Fortune Head is a minor headland located approximately 1.6 kilometres west of the town of Fortune on the southwest tip of the Burin Peninsula. A portion of the rock section exposed along the cliffs at Fortune Head has been designated by the International Union of Geological Scientists (IUGS) as a global stratotype representing the boundary between the Precambrian era and the Cambrian period, approximately 530 million years ago (Narbonne, 1987).
The Precambrian-Cambrian boundary marks a fundamental change in earth history, the first appearance of skeletal and bioturbating organisms. A significant "modernization" of marine biotas occurred during the Precambrian-Cambrian bounder interval. Trace fossils which exhibit a more efficient utilization of food became more diverse and abundant during this interval. Comparable trends in diversity and abundance during the appearance of skeletalized animals reflect the development of metazoan body plans and higher level taxa, ecological niches and protective stratrgies, that characterize Phanerozoic marine communities (Narbonne et al., 1988).
Three candidate stratotypes have been suggested for the Precambrian-Cambrian boundary. They include rock sections in Siberia (Russia), Meischucum (China) and Fortune Head (Newfoundland, Canada). The rock section exposed at Fortune Head has a number of distinct advantages: its extreme accessibility; the apparent absence of disconformities of marked faces changes in the boundary interval; and the presence of a distinctly Precambrian fossil assemblage in the underlying strata (Harleniella podolica zone). Because the siliciclastic deposits in which the fossils of Fortune Head occur are common for this period throughout the world, this horizon has excellent prospects for global correlation (Narbonne, 1987).
At Fortune Head, fossils are abundant and include trace fossils, small shelly fossils, vendotaenid algae, soft-bodied megafossils and microfossils. The Precambrian-Cambrian boundary is placed at the first appearance of complex Phanerozoic-type trace fossils (Phycodes pedum zone), which approximately corresponds with the first appearance in the section of simple small, shelly fossils (Sabellidites cambriensis interval).

Regional Geology

The Burin Peninsula of southeastern Newfoundland forms part of the Avalon Zone, one of four major tectonostratigraphic subdivisions of the Appalachian Orogen in the Province. The Avalon zone was originally part of Gondwana, a continental plate that included parts of Europe and Africa. This plate was situated along the southern side of the ancient ocean, Iapetus. The geology of the Avalon zone records the effect of Late Precambrian and Early Palaeozoic Appalachian Orogeny which has been linked to the generation and destruction of the ancient Iapetus Ocean (H. Williams, 1979).
There are stratigraphic variations across the Avalon zone. On the southern Burin Peninsula the sequence begins with the Burin Group, a series of mafic pillow lavas, volcanigenic sediments, shales and limestones. A 1500 metre thick sill, the Wadsworth Gabbro, dated at 763 +/- 2 Ma intrudes the Burin Group, a thick sub-aerial succession of bimodal volcanics, mostly acidic pyroclastics, and subordinate volcaniclastic sediments. On the northern part of the peninsula, rocks equivalent to those of the Marystown Group have been dated at 608 +/-25 Ma (U-Pb in zircons, Odom, 1980).
The Marystown Group is overlain disconformably by a continuous sequence of siliciclastic rocks that span the Precambrian-Cambrian boundary; in ascending order, the Roncontre, Chapel Island and Random formations. The redbed conglomerates, sandstones and shales of the Roncontre Formation were deposited in a variety of fluvial and marginal marine settings. The sandstones, siltstones and minor limestones of the Chapel Island Formation were deposited in nearshore and shelf environments. The sandstones, shales and quartzites of the Random Formation were deposited in tidally dominated nearshore and shoreline environments.
The Random Formations is the first regionally correlatable unit in the Newfoundland Avalon zone. It was deposited during a marine transgression (Anderson, 1981; Hiscott, 1982). In the Random Island area, the Random Formation is disconformably overlain by a sussion of fossilerous shales and carbonates of Early Cambrian area, comprising in ascending order: The Bonavista, Smith Point and Brigus formations. However, on the southwest corner of the Burin Peninsula, the Bonavista and Smith Point formations are absent and the upper part of the Brigus formation rests disconformably on the Random Formation (Myrow et. al., 1988). The Brigus Formation is overlain disconformably by the Camberlain's Brook Formation, which is Middle Cambrian in age and consists of marine siliciclastic sediments. The Manuels River Formation, composed predominantly of grey to black shales, lies conformably upon the Chamberlain's Brook Formation.

Geology of the Reserve Site

At the Fortune Head site a 410 metre thick section dating from the Late Precambrian to the Early Cambrian is superbly exposed in a continuous series of low coastal cliffs. The beds dip NW at 15 - 46 degrees with dip increasing upsection; a few faults are present, but marker horizons allow for easy correlation across them (Narbonne et al., 1988).

Stratigraphy

A continuous section through the uppermost part of member 1 and all of member 2 of the Chapel Island Formation is exposed at Fortune Head. This section is proposed as the global stratotpye for the Precambrian-Cambrian boundary. The boundary horizon is located 2.4 metres above the base of member 2 of the Chapel Island Formation.
Bengtson and Fletcher (1983) divided the Chapel Island Formation into five informal members numbered 1-5, based on outcrops studies at Grand Bank and Little Dantzic Cove. This same scheme was also used by Crimes and Anderson (1985) who studied the same outcrops. The first detailed measured sections of the formation were prepared by Myrow et al., 1988. Errors in thickness were corrected and member boundaries were defined. Based on these more detailed studies, the lower boundary of member 3 has been moved upward from its former position to a point much higher in the formation. The old boundary between member 2 and 3, which now lies within member 2, is retained for historical reasons and separates the member into two parts, 2A and 2B. The early estimate of 50 metres for member 2 have been increased to approximately 430 metres; 265 metres for 2A and 165 metres for 2B.
Studies by Narbonne et al., (1988) and Narbonne and Myrow (in Landing et al., 1988) indicate that three distinct statigraphically- restricted trace fossil zones can be identified in the Chapel Island Formation; Harlaniella podolica zone, Phycodes pedum zone and the Rusophycus avalonensis zone. Each zone can be readily correlated with Crimes (1987) global ichnofossil zones, and each is named for a common ichnospecies which is restricted to, or first appears in the zone.
The Harlaniella podolica zone characterized member 1 and the basal 2.4 metres of member 2. Fossils are sporadically present within this portion of the section. Trace fossils are simple forms consisting of sediment filled nonbranching burrows (Planolites), Irregularly branching burrows (Buthrotrephis), and meandering burrows (Gordia) as well as, the simple subhorizontal burrows (Harlaniella and Palaespascichnus), which are particularly valuable since they have a restricted range. Well preserved examples of carbonaceous mall shelly fossils Sabellidites cambriensis, and Vendotaenid algae Tyrasotaenia are also present. This fauna suggests a later Pre-cambrian Age.
The beginning of a Phycodes pedum zone can be identified 2.4 metres above the base of member 2 and is approximately 150 metres thick. This point has been proposed as a candidate stratotype point and the horizon of the Precambrian - Cambrian boundary (Narbonne, 1987). The proposed boundary occurs within a continuous and relatively uniform succession of subtidal strata. Typical Vendian ichnofossils, such as Harniella, and Palaeopascichnus, are absent from this zone. It includes arthropod traces (Monomorphichnus), vertical dwelling burrows (Skolithes Arenicolites), coelenterate resting burrows (Conichnus, Bergauria), the complex feeding burrow Gyrolithes and perhaps the most useful index fossil, Phycodes pedum, which defines the base of the zone and occurs at several higher levels.
Trace fossils of the Rusophyus avalonensis zone which first appear 135 metres above the base of member 2 and higher strata of the Chapel Island Formation. The zonal assemblage includes all ichnogenera (previously cited present in the Pycodes pedum zone. The zone is marked by the first appearance of additional arthropod traces (Rusophycus, Cruziana, Dimorphichus), dwelling burrows (Diplocraterion) and complex feeding burrows (Taphrhelminthopsis, Helminthoida). The organic-walled, worm like megafossils, Sabellidites cambriensis, first occurs a few metres below the top of member 1 and is found in the dark shaley units throughout the lowest 140 metres of member 2. The lowest occurence of small calcareous shelly fossils is in the upper part of member 2, approximately 600 metres above the base of the formation where thin tapering mud-filled tubes of the fossil Circotheca have been recovered (Narbonne et al., 1988). Carbonaceous impressions of vendotaenid algae and impressions of medusoid coelenterates occur sporadically throughout member 1 and the lower half of member 2.

Depositional Environment

The sediments of the entire Chapel Island Formation were deposited in shallow or relatively shallow water in a variety of environmental settings. Frequent or periodic subaerial exposure occurred in the basal succession where there are mud cracks, and member 4 contains stromatolites indicating intertidal or perhaps very shallow subtidal water depths. The intervening members were desposited in slightly deeper water but still within tidal tidal influence and include offshore deposits and a possible prograding delta-front sequence. The generalized environmental distribution of the trace fossils within this formation is depicted in Figure 9. In the higher energy environment, vertical burrows predominate (e.g. Arencolites, Skolithos) and a few complex feeding burrow systems (e.g. Phycodes) also occur in this setting. Quieter water, tidally influenced, off-shore and deltaic sediments have a variety of more complex traces normally only associated with much deeper waters (e.g. Helmnenthoda nereites) (Crimes and Anderson, 1985).