no direct successor. Some evidence survives for the elevations, which conform to orthodox Anglo-Norman ‘thick-wall’ technique. At main arcade level there was an alternating scheme in the straight bays, compound piers alternating with cylindrical ones; the piers in the apse hemicycle were all cylindrical and the base of one of them survives. The gallery was similar to that of the surviving transepts though of reduced height. The clerestory resembled the earliest design still evidenced in the transepts; the present clerestory design in the transepts results from a compromise introduced when the scheme for towers at the ends of the transepts was abandoned.

Production of mangrove litter in a macrotidal embayment, Darwin Harbour, N.T., Australia

Mangrove forests within a macrotidal embayment, Creek ‘H’, East Arm, in Darwin Harbour, northern Australia, can be divided into a number of assemblages corresponding to geomorphologically defined habitats. The most widespread of these are: (i) tidal creek, dominated by Camptostemon schultzii, Rhizophora stylosa and Bruguiera parviflora, with occasional massive Avicennia marina; (ii) tidal flat, dominated by mixed Ceriops tagal var. australis and Bruguiera exaristata; and (iii) hinterland margin, with a gently inclined colluvial substrate, dominated by almost monospecific Ceriops tagal var. australis. The average annual litter fall was estimated from two years of observations at a series of locations. The highest rates of litter fall, > 1000 g m −2 year −1 (dry weight), were recorded in tidal creek mangrove forests, with litter fall beneath 13 m tall Avicennia marina of 1400 g m −2 year −1. In tidal flat, mixed Ceriops tagal/Bruguiera exaristata litter fall was 745 g m −2 year −1, with approximately equal contributions from both species. In hinterland margin, litter fall varied between 686 g m −2 year −1 beneath taller Ceriops (up to 7 m) and 300 g m −2 year −1 beneath lower Ceriops (< 2 m). All locations showed seasonality of litter production with peak fall in the wet season (December–May), but the proportions and nutrient content of components varied from species to species and site to site. Despite the tropical location (12°30'S) and large tidal range (up to 7·8 m), litter fall is not especially high compared to other mangrove forests and shows considerable spatial variation. Leaf litter was generally low in nitrogen (N) content (</ 0·8%). Nitrogen and phosphorus showed marked decreases (ca. 50%) from green to senescent leaves on the tree, presumably as a result of nutrient translocation, but the percentages of these nutrients subsequently increased in litter from traps.

Operations for an Under-Ice Ecology Program

A pilot program tested the feasibility of conducting a study on the under-ice ecology of the Great Lakes. The east arm of Grand Traverse Bay, in the lower peninsula of Michigan, was chosen as the test area. The project was conducted in three phases; (1) a pre-ice cruise (open water), (2) an under-ice phase, and (3) a post-ice cruise (open water). Overall, an under-ice ecology program that could produce sound scientific results was found to be feasible. However, to be successful, detailed planning is essential and careful attention must be given to operational safety, proper winter clothing, and scientific coordination.

Depositional Dynamics of Cyclic Carbonates from the Interlake Group (Lower Silurian) of the Williston Basin

A set offour transgressive-regressive cycles is recognizable in the Lower to Middle Interlake Group of central Manitoba. The initial Silurian transgression reached its maximum with development of Virgiana (brachiopod) beds in the Fisher Branch Formation. Three subsequent transgressive peaks are identified by coral-rich layers in the upper part of the overlying Inwood Formation, the whole of theAtikamegFormation, and the middle part of the Cedar Lake Formation. These are separated by strata with flat to domal stromatolitic structures or other very shallow water features occurring in the lower to middle parts of the Inwood Formation, the upper part of the Moose Lake Formation, and much of the East Arm Formation. Regressive phases are marked by thin, reddish-brown shale beds bearing abundant frosted quartz grains. These marker beds are best known from middle and lower levels in the East Arm Formation (v and u2 horizons) and the middle of the Inwood Formation (ul horizon). A similar marker bed separates the Interlake Group from the underlying Stonewall Formation of latest Ordovician or earliest Silurian age. The number and spacing of these cycles suggest a correlation with other carbonate cycles found in the East Iowa, Michigan, and Anticosti Basins. While a Bahamian bank model of mosaic-facies deposition explains some aspects of the shallow-water Manitoban cycles, modification of the Israelsky wedge concept (facies in phase) enhances the interpretation of a vast, dynamically interconnected sea stretching far beyond the confines of the Williston Basin. During the first 10 m.y. of the Silurian, the Williston Basin was not an isolated topographic depression. Available evidence suggests that the region was still one of the shallowest parts of the periodically, extensively flooded North American platform.

U–Pb zircon ages from Athapuscow aulacogen, East Arm of Great Slave Lake, N.W.T., Canada

Athapuscow aulacogen is an Early Proterozoic intracratonic basin located in the East Arm of Great Slave Lake between the Slave and northwest Churchill provinces. Athapuscow aulacogen comprises three stratigraphic sequences, the Wilson Island Group, the Great Slave Supergroup, and the Et-Then Group. New U–Pb zircon ages provide constraints on the development of the aulacogen.The Blachford Lake Intrusive Suite consists of an older alkaline phase (Hearne Channel Granite) dated at 2175 ± 7 Ma and a younger peralkaline phase (Thor Lake Syenite) dated at 2094 ± 10 Ma, confirming the suggestion that the two phases may not be related. A felsite from the Wilson Island Group has an age of 1928 ± 11 Ma. The Wilson Island Group is intruded by epizonal granites (Butte Island Intrusive Suite), one of which has an age of 1895 ± 8 Ma. The Wilson Island Group and the Butte Island Instrusive Suite are entirely allochthonous with respect to the Slave craton. Rocks of the Great Slave Supergroup overlie mylonitized Wilson Island Group rocks and both were involved in northeast-directed thrusting. The Compton laccoliths intrude rocks of the Great Slave Supergroup, postdate thrusting, and are about 1865 Ma old.The Blachford Lake Intrusive Suite is significantly older than both the rift sequence in Wopmay Orogen (ca. 1900 Ma) and the Wilson Island Group; it probably is genetically unrelated. The age of the Wilson Island Group and Butte Island Intrusive Suite is considerably younger than previous estimates and is close to the minimum age of rifting in Wopmay Orogen. The Compton laccoliths are very similar to intrusive rocks in the Great Bear Magmatic Zone of Wopmay Orogen and may be related to east-dipping subduction beneath the aulacogen.The new ages strengthen the correlations between Athapuscow aulacogen and Wopmay Orogen and suggest a link with events in the Trans-Hudson Orogen to the south.

Lower Proterozoic red beds, evaporites and secondary sedimentary uranium deposits from the East Arm, Great Slave Lake, Canada

The East Arm graben contains a thick sequence of well-preserved, unmetamorphosed sedimentary rocks. Three units have been recognized: the Wilson Island Group ( c . 2400–2300 Ma); the Great Slave Supergroup ( c. 2300–1790 Ma), and the Et-Then Group ( c. 1700–1600 Ma). The Wilson Island Group contains red beds and overlies an apparently normal regolith formed on Archaean granites. The Great Slave Supergroup contains a normal regolith, numerous red beds, glauconitic horizions, biogenic hydrocarbons and sedimentary uranium deposits. There is also abundant evidence for the former presence of evaporites in that part of the sequence that was deposited within 30° of the palaeoequator. The uranium deposits originated during low temperature diagenesis by precipitation from oxidizing waters in areas of early accumulated hydrocarbons. These observations suggest that the Lower Proterozoic climate, atmosphere and hydrosphere were similar to modern ones and that uranium prospectors in such old terrains should look for chemical deposits as well as the classic placer types.

Ophiolite emplacement by collision between the Sula Platform and the Sulawesi Island Arc, Indonesia

Much of the tectonic complexity displayed in eastern Indonesia results from a series of Neogene collision events between island arcs, continental fragments, and the Australian continent. Here we examine the emplacement of a large ophiolite belt, resulting from the Miocene collision between the Sulawesi island arc and a continental fragment, the Sula platform. We present the results of several marine geophysical expeditions to the SW Molucca Sea and the NW Banda Sea, plus gravity and geology on the east arms of Sulawesi. The Batui thrust separates the ophiolite from sedimentary rocks deformed along the leading edge of the Sula platform. We mapped this thrust eastward from Sulawesi along the southern margin of the Gorontalo basin. The latter is floored by oceanic crust, and its south edge is uplifted against the thrust. Thus the Sulawesi ophiolite can be traced offshore to its origin as basement of the Gorontalo basin. The ophiolite is composed of harzburgite in the Southeast Arm and passes upward through a complex of gabbros and diabase dikes in the East Arm. Ophiolite melange underlies the harzburgites on the Southeast Arm beneath low‐angle thrust contacts where seen. Our local observations show the melange to be composed of thrust packets of both serpentine and red shale matrix varieties. The packets are several hundred meters thick, and the melange, where studied, has a moderate north to northeast dipping foliation. This orientation, if regionally representative of the melange fabric, is consistent with a significant northward component of movement of the lower plate, probably the Sula platform or its margin. Where the ophiolite is in contact with rocks of the central schist belt, it dips under the schist, but where it encounters melange, or Mesozoic or Paleogene sedimentary rocks, the ophiolite is thrust over them. The tectonic overlap sequence, from west to east, is schist over ophiolite over older sediments or melange. The ophiolite appears to have been emplaced by oblique convergence of the Sula platform along the southern edge of the Gorontalo basin. We suggest that the Gorontalo basin represents a forearc basin and the ophiolite is its basement, analogous to a number of other forearc settings. Deformation of the ophiolite may have occurred in part on the seafloor prior to emplacement, but we feel that much of the deformation occurred during emplacement. The Sulawesi ophiolite is only one of a number of ophiolites in the Indonesian region, each of which has a very different origin and tectonic history.

Akaitcho (ca. 1786-1838)

The Yellowknife Indian leader Akaitcho stepped upon the stage of Canadian history in the afternoon of 30 July 1820 when he met Captain John Franklin and affirmed his willingness guide and provision Franklin's expedition of exploration to the shores of the polar sea. ... Known in Franklin's time as Copper Indians, the Yellowknives were the northwesternmost division of the widespread Chipewyan peoples. ... Ranging broadly in the caribou lands from the East Arm of Great Slave Lake the Coppermine River, Akaitcho and the Yellowknives traded as meat provisioners into the North West Company post of Fort Providence on the North Arm of Great Slave Lake. For at least a decade the Yellowknives had pillaged furs, stolen women, and occasionally killed Dogrib and Hare Indians, their neighbours the west and northwest. Dogribs were forced avoid parts of their traditional hunting range during Akaitcho's years of aggressive leadership. ... When, after the terrible overland return from the arctic coast, the starving remnants of the Franklin expedition were rescued by Yellowknives, Akaitcho revealed another facet of his character. Treated with the utmost tenderness by their rescuers, Franklin and his party from Fort Enterprise were conveyed the camp of our chief and companion Akaitcho. ... In consequence of the merger of the North West Company and the Hudson's Bay Company in 1821, the post of Fort Providence had closed in 1823. Akaitcho and the Yellowknives now perforce had direct their trade into Fort Resolution in company with Chipewyans already attached that post. (Their intermarriage and absorption into that population brought the eventual disappearance of the Yellowknives as a distinct people.) Driven by vengeance or desperation over killings perpetrated by Yellowknives earlier in the year, in October of 1823 Dogribs attacked the Yellowknife Long Legs and his band, who were encamped in the area between Hottah Lake and Great Bear Lake. Thirty-four Yellowknives perished - four men, thirteen women, and seventeen children. This was a bitter reversal. Akaitcho refused join Franklin's expedition Great Bear Lake, sending word that he and his hunters would not go into the lands where their kinsmen had died, lest we should attempt renew the war. Peace took the form of mutual avoidance between Dogribs and Akaitcho's band. In 1829 a tense encounter, apparently the first since the destruction of Long Legs's band, was resolved without bloodshed. ...

Reconnaissance microearthquake survey of Sulawesi, Indonesia

Several hundred earthquakes were recorded during the operation of a two‐week five‐station portable seismograph network on Sulawesi, Indonesia. Forty one of these events were locatable and half of these occurred beneath the eastern Gorontalo Basin in a north dipping zone which extends from the Batui Thrust on the East Arm of Sulawesi to about 100 km depth beneath the Gorontalo Basin. The Batui Thrust is the site of thrusting of the Banggai Islands continental fragment beneath the East Arm ophiolite and oceanic crust of the Gorontalo Basin. The observation of a zone of earthquakes dipping to the north from this thrust zone suggests that the leading edge of the Banggai Island block was subducted to at least 100 km depth. The eastern Gorontalo Basin earthquake zone may connect with a deep seismic zone beneath the Celebes Basin. Beneath the western Gorontalo Basin, a very narrow zone of earthquakes dips to the south, probably within lithosphere of the Celebes Basin subducted at the North Sulawesi Trench. Three shallow earthquakes occurred near Lake Matano in central Sulawesi, possibly on the Matano Fault, and their composite focal plane solution suggests east‐west extension. The occurrence of only one earthquake on the Palu Fault and the lack of short S‐P times at a station operated on the fault indicate that this feature was very quiet during the survey period.

Rb–Sr isochron ages, magmatic 87Sr/86Sr initial ratios, and oxygen isotope geochemistry of the Proterozoic lava flows and intrusions of the East Arm of Great Slave Lake, Northwest Territories, Canada

Two Rb–Sr isochrons from the oldest (Wilson Island Group) and one of the youngest (Pearson Formation) Proterozoic volcanic units in the East Arm of Great Slave Lake give dates and initial ratios of 1810 ± 19 Ma, 0.7051 ± 0.0008, and 1809 ± 30 Ma, 0.7089 ± 0.0004, respectively. These dates restrict the Great Slave Supergroup entirely to the Aphebian. The Pearson Formation date is interpreted as magmatic, but it is considered to be rapidly followed by a significant metamorphic and tectonic event within the area. Both the above suites and the volcanic formations of intermediate age have undergone metamorphism up to and including epidote–amphibolite facies. A study of remnant clinopyroxene grains from these formations has indicated general increases in δ18O of whole rocks during epidote–actinolite facies metamorphism of 0.4–2.0‰. The tholeiitic volcanic rocks and intrusions all indicated δ18O values typical of modern continental tholeiites (5.9–7.0‰). The highest δ18O came from those suites that were suspected of having been contaminated by crustal material. The estimated initial 87Sr/86Sr ratios of the magmas, both from clinopyroxene separates and isochrons, indicate a mantle origin for early tholeiitic mid-Aphebian diabase (0.7016–0.7017), Union Island Group diabase (0.7021–0.7030), and Sosan Group alkali volcanics (0.7017). The later Jackson gabbro (0.7050–0.7054) and especially the Pearson Formation tholeiitic basalt (0.7089) both show the effects of significant crustal contamination. The evidence for the Wilson Island Group is less decisive but appears to indicate a mantle origin.

Prentice G. Downes (1909-1959)

Prentice G. Downes was one of the most singular men to travel in the North in the last years before the 1939-45 War. An able man in the wilderness and a gifted cartographer, ethnologist, and naturalist, he is best remembered as the author of Sleeping Island: Story of One Man's Travels in the Great Barren Lands of the Canadian North, a classic of northern canoe travel. ... In a letter to George Douglas in 1943, Downes remarked that his having read Napolean Comeau's Life and Sport on the North Shore of the St. Lawrence a great deal to do with my ever going north, as I was so interested that I set off for the North Shore to find the old gentleman. Comeau had died, but thus in 1935 Downes commenced his northern travels. In 1936 he took passage aboard R.M.S. Nascopie from Montreal to Churchill, during which trip he made copious notes on climate, geography, wildlife, Ungava Eskimo vocabulary, and northern society. From Churchill he flew to Pelican Narrows and with an Indian companion canoed to Reindeer Lake and back again. In 1937 the New England Museum of Natural History sponsored a solo trip by Downes to study the Eskimos of Boothia Peninsula, before which he made his way to Brochet at the north end of Reindeer Lake and investigated the histories, languages, and ways of the Crees and the Chipewyans. This fascination with northern Indians, and above all with the significance of dreams in their cultures, was central to Downes's travels. Crees named him The-man-who-talks-about-dreams. Two of Downes's unpublished writings are a Cree-Chipewyan dictionary and a volume titled The Spirit World of the Northern Cree: Contributions to Cree Ethnology. first of Downes's major canoe trips came in 1938, when he paddled alone from Waterways to Fitzgerald, after which he moved on the Great Slave, the Mackenzie, and Great Bear. ... Sleeping Island trip of 1939 - from Brochet to Nueltin Lake - was followed by another, less triumphant, venture into that region in 1940. Despondent as he was at his failure to reach Kasba Lake by way of the Little Partridge River, Downes could still confide in his journal: Three important routes and one previously unknown river have been worked out. Kasmere Lake is now plotted, both north and east arm. Actually, far more was accomplished than a successful trip through to Kasba would have afforded. Much of the North was as yet imperfectly mapped then, of course, and one of Downes's primary achievements was his meticulous mapping of every obscure route he followed. ...

Petrochemistry of late Aphebian (~ 1.8 Ga) calc-alkaline diorites from the East Arm of Great Slave Lake, N.W.T., Canada

The East Arm of Great Slave Lake is a 2.5–1.7 Ga graben connected to the contemporaneous Wopmay Orogen on the margins of the Archean (2.5 Ga) Slave craton. It contains three major groups of sedimentary and volcanic rocks. The two earlier ones are cut by ~ 1.79 Ga diorites, which outcrop over 220 km along the graben.The diorites were preferentially emplaced as laccoliths into a horizon of megabreccia thought to be the product of evaporite solution and collapse. The diorites are similar down the entire length of the East Arm. Main phases are usually plagioclase–hornblende poryphyritic, but younger and possibly high level phases contain biotite and quartz. The diorites are calc-alkaline but show no obvious chemical trends along the graben. They cannot be related directly to the proposed easterly dipping, late Aphebian subduction zone that generated the Wopmay Orogen.

Mineralogy and paragenesis of hydrothermal mineralisations in the East Arm of Great Slave Lake

Mineralogy and paragenesis of hydrothermal mineralisations in the East Arm of Great Slave Lake

Lead isochron dating of the Seton Formation, East Arm of Great Slave Lake, Northwest Territories

Dating of "spilitic" basalts from the Proterozoic Seton Formation by the Pb/Pb method yields an age of 1804 ± 23 Ma (2σ), in excellent agreement with Rb–Sr data on the same rocks, when the latter ages are calculated with currently accepted decay constants. It seems unlikely that spilitization could have reset both the Rb–Sr and Pb/Pb systems to yield exactly the same ages with no geologic scatter about the isochrons in either case, and, therefore, it may be that the isochrons represent the true age of deposition of the lavas.

Epipelic and epiphytic algal communities in Great Slave Lake

Seasonal changes in the densities of epiphytic and epipelic algae were determined during 1975 and 1976 at two sites on the north shore of Great Slave Lake. The epiphyton waxed rapidly in May and reached maximum densities of 5–6 × 108 μm3/cm2 in June. Although densities remained high during the summer, they fell rapidly in October, followed by a secondary peak in November. The main species were Tabellaria flocculosa, Achnanthes minutissima, Achnanthes linearis, Gomphonema intricatum, and Lyngbya diguetii. The epipelon, consisting largely of Nitzschia palea, Nitzschia dissipata, Fragilaria construens, and Oscillatoria spp., also showed spring and fall peaks in densities, with maximum populations of 4.2 × 108 μm3/cm2 being recorded in June and July. The growing season for both communities was limited by the formation of ice in the fall, whereas the maximum standing crop of the flora was at least partially restricted by low nutrient levels (total phosphorus, 0.008 mg/L; total alkalinity, 30 mg/L).Seasonal changes in the density of epipelon were monitored during 1977 and 1978 at a third site on the north shore of the lake, where total phosphorus and total alkalinity ranged up to 0.010 and 94 mg/L, respectively. Prevalent taxa included Cymatopleura elliptica, Epithemia turgida, Rhopalodia gibba, Cymbella spp., and Nostoc spp. The growth cycles of the flora were again characterized by spring and fall peaks in numbers. Algal densities were, however, high (up to 2.3 × 109 μm3/cm2), possibly a reflection of nutrient availability.Samples of epiphyton and epipelon were taken at an additional seven sites in the east arm of the lake during June and July 1975. The main epiphytic species were Cocconeis placentula, Achnanthes minutissima, and Tabellaria flocculosa, while prevalent epipelic taxa included Navicula graciloides, Fragilaria vaucheriae, Tabellaria flocculosa, and Achnanthes minutissima. Densities were less than those recorded from the three sites on the north shore, again possibly a reflection of nutrient availability.

Water movements, sediment transport and deposition, Western Port, Victoria

Western Port is a tidal embayment, and the characteristics of oscillatory tidal currents, overall net and small-scale circulation patterns, and aspects of the mode, frequency and mass transport of sediment are presented. The results of the studies reported in this volume are both corroborative and complementary, and their integration has resulted in a much better understanding of the bay than could otherwise have been obtained. Consideration of dynamic process, sedimentation patterns and morphology has allowed the distinction of four “systems” within the bay: 1. (1) High-energy channel/bank system (Western Entrance to just beyond the Confluence Zone), characterised by strong tidal currents, varying degrees of surface wave activity, and a sandy substrate. Middle Bank represents the largest individual sand body in Western Port and is associated with intense sediment movement. Sand transported from Middle Bank through the Confluence Zone supplies and maintains the sandy deposits and beaches in the Lower North Arm and East Arm. Seismic evidence coupled with estimates of mass transport of sand suggest that during the Holocene, 10 3 to 10 4 g cm −1 yr −1 have been transported from Middle Bank to Cowes Bank in order to produce the extensive sedimentary complex in that region. 2. (2) Ebb/flood differentiated segments with contrasting intertidal sediments (lower North Arm, Corinella Segment), characterised by clearly differentiated flood- and ebb-dominant zones, sandy and muddy intertidal sediment deposition. 3. (3) Subtidal, partly enclosed mud-deposition basin (Rhyll Segment), characterised as a shallow, subtidal, depositional basin collecting fine sediments. A central clay-size zone is flanked by coarser sediments. Circulation is weak and sediment transport is minimal. 4. (4) Extensive intertidal flat systems with minor channels (Embayment Head), characterised by a web of minor channels, and luxuriant seagrass meadows except in the regions of the Bunyip and Lang Lang Rivers. Local flood- and ebb-dominant transport paths are defined by sandy deposits of reworked or autochthonous Pleistocene sands.

Geology and petrochemistry of lower Aphebian (2.4–2.0 Ga) alkaline plutonic and hypabyssal rocks in the East Arm of Great Slave Lake, Northwest Territories

Two alkaline igneous complexes and three lines of diatreme breccias were emplaced in the East Arm of Great Slave Lake during the lower Proterozoic. Field relationships suggest that those rocks are broadly cogenetic and were emplaced about 2.1 Ga ago.One of the intrusions, the Easter Island dyke, was rotated subsequent to emplacement such that both top and bottom are now exposed. Field and petrographic data are indicative of progressive differentiation along (i.e., up) the dyke and are substantiated by chemical data. The differentiation history of the early gabbros of the Blachford Lake complex is similar. Late differentiates of both complexes closely resemble the igneous matrices of the breccias and petrographic and chemical data support the proposal of cogenesis and contemporaneity.The field data show that there was a period of significant faulting and concomitant alkaline igneous activity in the East Arm area in the lower Proterozoic.

Seasonal changes in the standing crop of an epilithic algal population on the north shore of Great Slave Lake

Seasonal changes in the density of epilithic algae were determined from June 1975 to November 1976 at three sites on the north shore of Great Slave Lake. Densities increased rapidly in May immediately after the disappearance of ice, showed only small fluctuations during the summer, and waned during October. Microscopic algae, mainly Fragilaria capucina, Gomphonema olivaceum, G. ventricosa, Cymbella ventricosa, and other diatoms, reached peak abundance (2 × 109 μm3/cm2) on irregular occasions (July, September, October) throughout the growing season. However, filamentous algae, mainly Ulothrix zonata, reached maximum densities (60 mg/cm2 dry weight) during July of both years.Additional survey collections of epilithic algae were made at 14 sites in the east arm of the lake during June and July of 1975. The most common species included Tabellaria flocculosa, Rhopalodia gibba, Cymbella angustata, C. microcephala, Anomoeoneis vitrea, Achnanthes minutissima, and Ulothrix zonata. Algal densities were considerably less than those recorded from the north shore, a reflection of lower nutrient and alkalinity levels. Maximum abundances for microscopic and filamentous algae were 1.05 × 109 μm3/cm2 and 4.1 mg/cm2 respectively.

Magnetite-apatite-amphibole-uranium and silver-arsenide mineralizations in lower Proterozoic igneous rocks, East Arm, Great Slave Lake, Canada

The East Arm of Great Slave Lake contains a great thickness of Aphebian (2,460-1,750 m.y.) sedimentary and igneous rocks that are little deformed or metamorphosed. Three separate periods of intrusive magmatic activity have been recognized: (1) early Aphebian ( approximately 2,100 m.y.) alkaline complexes and probably related breccia pipes; (2) middle Aphebian volcanism ( approximately 1,870 m.y.) with hypabyssal intrusions; and (3) late Aphebian ( approximately 1,790 m.y.) calc-alkaline diorite stocks and laccoliths. These intrusive rocks all contain examples of the Ag, Ni-Co arsenide ore-type and the later ones particularly contain magnetite-apatite-amphibole with uranium and rare earth element minerals. A model for the genesis of the arsenide ore fluid is postulated from the study of one of the early Aphebian complexes and involves essentially hydrothermal leaching principally of mafic minerals. Models for various modes of deposition are presented and principally involve fluid migration up early joints and deposition in open spaces. The magnetite-apatite-amphibole deposits were emplaced earlier in joints, sometimes passively and sometimes forcefully, and were probably generated as immiscible liquids.The similarity of the ore deposits in the rocks of different ages suggests a common petrochemical process for their formation. The metallogeny of the late Aphebian Great Bear batholith is also closely similar which justifies the assumption of some geotectonic relationship. It has been proposed that the two ore associations may serve as geotectonic fingerprints for ancient subduction zones. These data from the East Arm add no definite evidence for or against the hypothesis but appear only to confuse the issue.

The early history and tectonic significance of the East Arm graben, Great Slave Lake, Canada

The East Arm of Great Slave Lake contains a remarkably preserved sequence of Aphebian (2460–1750 m.y.) strata. The deposition, deformation and preservation of all these strata were controlled by intermittent movements on various splays of the MacDonald fault system. These faults have also controlled the loci of alkaline magmatism throughout the Aphebian. It is shown that the fault system was initiated in late Archean times as a right-lateral strike-slip system and that later movements were always in the same sense. The fault system has always been the fundamental boundary between two separately evolving parts of the Canadian shield—the Slave craton, stabilized at ~2500 m.y., and the Churchill province, stabilized at ~1750 m.y. The model of the East Arm graben as the ‘failed arm’ of a Proterozoic triple junction is found to be inconsistent with the new data concerning the early history of the graben. An alternative model involving ‘permobile tectonics’ is presented.