Geology of the Big Scrub Rainforest (Part 3)

Gondwana

The late Jurassic and Cretaceous marks a very important geological time. It is the time that the huge supercontinent of Pangea broke up. This when Gondwana became its own continent with present day Australia and Antarctica being a large part of a new smaller, but still impressive super-continent. Our region was actually quite close to the pole but, yet temperatures in our area were quite warm because the climate of whole of the earth was warmer and wetter than it is today. Life was abundant because summer days were nearly 24 hours long and the concentration of carbon dioxide in the atmosphere along with the warmth and moisture super-charged plant growth (in fact permanent ice did not exist anywhere on earth at this time). It was at this time that many of the plants synonymous with Australia and more specifically the ‘Bigscrub’ began their evolution (including the plant order Myrtales which includes eucalypts, bottlebrushes and lillypillys).

As the Jurassic gave way to the Cretaceous period big changes continued. India followed by Antarctica began to separate from Gondwana and finally at around 80Ma the Tasman Sea started to form. Prior to the Tasman Sea formation the landmass extended hundreds or even thousands of kilometers to the east before reaching the sea. The Tasman Sea caused major changes to the landscape. This area is known as Zealandia and as separation occurred it sank below the sea and is now known as the Lord Howe submarine rise. A bit like the mythical land of Atlantis, a huge land submerged under the ocean. The Australian continent now finishing only several of kilometres off the coast from the future Cape Byron. The detachment of Zealandia reduced the weight on the eastern side of Australia which then began to rise and form many of the inland ranges that now make up the Great Dividing Range. This continental crust “rebound” would progress over a long time and by the end of the Cretaceous the physical form of Eastern Australia was set for its last major changes through the actions of volcanoes.

Science, Philosophy and Politics

It is popular these days to start with a narrative and make the data fit in. Rarely do we think about alternative hypotheses other than our own already established professional narrative. Because researchers today rely on their reputation they must defend their theories or apparently loose credibility. Ironically, the most credible scientists are those willing to admit they may be incorrect.

Scientists today often delve into the areas of political intrigue. When this happens the scientists pet narrative becomes a fortress that must be protected. No one can criticize it because the scientist will see it as a personal affront since his whole credibility has been placed on the line. This in turn turns peers off from suggesting alternative hypotheses because they know if they make such a suggestion the whole thing will become a fight. Even suggesting an alternative that is not as popular as the politically expedient one can result in aggression, ostracism and public ridicule. No wonder scientists often find hiding in their laboratories the most rewarding experience!

When scientific concepts are backed by politics you know that you are onto ground that is not necessarily scientific. So with that in mind, here is a diagram that covers all the issues today that arise during scientific inquiry. This includes the moral, philosophical and political reasons for undertaking scientific research.

 

As a geological example that was publicized a couple of years ago in Nature on the problems with the mantle plume hypothesis. Even though the article recognizes many problems with the theory it demeans a group of scientists who have proposed alternatives that fit a wider range of data. It gently denegrates an alternative hypothesis by suggesting only a small number and therefore fringe group have advanced the hypothesis:

"No matter what the Réunion study finds, its results are unlikely to convince the few critics of the plume hypothesis."

Essentially, it is the anti-scientific idea of "consensus science" that is beginning to pervade all areas of inquiry. Some of these scientists responded  here is a link to a letter to the journal Nature. Unfortunately for the general public even to view this correspondence requires $18 just to read it (such is the nature of journals these days).

Geology of the 'Big Scrub Rainforest' (Part 2)

The story of the 'Big Scrub' s preserved in the rock unit known as the Neranleigh-Fernvale Beds. The first post in this series is about this rock, the foundations of the region. This post deals with the period of big sedimentary basins which corresponds with the age of the dinosaurs. Some 50 or 60 million years elapse from the Neranleigh-Fernvale Beds until we come to our next period of rock formation.

The Big Basins

Walloon Coal Measures overlain by Orara Formation at Bexhill

This period begins in the age of the dinosaurs during the end of the Triassic Period. During this time tectonic forces became extensional, that is, the east coast of Australia was pulled and twisted apart, very little compression occurred. The crust became thinner as the once colliding continental plates began to relax and lowland basin-shaped regions formed. The thin crust allowed more volcanism to occur and the first geological units of the Ipswich and Clarence-Moreton Basin were formed. The Chillingham Volcanics consisting of lavas and volcanic ash was laid down in after 229Ma and this was subsequently overlain by units of lake and river deposits including the Evans Head Coal Measures, Laytons Range Conglomerate, Walloon Coal Measures and many other layers. By the height of the age of the dinosaurs, during the Jurassic great river valleys spread out from the mountains of the New England over our region. These rivers laid down great expanses of alluvial sand which were further overlain by other units. The great expanses of river sand are called the Orara Formation.

Today, like the Neranleigh-Fernvale Beds the sediments of the Clarence-Moreton Basin in the ‘Bigscrub’ area are mostly obscured by younger rocks. However, some of the Chillingham Volcanics seem to present in the Blackhall Range behind Wardell though this is difficult to identify. Rocks of the Orara Formation are present at the edge of Meerschumvale but are most obvious at Bexhill, indeed at Bexhill the Walloon Coal Measures are evident in the old brickpit. The Walloon Coal Measures are overlain by a sub-unit of the Orara Formation called the Kangaroo Creek Sandstone. This Sandstone forms nutrient very well-draining but poor soils. Lovely examples of the Kangaroo Creek Sandstone can be seen at Bexhill Open Air Cathedral or in the creeks near The Channon. This means that the vegetation on these areas consists of different plants to that of the rest of the ‘Bigscrub’. The soils in these areas cannot support the lowland subtropical rainforest that is the biggest component of the ‘Bigscrub’ unless they are well sheltered in a gorge.

Even though the units of the Ipswich and Clarence-Moreton Basins are dated from the age of the dinosaurs (the Triassic, Jurassic and Cretaceous Periods) no one has yet found fossils of dinosaurs preserved in any of these units in our ‘Bigscrub’ area. However, abundant fossils of plants and fish do exist in many units of the Clarence-Moreton Basin and dinosaur footprints have been seen in rocks of the Walloon Coal Measures in the Queensland part of the basin. Some fossil fish have been observed in the creeks near Nimbin. Along with the abundant coal during the Jurassic shows there was a very large quantity of organic matter and plants growing at the time. This was a time rich in life.

Geology of the 'Big Scrub Rainforest' (Part 1)

Later this year it is intended that the Big Scrub Landcare group and many other contributors will release a book on people's connection with the 'Big Scrub'. It will be a multi-faceted book that introduces the emotional connection that people can have with a lovely part of the world. The book will even help picture the geological events that contributed to the formation of the amazing region. I understand that the book is intended to be launched in September at annual Big Scrub Rainforest Day. For those that would like some background information on the geology of the 'Big Scrub' I have provided a detailed outline of the rock types and events that went into building the foundations of this forest. There is a fair bit of information so I've broken this history into a series of blog posts. The book will have a different emphasis to this series of posts and will be a high quality visual feast. So I'll let everyone know more details about the book closer to the release date.

The oozy beginning

The history of the ‘Bigscrub’ starts a very long time in the past, yet it may be a surprise that by Australian standards the geology of our region is comparatively young. The oldest parts of Australia are 4400Ma (4400 million) years old but our little part of the Australian continent did not exist back then. It was not until 363-320Ma that the foundation rocks of our region were formed. These rocks are called the Neranleigh-Fernvale Beds

The Neranleigh-Fernvale Beds at Broken Head National Park near Byron Bay

The Neranleigh-Fernvale Beds are derived from the deep sea. In the Devonian Period sediments settled to the bottom of the deep sea crust. The sediments were at first very fine and mainly consisted of the microscopic silica based skeletons of algae called pelagic ooze. The amount of sediment was only slowly deposited but over a long time a great thickness accumulated. Occasionally thicker layers of mud would be deposited from submarine landslides which occurred at the edge of the far away continental land masses, rarely submarine volcanoes laid down lava that was quickly solidified by the deep, cold ocean water.

The process of plate tectonics means that oceanic plates move slowly under continental plates. Gradually, as this process continued during the Carboniferous our area came closer to the continental landmass as the oceanic plate was subducted (is pushed under) under the Australian continent. Tall active volcanic mountain ranges existed along the edge of the continent much like the mountain ranges of the Andes Mountains in South America today. As our area approached the continent it meant that the size of particles increased. Erosion of the mountain ranges and continuing submarine landslides created layers in the sediments called turbidite sequences. These are sequences where the bottom is coarse grained (usually sandy) and the top is fine grained (mud). This is because fine sediments take longer to fall out of the water. Subsequent nearby landslides would start a new layer with coarse grained sands followed by finer grained mud and so on.

Neranleigh-Fernvale Beds are quarried at many locations this is an old face at Teven

Eventually our part of the oceanic crust with its mass of deep sea muds and turbidite travelled to the zone of subduction where the collision with the Australian plate forced the oceanic plate under it. In this process the sediments that are on the oceanic plate are squashed and stuck onto the continent. Australia’s size grew as the process of accretion built up a thick wedge of submarine mud and turbidite. The pressure of the crustal plates sliding past each other squashed the sediments together and bent the layers into a complex arrangement of folds and faults. These are what we now know as the Neranleigh-Fernvale Beds.

As mentioned before, the Neranleigh-Fernvale Beds are the foundation rocks of our region. Today they form the ‘basement’ of the coastal Australian landmass from Gympie in Queensland to Broadwater on the Richmond River. However, in our region they are mostly obscured by younger rocks (discussed below) though it is possible to view outcrops of turbidite in several areas. These include on the Wilsons River at Laverty’s Gap, at Tintenbar and on the escarpment at Uralba between Ballina and Alstonville. Quartzite (from the silica rich sediment) is present on a hill near Nashua.

At the earlier stages of the carboniferous period the collision of the oceanic and crustal plates in our region stopped. The Neranleigh-Fernvale bed formation had been completed. The continent with its new additions ‘relaxed’ as compression eased and eventually stopped. Many more millions of years passed and what happened during that time is lost to history. No rocks are preserved in our region from the next time period known as the Permian. It is probable that the ‘Bigscrub’ area remained stable for a long time with only erosion being the most significant geological process.

Part 2.