Depositional Environment

Detailed examination of the New Mountain Sandstone and casual observations of the Windy Gully Sandstone, Terra Cotta Siltstone, Altar Mountain Formation and Arena Sandstone revealed no conclusive evidence of deposition in a marine environment as previously suggested by other workers (e.g., Bradshaw, 1981; Gevers and Twomey, 1982). The majority of the 250 m thick New Mountain sandstone was deposited in an eolian environment, with increasing evidence of water-lain deposition in the upper part of the formation. Deposition in the uppermost 50 m of the New Mountain Sandstone, was likely in a complex intermixture of eolian, fluvial and possibly marginal marine environments.

The lower 50 m of the New Mountain Sandstone contains alternating tabular units of small-scale (0.5 m thick by 1-2 m wide) trough cross-bedded sandstones with pebble lags and units of nested low-angle large-scale trough to tangential cross beds (1-2 m thick by several 10's of m wide) with very thin ('flaggy') page 3 foresets. The tabular sandstone units, up to 2 m thick, extend laterally from several hundred metres to several kilometers. Paleocurrents recorded from trough cross beds indicate sediment transport towards the southeast. The low-angle cross beds, on the other hand, have paleocurrent directions towards the west. The bottom sets of the trough-tangential cross beds as well as the lower 0.5 m of some foresets commonly contain abundant Heimdallia burrows. The remainder of the lower part of the New Mountain Sandstone, contains a complex arrangement of the nested low-angle large-scale trough to tangential cross beds, with rare interludes of the tabular units.

The flaggy foreset beds are overwhelmingly the most common type of cross stratification. The beds consist of well-sorted upper medium- to lower coarsegrained sandstone laminae, generally less than 5 mm thick, with rare cross laminae and preserved ripple forms (with coarse tops and finer bases). Exhumed foreset beds reveal the laminae are oriented at a very low angle across the foreset dip direction. These laminae are climbing translatent strata (Hunter, 1977), the product of wind generated ripples migrating across the slipface of eolian dunes. The flaggy character of the foresets is a result of pin stripe laminations (thin silt-rich laminae), also distinctive of eolian cross beds (Fryberger and Schenk, 1988). Other types of cross beds recognised are grain fall and grain flow laminae. The latter type result from avalanching of non-cohesive sand on dune slipfaces. Grain fall laminae result from settling of suspended particles on the dune slipface.

Both small-scale features and large-scale bedding geometries (cf. Trewin, 1993a and 1993b) suggest deposition in a eolian environment with periodic flooding by braided fluvial systems (tabular units of trough cross bedding).

The middle to upper part (except for the upper 50 m) of the New Mountain Sandstone consists of very large-scale (up to 8 m thick) cross beds. The scoop to gently curving geometry of the cross bed sets suggest some curvature to the crest of the bedform that deposited them, but often the sets can be traced laterally (both along and perpendicular to foreset dip) for at least two hundred metres. Internally the cross beds are also composed of flaggy foresets, with grain-flow and grain-fall beds less common. Slumped foresets and breccias of foreset beds are relatively common on the upper parts of the cross beds. Although generally on the order of metres in scale, one slumped cross bed, up to 5 m thick, could be traced laterally for over 1 km. Another cross-bed type is featureless or massive sandstones, up to 0.5 m thick. Lenticular to tabular shaped, these beds are devoid of sedimentary structures but their external geometries (including scour bases) and faint internal grading indicate they are depositional features and not the product of post-depositional alteration. Laterally these features can be traced into bottomset beds and are often interbedded with the features outlined below, but in some areas intervals of mostly massive beds, up to 10 m thick, are found.

Other bottomset beds consist of thin laminated ripple strata, disrupted and wavy thin laminae, adhesion ripples, wave ripples, mudstone beds (less than 20 cm thick) and desiccation cracks. Together with the massive beds these features suggest some deposition in shallow water as well as damp and dry substrates.

Similar interbeds of eolian strata, with common slumping, and massive sandstones have been reported by Gradzinski and Jerzykiewicz (1974) and page 4 Eschner and Kocurek (1986). The latter study describes coastal dune deposits flooded by trangressive seas. However, the New Mountain Sandstone contains no body fossils, and in the upper interval bioturbation is generally rare, except for trackways, which are generally found on (eolian) foreset beds. The formation described by Gradzinski and Jerzykiewicz (1974), interpreted as eolian deposits interbedded with sediments of intermittent rivers and lakes, is more akin to the New Mountain Sandstone beds. Thus it is inferred that the upper New Mountain Sandstone was deposited in a eolian setting with occasional inundation by a river system, resulting in flooding of the interdune areas and slumping of sand off the fronts of the eolian dunes into the ponded water.

The upper 50 m of the New Mountain Sandstone contains primarily low angle coarse beds, 1-2 m thick tangential cross bed sets and subhorizontal bedding. Skolithos bioturbation, commonly obliterating most of the sedimentary structures, is widespread in this interval. A regional and dramatic change in paleocurrent direction, back to an orientation similar to the lower tabular (fluvial) units, suggests a continued, but overwhelming, influence by the river system.