Berrara Beach
The beach is a nice place to visit, lots of rock pools, sand and viewing platforms. The rocks and sand also support some impressive history, notably the coastline of Gondwana. The EC is a low tide visit to see the thousands of fossils clearly visible on the rock shelf.
The Gondwana Coast:
It has been defined as that section of the SE Australian coastline that previously formed the coast of the ancient supercontinent of Gondwana. Early in the Permian period ~295Ma to ~269 million years ago (Ma), the sea level rose and began to cover the eastern edge of Gondwana and, as a result, fossiliferous sediments were deposited on the seafloor along the ancient Gondwana coast. The resulting rock sequence which attained a thickness of ~800m is known by geologists as the ‘Shoalhaven Group’. The Gondwana Coast’s abundant shallow water marine invertebrate fossils have been recognized by palaeontologists as ‘world class’.
How old are the rocks in the Harbour’s cliffs and rock platforms?
The rock unit exposed in the Harbour, the Wandrawandian Siltstone formation, is part of the Shoalhaven Group.
This rock unit is ~180m thick and is composed essentially of richly fossiliferous siltstone, with lesser amounts of mudstone and a few much harder, more massive, sandstone beds. The formation is known to extend from rock platforms south of Ulladulla, for almost 80km along the coast to Black Head at Gerroa in the north, and inland to Kangaroo Valley.
Its age has been determined as mid Permian by correlation with sedimentary strata (using fossils) at other locations where rocks had been dated radiometrically by determining the age of volcanic ash deposits above and beneath the fossiliferous layers. This isotopic dating confirms that the Wandrawandian Siltstone was deposited between ~273Ma and 269Ma.
The Permian period began at 299Ma and ended 251Ma with the biggest mass extinction of life recorded anywhere in the fossil record – 95% of all marine species (including many of the fossils found here) and 70% of all terrestrial organisms became extinct.
The spectacular landscape of the South Coast of NSW has largely been formed by erosional forces over a long period. The Southern Highlands plateau and sandstone escarpment west of Ulladulla was uplifted at about 60Ma. During the period ~80Ma – 60Ma the Tasman Sea opened up splitting off the New Zealand sub-continent. Uplift of the Highlands has been largely attributed to the rebounding of the edge of our continent following this stretching event. Since that time the land surface has been progressively worn down,although another uplift event is thought to have occurred at about 30Ma.
Like Sydney Harbour, Jervis Bay and Bateman’s Bay,Ulladulla Harbour, with its deep-water channel, attained its present shape largely as a result of erosion by running water. This occurred during the Pleistocene Ice Age when sea level was much lower because much of the world’s water was frozen in large ice caps at the poles. The two most recent glacial phases of this Ice Age peaked approximately 150,000yrs and 18,000yrs ago respectively (see Fig 2).
During these cold intervals coastal rivers eroded channels up to 140m below present sea level as they traveled much further to the east than the present coastline. Coastal features carved out in this way and subsequently invaded by the sea when sea level rose again are known as drowned river valleys.
The creatures lived on the sea floor 270 million years ago:
Animal groups that thrived in these silty conditions included colonies of Bryozoans (sea fans; Crinoids (sea lilies); and solitary Rugose corals (horn corals). All of these animals attached themselves to the silty bottom and fed by filtering food from the sea water. Animals that burrowed into the sediments on the seafloor include the Bivalve Molluscs, similar to modern day pippis and scallops; two distinct groups of Brachiopods (lamp shells), the spiriferids and, [ii] the often spiny-shelled, productids, both groups became extinct about 251Ma; Other animals that roamed the seafloor included the spiral shelled Gastropods (sea snails), and several types of worms that only left their burrows, tracks or trails as ‘trace fossils’.
How were these marine animals preserved in the sedimentary rock sequence to form such intricate moulds and casts?
Most sediment deposited on the seafloor here 270Ma ago was very fine grained. The silt and clay sized particles imply that deposition occurred in a very low energy environment where water was only moved about by gentle, low velocity ocean currents. Burial of dead organisms by further sediment must have been sufficiently rapid to limit bacterial decay of the soft animal tissues. Where the hard calcium carbonate shells (=exoskeletons) were subsequently dissolved by groundwater, their shape is generally preserved in exquisite detail forming both internal and external moulds with very sharply defined features.
In some cases, the cavity formerly occupied by the animal’s shell was later in-filled by another mineral substance (such as iron oxide or silica) that was deposited from circulating groundwater. Such fossils are called replacement casts and they reflect the animal’s true shape but don’t preserve the internal structures.
The Questions
Area 1:
- What is the length of the visible Sea Lilly?
- Are the fossils in the grey or brown rock?
Area 2:
- What is the direction of the large seam of iron embedded in the rock?
Please submit your answers as soon as you can. It's ok to post prior, I will check your findings and respond if there are issues. Feel free to add a photo of yourself at the site if you want too. Group visits are encouraged. I hope you enjoy the experience, this is a very special area.