Misplaced Pages

#446553

38-639: Initially, the three major stratigraphic units containing bedded chert found within the Warrawoona Group were correlated as one stratigraphic unit across the various greenstone belts that occur in the East Pilbara Terrane. This formation was named the Towers Formation after the type of area near the town of Marble Bar. With subsequent and more detailed geological mapping and geochronologic studies , geologists found that

76-561: A pebble and smaller than a boulder . Other scales define a cobble's size differently. A rock made predominantly of cobbles is termed a conglomerate . Cobblestone is a building material based on cobbles. Cobbles, also called cobblestones , derive their name from the word cob , meaning a rounded lump. The term is further related to the German Kopf , meaning head . Chester Wentworth referred to cobbles as cobble bowlders [ sic ] in his 1922 paper that would become

114-454: A lagoon or flooded volcanic caldera, by way of connections with the open ocean. The Dresser Formation stromatolites apparently formed predominantly within these hydrothermally influenced shallow-marine lagoonal conditions, with geological and geochemical evidence indicating that some also formed in close proximity to subaerial (land-based) hot springs. The Dresser Formation is weakly metamorphosed having been subjected only to peak temperatures in

152-481: A thick laterally extensive bed of wrinkly laminated microbial mats with domical and coniform stromatolites. A variety of lithologies, including local, channel-shaped beds of cobble conglomerate and edgewise conglomerate, overlie the stromatolite bed. Finally, facies 2 contains thick sections of interlayered gray and white chert. Beds of coarse breccia up to 3 m (9.8 ft) thick and with clasts up to 30 cm (12 in) in diameter, occur locally interbedded with

190-552: A variety of other distinctive rock types. For example, a widespread layer, up to 1 m (3.3 ft) thick, of rhythmically bedded carbonate-chert, called zebra rock , occurs near the base of facies 2. The bed of zebra rock often grades upward into a 20 cm (7.9 in) thick bed of rippled and cross-laminated carbonate sandstone that exhibits well-defined linear, to bifurcating, ripple crests . This cross-laminated sandstone also contains evaporative aragonite crystal splays. This cross-laminated sandstone is, in turn, overlain by

228-414: Is a 0–100 m (0–328 ft) thick, fining-up sequence of green volcaniclastic conglomerate, sandstone, and chert that unconformably overlies the lower facies. It consists mainly of clasts of komatiitic basalt and basalt. Facies 4 is preserved in sedimentary wedges that thicken southward against bounding growth faults. The basaltic rocks of the middle basalt member overlie the deposits of facies 4 of

266-407: Is a volume of rock of identifiable origin and relative age range that is defined by the distinctive and dominant, easily mapped and recognizable petrographic , lithologic or paleontologic features ( facies ) that characterize it. Units must be mappable and distinct from one another, but the contact need not be particularly distinct. For instance, a unit may be defined by terms such as "when

304-600: Is overlain disconformably by the Mount Ada Basalt. This formation is about 2 km (1.2 mi) thick. Morphologically diverse (spheroids and filamentous) carbonaceous microstructures have been reported from a layer of bedded chert within Mount Ada Basalt. However, they have not been confirmed by additional sampling. As reviewed by Buntin and Noffke (2021) and VanKranendonk (2019), the Dresser Formation contains an abundance of well-preserved evidence for

342-666: Is sandwiched between pillow basalts. The upper chert member also has been split apart by younger dolerite and felsic sills . The Dresser Formation lies conformably on the North Star Basalt. In the North Pole Dome region (Panorama greenstone belt) this formation is at least 2 km (1.2 mi) and composed of weakly metamorphosed massive and pillow basalt, komatiitic basalt, basaltic hyaloclastite, gabbro, and dolerite. A dense network of large chert-barite-pyrite-epithermal quartz hydrothermal veins that cut through

380-422: Is the uppermost member of the Dresser Formation, consists of centimeter-layered white-and-blue to black layered chert. It lacks the heterogeneity of the underlying North Pole chert, remains the same relative thickness along strike, and is not offset by the growth faults that affect the underlying members. The upper chert member is up to 35 m (115 ft) thick. This layered chert lacks any current indicators and

418-468: The North Pole chert , consists of hydrothermally altered, bedded chert, stromatolites, and volcanoclastic sedimentary rocks, with abundant barite. The middle member consists of metamorphosed pillow and massive basalt. The upper member, which lacks barite, consists of unfossiliferous, bedded gray and white chert and local interbeds of volcanoclastic sedimentary rocks. Previously, the unnamed upper member

SECTION 10

#1732772426447

456-859: The United States Department of Agriculture 's definition suggests a range of 75–250 millimeters (3.0–9.8 in) and the ISO standard 14688 names cobbles as ranging from 63–200 millimeters (2.5–7.9 in) in diameter. Various attempts have been made to refine the Udden–Wentworth scale, including its definition of cobbles. In 1968, D. J. Doeglas proposed subdividing the cobble designation into two fractions, small cobbles (for particles with diameters from 64–125 millimeters [2.5–4.9 in]) and large cobbles (for particles with diameters from 125–250 millimeters [4.9–9.8 in]). A 1999 paper by Terence C. Blair and John G. McPherson argued that

494-649: The greenschist facies range (~150–350 °C). At first, regional mapping studies suggested that this low metamorphic grade was the result of regional prehnite – pumpellyite to greenschist metamorphism caused by the widespread emplacement of granitic rocks such as the intrusive North Pole Monzogranite ~3300 Ma. More recently, it was proposed that that the prehnite–pumpellyite to greenschist metamorphism resulted from repeated episodes of hydrothermal circulation within volcanic packages. These volcanic packages were bound above and below by silicified sediments that acted as aquicludes and confined fluid circulation to within

532-414: The Dresser Formation was dated at about ~3490 Ma. An interpretation of U–Pb dates of detrital zircons from volcanoclastic sediments of the Dresser Formation concluded that they accumulated between 3481 and 3470 Ma. Initially, the Dresser Formation was interpreted as having accumulated in an enclosed, evaporitic marine setting influenced by syndepositional hydrothermal circulation . Later re-evaluations of

570-488: The Dresser Formation's depositional environments proposed that it accumulated in shallow-water, low-eruptive, caldera lagoon influenced by syndepositional magma -driven, hydrothermal circulation. Both models propose the presence of marine influence during the deposition of the North Pole chert. They both infer that seawater routinely intermixed with hydrothermal fluids within an enclosed body of shallow water, e.g.,

608-518: The North American Stratigraphic Code, and are permitted under International Commission on Stratigraphy guidelines only in exceptional circumstances. A supergroup is a set of two or more associated groups and/or formations that share certain lithological characteristics. A supergroup may be made up of different groups in different geographical areas. A sequence of fossil -bearing sedimentary rocks can be subdivided on

646-928: The North Pole chert varies from a minimum of 1–2 m (3.3–6.6 ft) to a maximum of 208 m (682 ft). The North Pole chert is the only fossil -bearing unit of the Dresser Formation The North Pole chert consists of four main facies . Facies 1 typically forms the base of the North Pole chert and varies from 0 to 20 meters (0 to 66 ft) thick. It consists of mixture of cross-bedded sandstones, massive sandstone, silicified mudstone , and volcaniclastic conglomerate, and breccia . The volcaniclastic conglomerates locally contain gravel -size clasts of bedded jaspilitic chert , coarse crystalline barite, and stromatolitic laminates. These gravels are surrounded by felsic tuffaceous matrix . Facies 2, which overlies facies 1, contains widespread stromatolite layers, putative hot spring deposits, and

684-582: The North Pole chert. A basal layer of pillow basalt directly overlies the North Pole chert throughout the northern part of the North Pole Dome and is absent in the south. A homogeneous, massive, fine-to-medium-grained basalt overlies the pillow basalts with an apparently conformable contact. This homogeneous basalt pinches out against listric growth faults that bound either side of the Dresser Mine. It locally contains large blocks of barite and jaspilitic-bearing sedimentary strata. The upper chert member, which

722-427: The North Pole chert. The close spatial association between stromatolites and biogenic carbon and sulfur isotopic biosignatures suggest the former presence of hyperthermophilic microbes . The estimated depositional age of the Dresser Formation is inferred to be between 3483 and 3479 Ma . It is older than the ~3470 Ma Mount Ada Basalt and younger than the ~3490 Ma North Star Basalt. Syngenetic galena from barite in

760-595: The North Star Basalt immediately underlying the Dresser Formation. These veins extend up to the base of and terminate within facies 1 and 2; to a lesser degree in facies 3; and not in facies 4 of the North Pole chert. The largest veins occupy major, long-lived listric growth faults. 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. Typically, these veins are only 1 km (0.62 mi) deep and 2–10 m (6.6–32.8 ft) wide. The Dresser Formation

798-820: The Strelley Pool Chert, a stromatolitic unit that occurs in several of the greenstone belts of the East Pilbara. 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 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. These lithologies three mappable, member-scale stratigraphic units . The lowermost member, often informally called

SECTION 20

#1732772426447

836-411: The Udden–Wentworth and Krumbein scales betrayed a historical emphasis on the study of sand grains while ignoring larger gravel grains. They proposed defining fine cobbles as those with diameters from 64–128 millimeters (2.5–5.0 in) (−6 to −7 φ) and coarse cobbles as those with diameters from 128–256 millimeters (5.0–10.1 in) (−7 to −8 φ). In 2012, Simon J. Blott and Kenneth Pye suggested that

874-485: The Udden–Wentworth scale, an unlithified fraction of cobbles is classified as gravel while a lithified sample primarily composed of cobbles is a conglomerate . The Committee on Sedimentation of the US National Research Council has recommended that in situ cobbles be identified by their process of origination, if possible (e.g., cobbles by disintegration , by exfoliation , etc.). In

912-540: The basis for the Udden–Wentworth scale. Within the widely used Krumbein phi scale of grain sizes , cobbles are defined as clasts of rock ranging from −6 to −8 φ. This classification corresponds with the Udden–Wentworth size scale which defines cobbles as clasts with diameters from 64–256 millimeters (2.5–10.1 in). On this scale, cobbles are larger than pebbles which measure 4–64 millimeters (0.16–2.52 in) in diameter and smaller than boulders, whose diameters range from 256–4,096 millimeters (10.1–161.3 in). On

950-524: The basis of the occurrence of particular fossil taxa . A unit defined in this way is known as a biostratigraphic unit, generally shortened to biozone . The five commonly used types of biozone are assemblage, range, abundance, interval and lineage zones. Cobble (geology) A cobble (sometimes a cobblestone ) is a clast of rock defined on the Udden–Wentworth scale as having a particle size of 64–256 millimeters (2.5–10.1 in), larger than

988-458: The boundaries do not need to be sharp. To be formally recognised, a formation must have sufficient extent to be useful in mapping an area. A group is a set of two or more formations that share certain lithological characteristics. A group may be made up of different formations in different geographical areas and individual formations may appear in more than one group. Groups are occasionally divided into subgroups, but subgroups are not mentioned in

1026-495: The case of a marker horizon . A member is a named lithologically distinct part of a formation. Not all formations are subdivided in this way and even where they are recognized, they may only form part of the formation. A member need not be mappable at the same scale as a formation. Formations are the primary units used in the subdivision of a sequence and may vary in scale from tens of centimetres to kilometres. They should be distinct lithologically from other formations, although

1064-487: The cherts. Angular to subrounded cobbles and boulders of wrinkly stromatolitic laminates , barite, and bedded chert occur in these breccias. Discontinuous, thin beds of well-rounded, unlaminated white chert cobble conglomerate occur interbedded with the gray and white chert. Facies 3, which overlies facies 2, consists largely of centimeter-layered chert. The layered chert consists of white-gray layered chert and hematitic chert or jaspilite. Barite veins locally cut across

1102-422: The cobble designation be eliminated altogether, replaced by very small boulder and small boulder designations equivalent in size to Blair and McPherson's fine and coarse cobbles, respectively. When occurring in streams, cobbles are likely to be found in mountain valley streambeds that are moderately steep. Cobbles are also transported by glaciers and deposited as with other grades of sediment as till . If

1140-495: The existence of microbial life during the Paleoarchean about 3480 Ma. This evidence consists of biolamites and domical, stratiform, and coniform stromatolites ; microbially induced sedimentary structures (MISS); microfossils; microbial mat fabrics; and biosignatures. These fossils and biosignatures are preserved within hydrothermal dikes , barite mounds, siliciclastic sediments, and siliceous and ferruginous carbonates of

1178-534: The late 1800s and early to mid-1900s, prior to the Udden–Wentworth scale's widespread adoption, size classifications tended to group all particles larger than 2 millimeters (0.079 in) together as gravel or stones . Other scales have defined the size of a cobble slightly differently than the Udden–Wentworth; the British Standards Institution denotes a cobble as any clast ranging in diameter from 60–200 millimeters (2.4–7.9 in) while

Dresser Formation - Misplaced Pages Continue

1216-414: The layered cherts. The jaspilitic chert is typically laminated and lacks any evidence of being reworked by currents. 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 . The thickness of facies 3 is not published. Facies 4

1254-592: The major bedded cherts of the Towers formation belonged to one of three different and distinct stratigraphic units. The oldest of these bedded cherts are the stromatolitic bedded chert-barites of the Dresser Formation in the Panorama greenstone belt. The second oldest are the slightly younger Marble Bar Chert Member of the Towers Formation in the Marble Bar greenstone belt. The youngest of these three bedded cherts of

1292-597: The sandstone component exceeds 75%". Sequences of sedimentary and volcanic rocks are subdivided on the basis of their shared or associated lithology . Formally identified lithostratigraphic units are structured in a hierarchy of lithostratigraphic rank , higher rank units generally comprising two or more units of lower rank. Going from smaller to larger in rank, the main lithostratigraphic ranks are bed, member, formation, group and supergroup. Formal names of lithostratigraphic units are assigned by geological surveys . Units of formation or higher rank are usually named for

1330-474: The till is water-laid, finer particles like sand and pebbles may be entirely washed away, leaving a deposit of only boulders and cobbles. The term shingle beach refers to a beach covered with small- to medium-sized cobbles or pebbles (as opposed to fine sand). Glacially transported cobbles tend to share several identifying features including a tabular shape and downward diagonal striations on lateral facets. Cobble conglomerates may be alluvial in origin or

1368-467: The unit's type location , and the formal name usually also states the unit's rank or lithology. A lithostratigraphic unit may have a change in rank over a some distance; a group may thin to a formation in another region and a formation may reduce in rank for member or bed as it "pinches out". A bed is a lithologically distinct layer within a member or formation and is the smallest recognisable stratigraphic unit. These are not normally named, but may be in

1406-505: The volcanic packages. The effects of hydrothermal alteration in the Dresser Formation and other stratigraphic units throughout the Warrawoona Group and the local preservation of low-temperature hydrothermal products such as hydrothermal kaolinite are well-documented. An amphibolite-facies metamorphic aureole surrounds the North Pole Monzogranite in the core of the dome. Stratigraphic unit A stratigraphic unit

1444-667: Was subdivided into as many as three individual subunits separated by interbedded basalt. 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. 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. The thickness of

#446553