MARITIME SEDIMENTS AND ATLANTIC GEOLOGY 37 Trace fossils in a Lower Palaeozoic submarine canyon sequence - the Siegas Formation of northwestern New Brunswick, Canada R.K. Piakerill3 Department of Geology University of New Brunswick, Fredericton, New Brunswick* Canada E3B 5A3 The Siegas Formation (Silurian - early Llandovery) of northwestern New Brunswick represents, In part, an ancient example of a submarine canyon sequence, erosive into and partially coeval with the Carys Mills For- mation of late Ordovician—middle Llandovery age. The canyon succession contains a moderately diverse ichno- fauna consisting of Buthotrephie, Chondrites, Cochliohnue anguineue, Diplichnitee, Fucueopeie, Gordia arauata, Gordia, Gyrochorte, Helminthoida labyrinthica, Helminthopeie of. abeli, Neonereitee bieerialie, Neonereitee unieerialie, Planolitee beverleyeneie, Protopaleodictyon, Scalarituba mieeourieneie and Skolithoe. These trace fossils are described in detail. Contrary to previous predictions that dwelling burrows and associated traces would predominate in sub- marine canyon sequences, the Siegas Formation contains traces produced essentially by vagile sediment eaters. This Is perhaps best explained by the absence within the canyon, at least for the majority of time, of suf- ficiently strong bottom currents, which precluded inhabitation by an active and abundant suspension feeding population. Instead, the sediments probably contained sufficiently abundant organic detritus to support an active population of infaunal sediment feeding organisms. Situee au nord-ouest du Nouveau-Brunswick, la formation de Siegas (Silurien — Llandovery inferieur) rej>- resente, en partie, un exemple ancien de sequence de canyon 60us-marin; elle recoupe la formation de Cary6 Mills (Ordovicien superieur —Llandovery moyen) et est en partie contemporaine a cette derniere. On rencon- tre dans la sequence une ichnofaune moderement diverse comprennant Buthotrephie, Chondrites, Cochliohnue anguineue, Diplichnitee, Fucueopeie, Gordia arauata, Gordia, Gyrochorte, Helminthoida labyrinthica, Helmin- thopeie of. abeli, Neonereitee bieerialie, Neonereitee unieerialie» Planolitee beverleyeneie, Protopaleo- dictyon, Scalarituba mieeourieneie et Skolithoe. Ce6 traces fossiles sont decrites en detail. Selon les predictions anterieures, les terriers d'habitation et les traces connexes devraient predominer dans les sequences de canyon sous-marin mais contrairement a ceci, la formation Siegas contient plutot des traces produites essentielleinent par des ingesteurs de sediment vagiles. Au mieux, ceci pourrait s'expliquer par 1*exclusion d'une population active et abondante d'animaux filtreurs, due a 1*absence dans le canyon, au tnoins la plupart du temps, de courants de fond significatifs. Au lieu de cela, les sediments devaient con- tenir suffisamment de detritus organiques pour accomoder une population active d* organismes endobiontes limnivores. (Traduit par le journal] INTRODUCTION associations and sediment emplace- Oceanographic research has de- ment mechanisms (e.g. Stanley and monstrated that penetration and Bertrand 1979). stirring of sediment by benthic One exception is the recent do- and other organisms is a common cumentation by Hayward (1976) of phenomenon in modern submarine the trace fossils from a Miocene canyons (e.g. Shepard and Dill submarine canyon sequence in New 1966, Trumbull and McCamis 1967, Zealand. Hayward described Tigil- Dillon and Zimmerman 1970, Heezen lites, Saalarituba3 Planolite s and Hollister 1971, Scott and and branching "Planolites"*, from Birdsall 1978, Valentine et al. the unconsolidated sediments of 1980). Yet little research has the canyon floor and Tigillites, thus far been undertaken on the Rhizooorallium and several varie- nature of trace fossils and trace ties of horizontal and inclined fossil assemblages in ancient sub- branching burrows from the semi- marine canyon sequences. In part consolidated but unlithified can- this has arisen from the inherent yon walls. More recently Crimes difficulties of recognizing an- (1977) produced a table which, in cient submarine canyons but also part, recorded both the known and by research workers placing more anticipated ichnofaunas of ancient emphasis on their detailed facies submarine canyon sequences. This * Strictly speaking branching forms are I available for the ichnogenus Planolites (Hantzschel 1975). MARITIME SEDIMENTS AND ATLANTIC GEOLOGY 17, 36-58 (1981) 38 R.K. PICKERILL paper is therefore intended to Clastic Belt and/or Aroostoock- document the trace fossils occur­ Matapedia Carbonate Belt by Ayrton ring in an ancient submarine can­ et at. (1969). In terms of re­ yon sequence and to add to the cently proposed plate tectonic data presented by Crimes {ib id .) models (Rast and Stringer 1980) so that eventually more realistic the trough represents a remnant and predictable models will be of the Lower Palaeozoic Iapetus realized. The definition of a Ocean. submarine canyon adopted here is The Siegas Formation is of res­ the same as that proposed by tricted areal extent, persisting Heezen and Hollister (1971) i.e. some 11 km to the north and north "any persistent valley found on west of Siegas and 9.5 km to the the continental margin regardless south and southeast. In thickness of shape or size", or alternatively it ranges from between c. 107-244 but less specifically, following m in the Siegas area to zero at Bates and Jackson (1980), a "gen­ its northwestern and southeastern eral term for all valleys of the limits where it passes into coeval deep-sea floor". strata of the Carys Mills Forma­ tion (Hamilton-Smith 1971a).Both STRATIGRAPHIC BACKGROUND this latter formation and the Strata under consideration in Siegas Formation are conformably this paper are referred to as the overlain by the Perham Group of Siegas Formation of Hamilton- post-early Llandovery to early Smith (1970), which crops out in Ludlow age (Hamilton-Smith 1970, the Siegas area of northwestern Roy and Mencher 1976). New Brunswick (Fig. 1). The for­ Fragmentary brachiopod faunas mation occurs within a (now) rela­ recognized by Ayrton et at. (1969) tively narrow northeast-southwest have indicated an early Llandovery elongate trough which extends from age for the Siegas Formation. Gaspe through New Brunswick and into Maine and termed the Central SEDIMENTOLOGY AND DEPOSITIONAL ENVIRONMENT The sedimentology and deposi- tional environment of the Siegas Formation has been studied in de­ tail and succinctly described in a series of papers by Hamilton- Smith (1971a, 1971b, 1980). This section presented below therefore summarizes the major conclusions of Hamilton-Smith together with observations made by the present author. In view of the purpose of this paper, only a brief sum­ mation will be presented here, and the interested reader is re­ ferred to the above papers for a more detailed appraisal and for detailed geological maps of the area. Detailed mapping of the Siegas west New Brunswick and northeast Maine. area by Hamilton-Smith (1970) de- MARITIME SEDIMENTS AND ATLANTIC GEOLOGY 39 monstrated that in the north and erosive (a maximum of 2 m having northwestern part of the area the been observed), normally graded Siegas Formation was erosive into and/or imbricated and contain the underlying Carys Mills Forma- clasts up to 80 cm in diameter. tion. The depth of erosion was In others, the preferred clast estimated to be in the order of orientation is essentially paral- between 25-50 m, i.e. comparable lel to bedding, similar to those to known erosional depths of many recently described by Keith and present day small submarine can- Friedman (1977) from canyon and yon systems (see Shepard and Dill slope channelized and, or, sheet 1966, Whitaker 1976, etc.), and flow deposits from the Cambrian much larger than reported modern of New York and Vermont. and ancient inner and middle fan channels (see Walker and Mutti The graded and imbricated varie- 1973). This, combined with a ties are interpreted as the pro- detailed sedimentological apprai- duct of deposition from turbulent sal, led Hamilton-Smith (1971b) flows (turbidity currents) whereas to suggest that in the north and the other varieties were probably northwestern part of the area, produced by various forms of mass the Siegas Formation represented flow, such as debris flow or from a submarine canyon-fill. Indeed, incoherent slumping to a complex the absence of thinning and fin- sliding of relatively coherent ing upward sequences and thicken- lenticules one over another\with ing and coarsening upward sequen- most of the shear stress concen- ces, characteristic of mid-fan trated near lenticule margins and suprafan lobes respectively, Hamilton-Smith 1980). Conglomer- combined with the nature of the ates resembling those described sediments, do suggest deposition briefly here have been mentioned in an extremely 'proximal' environ- in the literature quite exten- ment comparable to many modern- sively (see Walker and Mutti 1973 day and ancient submarine canyon and Walker 1975 for a review) and systems (see Middleton and Bouma are considered by the majority of 1975, Whitaker 1976, Stanley and authors to be characteristic of Kelling 1978 and Doyle and Pilkey submarine canyon or inner fan 1979). channel deposits. This lithofacies association constitutes some 13% of the succession (Hamilton-Smith Although it is difficult to 1970). generalize on the nature of these (ii) Medium - coarse sandstones sediments, particularly as there up to 2.75 m in thickness and are demonstrable proximal-distal