These are some personal observations, just thinking aloud, and part of my learning process and fascination with geology. I have a lot of questions to ask about the rocks at Port Douglas on the Queensland coast in Australia! They are marked by a GeoCache site which says they are a batholith. A batholith is formed deep under the earth’s crust where molten magma from superheated, melted rocks, cools slowly and forms granite made up of fairly large crystals. In eastern Australia the batholith formations, known collectively as the Kennedy Province, were created between 330 and 255 million years ago – following the earlier formation of the Hodgkinson Province which was created between 440 and 360 million years ago, and into which the batholith magma eventually intruded.
I have seen a batholith before – at Peggy’s Cove in Nova Scotia – and the the rocks I photographed at Port Douglas are very different from the rocks exposed at Peggy’s Cove – at least the ones around the edges of the formation in Port Douglas, notwithstanding that the two outcrops of bedrock are in two separate continents and have been subjected to very different erosional and weathering processes. Peggy’s Cove rocks have been smoothed and polished by ice sheets (glaciated) and their surfaces remain clean in a temperate climate. At Port Douglas, on the other hand, the rocks have eroded out and weathered in the wet tropics climate which has led to different erosional characteristics and a surface obscured in many parts by black bio-film.
Batholiths are made of granite. I think I can see granite in some areas of the Port Douglas outcrop on which the Lookout stands. A lot of what I believe to be granite is covered in black biofilm (possilbly cyanobacteria and lichen) and is colonised by organisms like barnacles so that the details are obscured. However, most of the detailed close-up shots I took of the rocks, particularly those around the edges of the feature, including loose boulder lying on the waters’ edge, did not seem at all like granite to me. There are various colours, textures and features as shown in the photographs in this post. I have been wondering to myself, speculating, whether these rocks may represent the junction between the granite of the Kennedy Province batholith and the Hodgkinson Province rocks into which they intruded, showing further changes to the earlier overlying (and already much altered, stretched, compressed and vulcanised) metasedimentary rocks.
The geological map of the area describes the outcrop of bedrock in the Port Douglas environs as Larramore Metabasalt Member which is part of the Hodgkinson Province rocks. I suspect that some of my photographs may be showing these metabasaltic rocks, or metasedimentary rocks with evidence for explosive volcanic activity and volcanic intrusions. I have read Rocks, Landscapes and Resources of the Wet Tropics by Berndt Lottermoser et al (2008) published by the Geological Society of Australia, Queensland Division, and have found it very useful. However, a more relevant account of the port Douglas geology might be given in another book which I have been trying to track down: Rocks and Landscapes of the Cairns District by W. F. Willmott and P. J. Stephenson (1989) published by The Queensland Department of Mines and Energy but it is out of print. I think it might be useful in helping me answer some of the questions I’m posing.
5 Replies to “Rocks at Port Douglas, Queensland”
It is a very difficult subject I’ve found in my limited investigation of it. So many places I’ve been at and then read about have rocks looking completely different, yet formed by almost identical processes.
The other thing that strikes me is how many great images there are in this one post! Landscapes, wildlife and geological close ups – you’re very prolific to be able to put so much into one post.
Understanding the rocks is fascinating and trying in equal measures. So many processes to grasp.
I do take lots of photographs! It is a way of capturing the entirety of the experience, noticing and remembering the details of it. It is amazing how much a person can forget. It also enables me to find out more about the place afterwards by doing some additional research so that I truly get the most from the visit while reliving the experience.
Very true Jessica, I have much more detailed memories of places where I have photographs to prompt me.
I got a bit lost here, Jessica, while enjoying the photographs with the colours and patterns of the rocks nonetheless. So I can’t quite tell if your observation of this place amounts to a breakthrough discovery that may lead to the reclassification of the entire area’s geology… or not…
Well, you never know! Not really likely, though. It is probably just the result of my ignorance multiplied by lack of readily available information about that location. It doesn’t stop me being curious and asking questions about my observations. I think the post can be best summed up by saying that although the Geocache information for the spot says that the rocks are a batholith, there are rocks around the margin of the feature (at least) that do not look (to me) typical of such a structure but bear a greater resemblance to the rocks that might have originally overlain the batholith – though maybe altered by the great heat from a batholith intrusion. I have no doubt that eventually someone, who really knows all about the geology of the area, will get in touch and give me a professional opinion.