Fossils down under or finding fossils in boreholes – Deposits

Dr Susan Parfrey (Australia)

It’s possible you’ll be aware of accumulating fossils from eroded rock surfaces, a riverbank, a highway or rail reducing, a cliff face or a recent lower floor comparable to a quarry. However there may be one other approach fossils might be recovered – from boreholes. Boreholes have been drilled in Queensland for a few years for exploration and to research the regional geology. Since Queensland is a comparatively flat a part of Australia, outcrops might be onerous for geologists to seek out. Subsequently, drilling gives a approach of learning sub-surface geology that assists within the understanding of the stratigraphy of the State. Usually, drilling does not produce usable macrofossils.

The process of drilling normally involves pushing a mud mixture down the borehole and over the bit, for cooling and lubrication. In this process, larger fossils are forced to the surface in the drilling mud and are broken into small pieces making them impossible to identify. However, microfossils can be recovered, as they are so small they are undamaged by the drilling process and are recovered at the surface to be identified and used in biostratigraphy (to date and correlate rocks). However, in Queensland, drilling was undertaken by the Geological Survey that retrieved lengths of core which provided access to deeply-buried strata and allowed recovery of both micro and macrofossils. Exploration companies also often retrieved short lengths of core at specific levels of interest, which sometimes contain fossils.

Before a borehole is drilled, considerable geological mapping of the surface is undertaken. Only then does the geologist flag a place where the borehole should be started or “spudded”. The drilling rigs are normally transported to the site on trucks.

The usual diameter of a borehole is 76mm, which produces a core diameter of 46.5mm, often called an NQ core. Larger cores are better if you want to see the rock structure and collect fossils, but the size of core has to be balanced against the capacity of the rig and the higher cost of drilling.

The advantage of retrieving fossils from cores is that you can see specimens that might have taken millions of years to be uplifted and exposed at the surface. They are fresh, unweathered and have not been transported after exposure. The disadvantage of boreholes is that their relatively small diameter means fossils are often cut in half. Although the fossils retrieved are often incomplete, they have allowed some of the most extraordinary fossils to be recovered, providing information that would not have been otherwise available.

One of the luckiest finds was a 100 million-year-old fish that was split out of a core that came from 615m below the surface from a bore drilled near Maneroo in western Queensland (Fig. 2). The tiny fish was even curled around, so it neatly fitted into the 63.5mm diameter core. It is rare to find a fish that is so complete. When it first came out of the rock, it was gold in colour as it was preserved as pyrite. The fish is a new Actinopterygii (ray-finned fish), which is currently being described. In contrast to today’s vast open plains, the sea covered this area of western Queensland during the Early Cretaceous (Albian).

Beautiful Cretaceous ferns were recovered from Government bores that were drilled before 1919 in the search for coal in central Queensland. These include the graceful fronds of ferns such as Sphenopteris (Fig. 3) and Cladophlebis (Fig. 4), which have an almost worldwide distribution.

Brachiopods occurred in large numbers during the Permian. This abundance can be seen in drill core that cut through these beds.

The large number of shells must have formed wave-resistant banks, equivalent to today’s reefs. A core taken from one of these accumulations can also give information about the environment by considering the position in which the shells are deposited.

The shells of brachiopods such as Echinalosia (Fig. 6.) shown in the core disarticulate quickly after death.  At the top of the core there are overturned shells that are both articulated and disarticulated, which indicates they were affected by current action. In the middle of the core the environment must have been higher energy to produce shells that are broken and disarticulated. The lower part of the core contains shells that must have been buried rapidly as they are articulated and in life position.

The preservation of long spines attached to a shell also means it could not been moved by wave action after death, as spines are easily abraded (Fig. 7). The large brachiopod, Taeniothaerus (Fig. 8), has been cut down the middle by the drill bit. It shows a cross-section of the two valves.

The section shows a broad body cavity and that the shell bends at a distinct angle along its length, forming a long trail. As this type of brachiopod was a filter feeder that lived on the surface of the mud, it had to find a way of keeping sediment out of the shell when it fed. The long, flattened extension of the shell displays various ways it used to exclude the sediment. The large spines on the outside excluded large particles, the long flat section would allow medium size particles to settle and, on the inner surface, are small spines that would trap the small particles (Fig. 9), allowing only food to enter. The large rhizoid (root-like) spines on other parts of the valve were used to stabilise the shell and stop it from sinking in the mud.

The strongly ribbed, Spiriferida, Tomiopsis (Fig. 10), is larger than the core diameter and so was cut in three places by the drill bit. Its high, central fold would have allowed it to open the valves and filter feed without allowing mud to invade the shell.

The trilobite shown in Fig. 11 appears that the bit has cut through this Late Cambrian trilobite leaving only the pygidium (tail section). In fact, that was all that was there – perfect, complete specimens are rare. To grow, trilobites had to moult their calcite exoskeleton periodically, so separate segments of their shell are the most common things found. This specimen shows marginal spines apparently used for protection.

Reference

Parfrey, S.M. 1990 Catch of the day. Queensland Government Mining Journal 91, 276.