Rocks at Port Douglas, Queensland

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.