Trace Fossils

Inspection of the lower Taylor Group resulted in several important observations that complement previous work of others, primarily the extensive study of Bradshaw (1981). Previously reported Skolithos, vertical to steeply inclined linear cylindrical burrows about 3-5 mm diameter, were discovered to be commonly (20-40%) curved. Strongly curved burrows may deviate from the vertical by as much as 45° for lengths up to 5 cm long. Because of the bending and grouping of burrows a braided pattern is commonly observed. However, whether the burrows are actually branched or whether they appear to branch because of the intersection of two or more burrows, could not be determined. Research into previously described Skolithos is currently being undertaken to determine if the curved tubes can be considered a feature of this type of trace fossils, or whether the burrows are of another variety. Furthermore, in several instances Skolithos was found in the same bedding units as Diplichnites, an occurrence not previously reported.

Diplichnites trackways occur throughout the New Mountain Sandstone, primarily on foresets (cross beds), some with up to 30° dip. In several examples the trails are very well preserved, with well-defined imprints (common), pushed piles of sediment at edges of individual prints (common), slide marks of the imprints (abundant), and tail drag (rare) indicating the trackways are primary (non-reworked) features. However, in many trackways, recent weathering of the outcrops has removed much detail. These conclusions differ from Bradshaw (1981), who suggested current reworking of trails. Furthermore, we believe that the preservation and clarity of many trackways is a result of eolian depositional processes (i.e., firm substrates a result of wind hardening) and not binding of subaqueous marine sands by algae (??Bradshaw, 1981; Gevers and Twomey, 1982). The trackways are nearly always preserved on foresets of ripple translatent strata, which apparently consists of very well packed sand. In a few instances, trackways were found on grain-flow cross beds, and the relatively loose-packing of the bed is manifested by the relatively deep imprint and the page 5 diffuse edges of the trail.

A new type of trackway has been discovered which consists of prints previously described by Bradshaw (1981) as 'large isolated prints'. The trails, about 5 cm wide, consist of tear- to horseshoe-shaped prints up to 3 cm wide and 1 cm deep, that are tapered towards the inside of the trackway. Whether this type of trail has been previously described elsewhere is currently being investigated.

Another previously unrecognised trace fossil, 'pit and pile' structures, are common in bottomset beds of the very large-scale foresets in the New Mountain Sandstone. These features consist of a elongate pit (about 2-3 cm deep, 4-5 cm wide and up to 20 cm long) with a small (less than 2 cm tall by about 5 cm diameter) adjoining pile of sediment. Most occurrences were isolated, but on one bedding plane exposure about 20 of the features were found. The form of these clearly biogenic structures suggest they are excavation features. No trackways or other trace fossils appear to be associated with the pit and pile features.

Heimdallia burrows usually occur in the interdune areas, and also along reactivation surfaces on the foreset beds (extending upwards to less than 20 cm). The intensity of bioturbation attests to the slow rates of deposition of the New Mountain Sandstone. Heimdallia is associated with Agrichnium, epichnial grooves (Bradshaw, 1981), 'hairpin' beds (type H trace fossils of Plume, 1976), Diplichnites, cylindricum, and Skolithos (rare). Although frequently homogenised by bioturbation, the interdune deposits in the Heimdallia-bearing interval are generally devoid of mudstone beds and thick (<5 cm) massive or featureless sandstones. This may indicate that the organism(s) responsible for creating the Heimdallia burrows was not tolerant of subaqueous environments.