The Eromanga Sea was an

inland sea An inland sea (also known as an epeiric sea or an epicontinental sea) is a continental body of water which is very large in area and is either completely surrounded by dry land (landlocked), or connected to an ocean by a river, strait or " arm of ...

across the

Australian continent The continent of Australia, sometimes known in technical contexts as Sahul (), Australia-New Guinea, Australinea, or Meganesia to distinguish it from the country of Australia, is located within the Southern and Eastern hemispheres, near t ...

that formed in the

Early Cretaceous The Early Cretaceous (geochronology, geochronological name) or the Lower Cretaceous (chronostratigraphy, chronostratigraphic name) is the earlier or lower of the two major divisions of the Cretaceous. It is usually considered to stretch from 143.1 ...

. The sea extended from the

Eromanga Basin The Eromanga Basin is a large Mesozoic sedimentary basin in central and northern Australia. It covers parts of Queensland, the Northern Territory, South Australia, and New South Wales, and is a major component of the Great Artesian Basin. The Er ...

northward to the Carpentarian Basin. Its southern extents comprised lagoons and rivers, and to the east it reached Surat Basin, a bay. The sea covered large parts of what is now Queensland and

Central Australia Central Australia, also sometimes referred to as the Red Centre, is an inexactly defined region associated with the geographic centre of Australia. In its narrowest sense it describes a region that is limited to the town of Alice Springs and ...

at least four times during the early

Cretaceous The Cretaceous ( ) is a geological period that lasted from about 143.1 to 66 mya (unit), million years ago (Mya). It is the third and final period of the Mesozoic Era (geology), Era, as well as the longest. At around 77.1 million years, it is the ...

. The present-day

Winton Formation The Winton Formation is a Cretaceous Formation (geology), geological formation in central-western Queensland, Australia. It is late Albian to early Turonian in age. The formation blankets large areas of central-western Queensland. It consists of ...

represents remnants of the river plains that filled the basin left by the Eromanga Sea. The formation is a major source of dinosaur fossils.

Opals

The

Great Artesian Basin The Great Artesian Basin (GAB) of Australia is the largest and deepest artesian basin in the world, extending over . Measured water temperatures range from . The basin provides the only source of fresh water through much of inland Australia. ...

(GAB) was flooded by the Eromanga Sea and filled with

volcaniclastic Volcaniclastics are geologic materials composed of broken fragments (clasts) of volcanic rock. These encompass all clastic volcanic materials, regardless of what process fragmented the rock, how it was subsequently transported, what environment it ...

sediments eroded from the Cordillera's volcanic arc. A theory was proposed explaining the abundance of

opal Opal is a hydrated amorphous form of silicon dioxide, silica (SiO2·''n''H2O); its water content may range from 3% to 21% by weight, but is usually between 6% and 10%. Due to the amorphous (chemical) physical structure, it is classified as a ...

s in GAB. It was suggested that the Eromanga Sea was shallow, cold, muddy, and stagnant, which have led to little amount of

carbonate A carbonate is a salt of carbonic acid, (), characterized by the presence of the carbonate ion, a polyatomic ion with the formula . The word "carbonate" may also refer to a carbonate ester, an organic compound containing the carbonate group ...

s in sediments in the Eromanga Basin. However the abundance of iron-rich and organic sediments have led to an anoxic sub-seafloor well-suited for anaerobic, pyrite-producing bacteria. It was suggested that during the periods of uplift, erosion, and denudation opals were formed due to acidic oxidative weathering during 97 to 60 Ma.Tao Hsu, Andrew Lucas, and Vincent Pardie
SPLENDOR IN THE OUTBACK: A VISIT TO AUSTRALIA’S OPAL FIELDS
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References

{{coord, 26, S, 140, E, display=title Geology of Australia Former lakes of Oceania