It’s almost impossible to visit the Kalbarri region and not appreciate the spectacular rocks, not only are the colours spectacular the formations are undeniably pleasing to the eye.
For those of you who are curious and want to know more we have enlisted the help of a local Geologist.
The area around Kalbarri is transitional between two major basins filled by sedimentary rocks that span the western margin of Western Australia, the Perth and Southern Carnarvon Basins. The rocks and sediments in these basins are less than about 500 million years old, quite young in comparison to the crystalline and deformed rocks to the east in the west Australian shield.
Triassic rocks (~250 m.y. old) of the Perth Basin extend north almost to Kalbarri, and Devonian (~400 m.y.) and Cretaceous rocks (~140-66 m.y.) of the Southern Carnarvon Basin thin towards the south and are absent 50 to 90 km south of Kalbarri. The river and coastal gorges are cut into the Tumblagooda Sandstone, named after Tumblagooda Hill just north of Kalbarri. This red and white banded and bedded sandstone is the oldest exposed rock unit to be deposited in the Perth and Southern Carnarvon Basins, 400 to 450 million years old.
Sedimentary rocks are either deposited bed by bed, by detrital grains eroded from pre-existing rocks and ranging from mud to massive blocks metres across, or grow like coral reefs, in both cases at the Earth’s surface. The former are clastic rocks, the latter are organic or chemical rocks. Sandstone for example consists of sand-sized grains, or clasts, that have been bound either by finer material like mud or cemented by minerals precipitating out from fluids percolating through the sand between grains.
Sandstones are composed predominantly of grains of sand, each grain between 0.125 mm and 4 mm diameter. If finer grained, the rock is a siltstone or mudstone, if coarser, a conglomerate. The grains are bound (held together) by either a crystalline cement (mostly calcite or silica), or silt- and mud- sized material filling spaces between grains (termed a matrix, much like mud brick) – if neither it is still an unconsolidated sand, not stone. If a matrix binds the sand grains, not much permeability may be left, as the finer silt and mud may clog the pore spaces between grains (porosity) and hinder or stop fluids percolating between grains (permeability).
The Tumblagooda Sandstone is characteristically red and white spotted, mottled and striped. The red is due to fluids rich in dissolved iron percolating through between the grains after deposition, either before or after lithification, but probably soon after deposition because bedding and fine structures are so well delineated. Within the areas now white or pale, the fluids could not percolate (silt, mud or cement blocking permeability) or there was material that reduced the availability of oxygen, so that hematite and limonite (both iron oxides) could not form. This may have been organic material – fecal material in or lining burrows, microbial and algal material, even soft-bodied worms. The white spots are called, logically, reduction spots. The lack of body fossils in the Tumblagooda Sandstone, given the evidence of abundant life provided by burrows, tracks and trails, suggests the environment was strongly oxidizing overall, so that any skeletal remains broke down rapidly.
The Tumblagooda Sandstone around Kalbarri was deposited by rivers and by tidal processes on coastal sand flats and river flats that sloped gently to the northwest, in an arid tropical climate. Land plants were very primitive at that time – colonization of the land had just begun – so rivers flowed in floods as vast sheets rather than in defined channels like the present day. Without clay and mud formed by plants and animals breaking down sand, and few if any roots from plants, channel banks were unstable, much as in desert and sub-arctic regions today.
When the present carpark for Natures Window was being ripped and levelled, a distinctive white siltstone containing a large ammonite coiled nautiloid shell impression about 20 cm diameter was unearthed. The fossil and the rock are about 120 million years old, the same age as the lower part of the white cliffs above Tumblagooda Sandstone north of the river nearer Kalbarri. The siltstone was deposited in a quiet normal-salinity marine setting. Their discovery at Natures Window, well below the top of the gorge just above the central notch of the Murchison Gorge, indicates a seaway extended up a pre-existing broad valley at least as far as the Loop about 120 million years ago. That valley had the same shape as the present-day gorge, including the meander of the Loop, but probably not the central incised gorge notch. The broad valley presumably developed as a river valley earlier. Incision of the central notched gorge probably began about 20 million years ago, following collision of the Australian continent against Asia as Australia moved northwards after separating from Antarctica about 80 million years ago.
What can you see at Natures Window? Here you can see the details of the tidal sandflat deposits of the Tumblagooda Sandstone – the sedimentary structures, the ripples, the smaller tracks, trails and burrows – as well as some larger scale features.
Trackways with discrete foot imprints range in width from about 20 cm to less than 5 mm, reflecting the size of the creature walking over the sand and silt. They range from quite blurred to very distinct, reflecting whether the creature’s legs were submerged, or moving on damp emergent sediment. Types of burrows include intensely burrowed beds, uncommon solitary short vertical cylindrical burrows, probable hunting burrows that spear through intensely burrowed beds, scattered ‘birds nests’ of mining or feeding burrows. A single crescent-moon shaped burrow (Selenichnites) is present at the edge of a ripple washout in a slab in the floor of the shelter 100 m up the path from Natures Window. We didn’t collect coords for this feature as we didn’t know about it till after our visit, if anyone manages to find it and would like to take coords to share that would be appreciated.
Ripples are mostly relatively small, near symmetrical, and formed in water a few centimetres deep or less. Interference ripples show flow in different directions, and reworking of earlier rippled surfaces. Flat-topped ripples formed when water depth decreased rapidly and water flow abruptly changed from spiral eddies in ripple troughs to swash-like planar flow across ripple crests. Double-crested ripples developed when water depth decreased enough that a single spiral eddy could not continue along a ripple trough, so that a double eddy developed forming a secondary crest along the trough. Ripple washouts, such as are found on present day river banks and tidal flats, are also common.
Features that indicate the sediment surface emerged from the water intermittently include the shapes of ripples and rippled surfaces, degree of preservation of detail in trackways, and sedimentary structures called adhesion surfaces and wind warts. Adhesion surfaces and wind warts form when wind carrying sand blows across a damp emergent surface. Sand grains stick to the damp surface, and gradually build up clusters of small warts, with the steep side facing into the wind, or extensive knobbly surfaces. Ripple blow-outs form when pre-existing rippled surfaces dry out and are partly blown out, just as on present-day beaches. Looking directly through the Window, there is a double opened joint about 500 m upstream at the bend in the gorge. This joint largely controls the course of the river between there and the Window, as a similar joint does at the Z Bend. Joints are very prominent along much of the gorge , and locally control the course of the river and tributary creeks. Some have opened, as at the Loop, and some may have small amounts of movement on them. Many act as groundwater conduits, with springs and seeps developed where they emerge into the gorge.
To log this cache we require you to visit Nature Window and message us with the following answers to the best of your ability;
1. Can you see evidence of continual erosion of the window?
2. How thick would you say each layer is, how many distinct layers make up the window (you might need your fingers and toes for this one ;) ?
3. Can you see any of the features we described above, if so identify them?
4. A photo of your team or GPS near GZ with your log and answers.
You are welcome to log your answers straight away to keep your TB's and Stats in order but please message us with your answers within 1 week of your log. Cacher’s who do not fulfil the Earth Cache requirement will have their logs deleted.
A massive thank you to Gelogist Roger Hockings for his invaluable assistance on this Earth Cache.
