The Colonization Of Deeper Water During The Vendian

The Vendian era, extending from ~610-545 Ma (Grotzinger et al. 1995), commences with the Varanger tillites and their equivalents and is the first to yield relatively common and diverse undisputed body fossils and trace fossils.

The oldest Vendian biota, consisting of Nimbia, Vendella?, and Irridinitus?, was found in the intertillite Twitya Formation of the Mackenzie Mountains, Canada (Hofmann et al. 1990). This sequence comprises siliclastic turbidites associated with major channel-fill conglomerates and is considered to be relatively deep-water (Hofmann et al. 1990).

The majority of post-tillite Vendian biotas have been found in shallow-water sequences, apparently deposited above fair-weather wave base, and in some regions (e.g., Australia, Namibia, Russia, Ukraine), remarkably abundant, diverse, and well-preserved faunas have been found (see reviews in Glaessner 1984; Sokolov and Iwanowski 1985; Fedonkin 1992; Jenkins 1992). Indeed, in some sequences deposited

Proterozoic Ocean
Figure 13.1 "Snapshots" of the ocean floor faunas for Middle Proterozoic, Vendian, and Cambrian, showing the progressive colonization of deeper water based on body fossils.

under varied depths of water, fossils occur only in the shallower-water lithologies. For example, in the Tanafjorden area of Norway, the Vendian Innerelv Member consists of two shallowing-upward sequences, each representing a transition from offshore marine (quiet basin, below wave base) to wave-influenced, shallow, subtidal and intertidal deposition (Banks 1973), but a biota consisting of Cyclomedusa, Ediacaria?, Beltanella, Hiemalora, and Nimbia? occurs only in sediments interpreted as representing a current-swept, wave-influenced environment (Farmer et al. 1992).

There are, however, a few well-documented examples in which body and/or trace fossils do occur in deeper-water deposits (figures 13.1B and 13.2A). In the case of body fossils, it might be possible to claim that they have been transported from shallow water, but such an argument cannot be applied to trace fossils, which reflect life activity at the precise location where they are now found.

In the Wernecke Mountains, Canada, Narbonne and Hofmann (1987) record a fairly extensive Ediacara fauna, most of which comes from Siltstone Units 1 and 2, deposited under shallow-water conditions. This includes the body fossils Beltanella, Beltanelliformis, Charniodiscus, Cyclomedusa, Kullingia?, Medusinites, Nadalia, Spriggia, and Tirasiana, as well as the trace fossils Gordia, Neonereites?, and Planolites. However, Charniodiscus was also recorded from the Goz Siltstone, which includes slump and load structures and was deposited on a slope in a deeper-water setting.

A more extensive deeper-water biota has been described by Narbonne and Aitken (1990) from the Sekwi Brook area of northwestern Canada, where the Sheepbed and Blueflower formations include turbidity current-deposited sandstones and common slump deposits and are interpreted as representing a deep-water basin slope setting, below storm wave base. The biota includes the body fossils Beltanella, Charniodiscus?, Cyclomedusa, Ediacaria, Eoporpita, Inkrylovia, Kullingia, Pteridinium, and Sekwia and the trace fossils Aulichnites, Helminthoida, Helminthoidichnites, Helminthopsis, Lockeia, Neonereites, Palaeophycus, Planolites, and Torrowangea. More recently, Hiemalora and Win-dermeria have been reported from the same sequence (Narbonne 1994).

Pteridinium has also been recorded from the South Carolina Slate Belt in deep-water, thinly bedded to finely laminated pelites and siltstones of the Albermarle Group, which may have been deposited between 586 and 550 Ma (Gibson et al. 1984). This sequence has also yielded the trace fossils Gordia, Neonereites, Planolites, and Syringomorpha (Gibson 1989).

Surfaces covered with numerous predominantly frondlike and bushlike Ediacaran body fossils, including Charnia and Charniodiscus, occur within volcaniclastic turbi-dite sequences interpreted as deep-water submarine fan and slope deposits (Myrow 1995) within the Conception Group on the Avalon Peninsula, Newfoundland, Canada (see Anderson and Misra 1968; Misra 1969; Anderson and Conway Morris 1982; Conway Morris 1989a; Jenkins 1992). Taphonomic and sedimentological data indicate that this is an in situ life assemblage that suffered rapid burial by volcanic ash at

Figure 13.2 "Snapshots" of the ocean floor faunas for Vendian, Cambrian, and Ordovician, showing the progressive colonization of deeper water based on ichnofossils.

some horizons (Jenkins 1992; Seilacher 1992; Myrow 1995). The turbidites may not have formed at truly oceanic depths but perhaps on a continental terrace (Benus 1988; Jenkins 1992). A broadly similar setting has been postulated for the occurrence of Charnia, Charniodiscus, and Pseudovendia within a Vendian sequence at Charnwood Forest, Leicestershire, England, where Jenkins (1992) suggests that the frequency of slumps, together with some current rippling and an absence of oscillation ripples, implies deposition on a slope environment below storm wave base. Boynton and Ford (1995) record three new genera from this sequence (Ivesia, Shepshedia, and Black-brookia), but conclude that, despite the presence of graded bedding and absence of shallow-water indicators, water depth may be little more than wave base.

The classic sequence at Ediacara, Australia, which has yielded an abundant and diverse nonskeletal fauna, has been interpreted by Gehling (1991) as deposited in an outer shelf setting below fair-weather wave base, with burial of the organisms by storm surge sands. Seilacher (in Jenkins 1992: 152) considers that the common occurrence of wave oscillation and interference ripples suggests deposition on the shoreface, albeit perhaps by storm events, and a shallow-water tidal environment also seems indicated by the large polygonal desiccation cracks in the highly fossiliferous parts of the section (Jenkins 1992: 153).

Evidence of life at truly bathyal depths is largely absent during the Vendian, although records of the trace fossil Planolites within the deep-sea turbidite sequence of the South Stack Formation of the Mona Complex on Anglesey, Wales, by Greenly (1919) have been substantiated during recent fieldwork. The age of these rocks is debatable, but radiometric dates on intrusive granites suggest that it is greater than 600 Ma (Shackleton 1969).

The conclusion appears to be that while most Ediacarian body and trace fossils from the prolific localities in Australia, Namibia, Russia, and Ukraine occurred in shallow-water environments at or above wave base, other localities, including Charnwood Forest, Newfoundland, Sekwi Brook, and Wernecke Mountains, show features suggestive of a slightly deeper-water environment below storm wave base, mostly on the continental slope. There is not, however, any evidence of significant colonization of truly oceanic depths during the Vendian.

Such colonization as took place in intermediate water depths was dominated by sessile body fossils (e.g., Cyclomedusa, Ediacaria) and detritus-feeding animals that produced traces either on muddy substrates (e.g., Helminthoida, Helminthopsis) or at very shallow depths (e.g., Paleodictyon). Significant bioturbation did not occur until the Early Cambrian (Crimes and Droser 1992). In present-day oceans, faunas inhabiting muddy substrates are more abundant and diverse than those of sandy areas (Menzies et al. 1973), whereas in these ancient seas, the absence of algae and the scarcity of large animals increased the survival possibilities of the detritivorous trophic group (Sanders and Hessler 1969; Sokolova 1989).

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