Dresser Formation consists of metamorphosed, blue, black, and white bedded chert; pillow basalt; carbonate rocks; minor felsic volcaniclastic sandstone and conglomerate; hydrothermal barite; evaporites; and stromatolites.
The lowermost of three stratigraphic units that comprise the Dresser Formation contains some of the Earth's earliest commonly accepted evidence of life such as morphologically diverse stromatolites, microbially induced sedimentary structures, putative organic microfossils, and biologically fractionated carbon and sulfur isotopic data.
[5][6] With subsequent and more detailed geological mapping and geochronologic studies, geologists found that the major bedded cherts of the Towers formation belonged to one of three different and distinct stratigraphic units.
With time and additional research, the assigned stratigraphic position of these and other stratigraphic units within the Pilbara Supergroup have changed and been rearranged numerous times[7][8] The Dresser Formation consists primarily of komatiitic basalt transected by silica-rich veins; fossiliferous, interbedded chert and barite; and pillow basalt with interbedded chert and diabase.
The upper member, which lacks barite, consists of unfossiliferous, bedded gray and white chert and local interbeds of volcanoclastic sedimentary rocks.
However, detailed mapping has shown that these chert subunits all belong to a single member that has been separated by younger, dolerite, and felsic sills at significant period of time after deep burial.
[1] The thickness and lithology of the North Pole chert, the basal member of the Dresser Formation, vary greatly over short distances.
Rapid lateral facies variations and sedimentary thickness changes take place over short distances (50 to 1,000 m (160 to 3,280 ft)), along-strike due to growth faults filled by hydrothermal chert-barite veins.
This cross-laminated sandstone is, in turn, overlain by a thick laterally extensive bed of wrinkly laminated microbial mats with domical and coniform stromatolites.
Drill cores from beneath the zone of surficial weathering demonstrate that some of the gray and white layered chert of this assemblage are the silicified equivalents of laminated carbonate, a mixture of ankerite, siderite, and calcite.
[10] A dense network of large chert-barite-pyrite-epithermal quartz hydrothermal veins that cut through the North Star Basalt immediately underlying the Dresser Formation.
The main veins extend as much as 2 km (1.2 mi) into the North Star Basalt beneath the base of the Dresser Formation and form swarms as much as 300 m (980 ft) wide.
[1][11] As reviewed by Buntin and Noffke (2021)[9] and VanKranendonk (2019),[1] the Dresser Formation contains an abundance of well-preserved evidence for the existence of microbial life during the Paleoarchean about 3480 Ma.
These fossils and biosignatures are preserved within hydrothermal dikes, barite mounds, siliciclastic sediments, and siliceous and ferruginous carbonates of the North Pole chert.
[4] Initially, the Dresser Formation was interpreted as having accumulated in an enclosed, evaporitic marine setting influenced by syndepositional hydrothermal circulation.
[15][16] Later re-evaluations of the Dresser Formation's depositional environments proposed that it accumulated in shallow-water, low-eruptive, caldera lagoon influenced by syndepositional magma-driven, hydrothermal circulation.
They both infer that seawater routinely intermixed with hydrothermal fluids within an enclosed body of shallow water, e.g., a lagoon or flooded volcanic caldera, by way of connections with the open ocean.