Cobbold Gorge

Warwick Willmott, in his book “Rocks and Landscapes of the National Parks of North Queenlsand”, explains the geology of Cobbold Gorge. The section, attributed to Ian Withnall, is a great companion to the images that feature on my facebook page.

"COBBOLD CREEK GORGE NATURE REFUGE

with lan Withnall

.....

Geological setting and history

The gorge is on the western side of the Robertson River, which marks a major break separating two different rock units. On the eastern side are metamorphic rocks of the Etheridge Group, which were deposited as fine sand and mud in shallow seas about 1640 to 1700 million years ago in the Precambrian era. These were subsequently heated, compressed and deformed to metamorphic rocks about 1580 to 1550 million years ago, probably beneath a mountain chain.

These rocks are now schists, composed of fine layers of quartz and mica minerals. They include thick bands of dark black dolerite (the Cobbold Metadolerite), which were probably basalts erupted on to the sea floor or intruded as dykes. They are also cut by numerous veins and dykes of white quartz, which were either squeezed out of the metamorphic rocks or came from nearby granites intruded about 1550 million years ago.

Robertson River Fault

On the western side of the river the much younger Hampstead Sandstone is exposed. This was deposited in the Jurassic period about 180 million years ago, from large river systems probably similar to the Gulf rivers of today . They carried coarse sand and sometimes gravel derived from highlands of granite and metamorphic rocks to the southeast and east. Dinosaurs probably roamed the landscape at the time but none of their remains have been found. As the rivers meandered back and forth across the flood plains, the sand was deposited as a sheet that eventually covered the entire area. With time the sand hardened into sandstone. Subsequently, in the Cretaceous period about 120 million years ago, the sea flooded much of western and northern Queensland. Marine sands and muds were deposited on top of the Jurassic rocks, and even on top of the rugged Newcastle Range to the east of Forsayth.

After the sea withdrew, the region underwent a long period of weathering. Between about 20 and 4 million years ago, in the Tertiary period, the land was gently pushed upwards. The rate of erosion increased, and the sandstone sheet began to be stripped away, exposing the rocks underneath. This process is continuing today, and in most of

the Forsayth area remnants of the sandstone sheet are preserved as flat-topped hills or mesas.

However, around Cobbold Creek, a large area of sandstone is preserved, and unlike elsewhere, it does not cap the highest hills. The hills of schist to the east of the Robertson River are higher than the sandstone at Cobbold Creek Gorge. This is because of a downward movement of about 200 m on the southwestern side of a large fault, the Robertson Fault. Because the sandstone that originally capped the rocks to the northeast of the fault was at a higher level, it has been completely eroded away.

Most of the movement on the fault probably happened when the area was uplifted in the Tertiary, but it has had a long history. The relatively straight northeastern margin of the sandstone marks the line of the fault, which is followed by this part of the Robertson River.

Cutting across the sandstone are numerous fractures (or joints), which formed after the sand hardened into rock. The sandstone is about 50 m thick but extends over hundreds of kilometres. It behaved like a brittle sheet and even small flexing of the Earth's crust caused it to fracture. The fractures are often aligned in particular directions and can be seen from the air as sets of parallel lines etched out by weathering to give deep gullies. The most prominent of these trend northeast as can be seen on the aerial photo on the previous page.

Formation of the gorge

Until very recently (in geological time), Cobbold Creek joined the Robertson River at Stone Yard about 1.5 km upstream of where it does now. Above this old junction it flowed east to southeast through a relatively wide gorge, which is now an abandoned dry valley. At some past time all the water from upper Cobbold Creek and its tributaries became diverted and was funnelled through a very narrow slit in the sandstone that we know now as Cobbold Creek Gorge. The exact reason is uncertain.

One possible process is stream capture. The gorge may have started as a gully eroded along one of the fractures which flowed northeast into the river. With time, erosion caused the gully head to retreat until it met Cobbold Creek on the west. Because its mouth was at a lower point than the old mouth of Cobbold Creek, the gully captured all the water flowing down Cobbold Creek and then was scoured out more deeply.

However, the proximity to a large fault (which is known to have been active spasmodically over a long time), introduces the possibility that minor movement on the fault in very recent times could have contributed to the change in the course of Cobbold Creek. Slight movement on the fault nearby of as little as a few metres could have tilted the area northwards so that the old course was now uphill. This would have forced the creek to find an alternative course through one of the gullies cutting the sandstone to the east.

The narrowness of the gorge indicates that it is a very young feature. Because the diversion of Cobbold Creek happened only recently (perhaps even in the last 10,000 years), erosion has not yet had time to widen it. Below water level, the wall are undercut and extend back many metres. This is caused by abrasion by boulders and pebbles moving along the bed during floods. Eventually the walls will collapse, and the gorge will progressively widen. "