Description
The Roxbury Conglomerate comprises the lower part of the Boston Bay Group, which is a 5,000-meter-thick (3 miles) sequence of sedimentary rocks that fill the Neoproterozoic Boston Basin in eastern Massachusetts. The upper part of the Boston Bay Group consists of the Cambridge Argillite, which overlies the Roxbury Conglomerate. The Roxbury Conglomerate traditionally has been subdivided into three subdivisions; (1.) basal Brookline Member (conglomerate and sandstone), (2.) medial Dorchester Member (mostly sandstone with minor conglomerate) and (3.) upper Squantum Member (largely diamictite). However, these three subdivisions of the Roxbury Conglomerate complexly interfinger with each other and lack the simple layer-cake distribution that past studies have described. [6] [7] [8] [9] [10]
The Brookline Member of the Roxbury Conglomerate is the classic ‘puddingstone’ that is typically discussed and illustrated in popular web pages, articles, and other publications. It is about 150–1,300 m (490–4,300 ft) thick and consists of massive clast-supported pebble and cobble conglomerate beds interbedded with beds of argillite and sandstone. The conglomerates consist of grey feldspathic sand and well-rounded pebbles and cobbles of quartzite, granite, felsite, and quartz monzonite. The 'puddingstone' of the Brookline Member is complexly interbedded with layers of laminated and graded argillite and sandstone and massive diamictite. [8] [9] [10] [11] [12]
The Dorchester Member consists of purplish, greenish and grey siltstone, sandstone, and medium-to fine-grained argillite. As traditionally defined, it is 180–500 m (590–1,600 ft) thick and dominated by medium-to fine-grained argillite. It contains lesser amounts of sandstone and conglomerate than the Brookline Member. The sandstone beds within this member commonly exhibit full or partial Bouma sequences. The beds within this member commonly exhibit evidence of penecontemporaneous deformation due to downslope slumping. [8] [9] [10] [11] [12]
The well-known Squantum Member is also known as the Squantum diamictite, Squantum 'Tillite', Squantum Tillite, or the Squantum Tilloid. It consists largely of diamictites that are a heterogeneous and poorly sorted admixture of rare boulders up to 1.2 m in diameter, pebbles, cobbles, and sand in a silty-clay matrix. These diamictites occur as beds, which range in thickness from 18 to 215 m (60 to 705 ft) and are typically interbedded with purplish, greenish and grey siltstone, sandstone, and medium-to fine-grained argillite. Typically, the diamictites are mostly massive and lenticular in form; some are crude to moderately well sorted; and some exhibit normal grading. Most of the diamictite outcrops exhibit chaotic bedding in the form of contorted and folded patches of sand, local clusters of gravel, and coherent slump blocks of mudstone. In outcrops, lapilli tuff beds have also been identified in close association with the diamictite of the Squantum Member. At Squantum Head, the diamictite is interbedded with laminated mudstone beds that are 2 to 10 cm (0.8 to 4 in) thick. Some of laminated mudstone beds contain a few outsized pebbles or cobbles that have depressed the underlying laminae and giving them the appearance of dropstones. [7] [8] [9] [10] [11] [12] [13] [14]
The gravel-size portion of the Squantum Member diamictites consists of range from sub-rounded to angular clasts, 5–60 cm (2–24 in) in diameter, to well-rounded clasts 3–8 cm (1.1–3 in) in diameter. They are composed of multicoloured, locally derived felsic and mafic volcanic rocks, granodiorite, quartzite and massive, graded and laminated sandstone and siltstone. The sand- and gravel-sized fraction of the diamictites consist of volcanic, granitic and metasedimentary lithic fragments that have the same composition as the sediments of the Brookline and Dorchester members. Glacially striated pebbles, chattermarked quartz grains and dropstones have been reported from these diamictites. However, none of these reports have been substantiated by later research. For example, previously identified dropstones have been re-interpreted as having been emplaced by lateral sediment-gravity or current processes. [7] [8] [10] [15] [16]
The Roxbury Conglomerate has been significantly altered by metamorphism. Metamorphism has altered its sedimentary rocks to subgreenschist facies and created a slaty, well-developed, spaced cleavage that oriented approximately perpendicular to bedding within it. Typically, tectonism has flattened, stretched, indented, and fractured the pebbles and associated matrix of the Roxbury Conglomerate to the point that it often has the appearance of flow structure. [10] [17]
The strata of the Boston Bay Group lie within a faulted bounded basin. They are bounded by normal faults to the north and west. Within the Boston Basin, Roxbury Conglomerate lies unconformably upon Dedham Granite, Westwood Granite, and much older Middlesex Fells Volcanic Complex. To the south, the Roxbury Conglomerate unconformably overlies Mattapan Volcanic Complex, which, in turn, overlies the Dedham Granite. The Dedham Granite, Westwood Granite, and older rocks are overlain by the calc-alkaline Brighton Volcanic rocks, which consist of the altered basalt and andesite flows, pyroclastic rocks, breccia, tuff, and intrusive rocks. The upper part of the Brighton Volcanic rocks sporadically interfinger and interbedded with the Brookline and Dorchester members throughout the basin in the southern portion of the basin. [10]
Overlying the Squantum Member, the Cambridge Argillite consists of up to 5 km (3 miles) of laminated, dark to olive grey, graded, turbiditic siltstone and sandstone beds. Graded beds, starved ripples, scour marks, load casts and micro-faults are numerous. Soft sediment deformation structures, such as mega slump folds many meters in amplitude, and pinch and swell bedding, are also common. Discrete ash beds measuring a few centimeters to tens of centimeters in thickness have also been documented in this unit. [8] [10] [11]
Conglomerate is a clastic sedimentary rock that is composed of a substantial fraction of rounded to subangular gravel-size clasts. A conglomerate typically contains a matrix of finer-grained sediments, such as sand, silt, or clay, which fills the interstices between the clasts. The clasts and matrix are typically cemented by calcium carbonate, iron oxide, silica, or hardened clay.
The Dorchester Pot was a metal vase-like object that was recovered in two pieces after an explosion used to break up rock at Meeting House Hill, in Dorchester, Massachusetts in 1851. According to text reprinted from the Boston Transcript, a local paper, in the June 5, 1852 Scientific American, the two pieces were found, loose among debris thrown out by the explosion. Apparently, it was inferred from the locations of the two pieces of this pot among the explosion debris that this pot had been blasted from solid puddingstone (conglomerate), which is part of the Roxbury Conglomerate, from about 10 feet below the surface of Meeting House Hill. The story has been used by creationists and fringe theorists as evidence that conventional models of geology or the length of the human presence on earth are wrong. Mainstream commentators identify it as a Victorian era candlestick or pipe holder.
Diamictite is a type of lithified sedimentary rock that consists of nonsorted to poorly sorted terrigenous sediment containing particles that range in size from clay to boulders, suspended in a matrix of mudstone or sandstone. The term was coined by Richard Foster Flint and others as a purely descriptive term, devoid of any reference to a particular origin. Some geologists restrict the usage to nonsorted or poorly sorted conglomerate or breccia that consists of sparse, terrigenous gravel suspended in either a mud or sand matrix.
Puddingstone, also known as either pudding stone or plum-pudding stone, is a popular name applied to a conglomerate that consists of distinctly rounded pebbles whose colours contrast sharply with the colour of the finer-grained, often sandy, matrix or cement surrounding them. The rounded pebbles and the sharp contrast in colour gives this type of conglomerate the appearance of a raisin or Christmas pudding. There are different types of puddingstone, with different composition, origin, and geographical distribution. Examples of different types of puddingstones include the Hertfordshire, Schunemunk, Roxbury, and St. Joseph Island puddingstones.
The San Juan Basin is a geologic structural basin located near the Four Corners region of the Southwestern United States. The basin covers 7,500 square miles and resides in northwestern New Mexico, southwestern Colorado, and parts of Utah and Arizona. Specifically, the basin occupies space in the San Juan, Rio Arriba, Sandoval, and McKinley counties in New Mexico, and La Plata and Archuleta counties in Colorado. The basin extends roughly 100 miles (160 km) N-S and 90 miles (140 km) E-W.
The geology of Tasmania is complex, with the world's biggest exposure of diabase, or dolerite. The rock record contains representatives of each period of the Neoproterozoic, Paleozoic, Mesozoic and Cenozoic eras. It is one of the few southern hemisphere areas that were glaciated during the Pleistocene with glacial landforms in the higher parts. The west coast region hosts significant mineralisation and numerous active and historic mines.
The Triassic Stockton Formation is a mapped bedrock unit in Pennsylvania, New Jersey, and New York. It is named after Stockton, New Jersey, where it was first described. It is laterally equivalent to the New Oxford Formation in the Gettysburg Basin of Pennsylvania and Maryland.
The Triassic Lockatong Formation is a mapped bedrock unit in Pennsylvania, New Jersey, and New York. It is named after the Lockatong Creek in Hunterdon County, New Jersey.
The geology of Massachusetts includes numerous units of volcanic, intrusive igneous, metamorphic and sedimentary rocks formed within the last 1.2 billion years. The oldest formations are gneiss rocks in the Berkshires, which were metamorphosed from older rocks during the Proterozoic Grenville orogeny as the proto-North American continent Laurentia collided against proto-South America. Throughout the Paleozoic, overlapping the rapid diversification of multi-cellular life, a series of six island arcs collided with the Laurentian continental margin. Also termed continental terranes, these sections of continental rock typically formed offshore or onshore of the proto-African continent Gondwana and in many cases had experienced volcanic events and faulting before joining the Laurentian continent. These sequential collisions metamorphosed new rocks from sediments, created uplands and faults and resulted in widespread volcanic activity. Simultaneously, the collisions raised the Appalachian Mountains to the height of the current day Himalayas.
The Unkar Group is a sequence of strata of Proterozoic age that are subdivided into five geologic formations and exposed within the Grand Canyon, Arizona, Southwestern United States. The 5-unit Unkar Group is the basal member of the 8-member Grand Canyon Supergroup. The Unkar is about 1,600 to 2,200 m thick and composed, in ascending order, of the Bass Formation, Hakatai Shale, Shinumo Quartzite, Dox Formation, and Cardenas Basalt. Units 4 & 5 are found mostly in the eastern region of Grand Canyon. Units 1 through 3 are found in central Grand Canyon. The Unkar Group accumulated approximately between 1250 and 1104 Ma. In ascending order, the Unkar Group is overlain by the Nankoweap Formation, about 113 to 150 m thick; the Chuar Group, about 1,900 m (6,200 ft) thick; and the Sixtymile Formation, about 60 m (200 ft) thick. These are all of the units of the Grand Canyon Supergroup. The Unkar Group makes up approximately half of the thickness of the 8-unit Supergroup.
The Bass Formation, also known as the Bass Limestone, is a Mesoproterozoic rock formation that outcrops in the eastern Grand Canyon, Coconino County, Arizona. The Bass Formation erodes as either cliffs or stair-stepped cliffs. In the case of the stair-stepped topography, resistant dolomite layers form risers and argillite layers form steep treads. In general, the Bass Formation in the Grand Canyon region and associated strata of the Unkar Group-rocks dip northeast (10°–30°) toward normal faults that dip 60+° toward the southwest. This can be seen at the Palisades fault in the eastern part of the main Unkar Group outcrop area. In addition, thick, prominent, and dark-colored basaltic sills intrude across the Bass Formation.
One of the major depositional strata in the Himalaya is the Lesser Himalayan Strata from the Paleozoic to Mesozoic eras. It had a quite different marine succession during the Paleozoic, as most parts of it are sparsely fossiliferous or even devoid of any well-defined fossils. Moreover, it consists of many varied lithofacies, making correlation work more difficult. This article describes the major formations of the Paleozoic – Mesozoic Lesser Himalayan Strata, including the Tal Formation, Gondwana Strata, Singtali Formation and Subathu Formation.
The geology of Ghana is primarily very ancient crystalline basement rock, volcanic belts and sedimentary basins, affected by periods of igneous activity and two major orogeny mountain building events. Aside from modern sediments and some rocks formed within the past 541 million years of the Phanerozoic Eon, along the coast, many of the rocks in Ghana formed close to one billion years ago or older leading to five different types of gold deposit formation, which gave the region its former name Gold Coast.
The Bellingham Conglomerate is one of several Pennsylvanian stratigraphic units in eastern Massachusetts. It is mainly conglomerate and greywacke, exclusive to the Bellingham Basin. The conglomerate has granite and quartzite pebbles in a mica matrix, sourced from the neighboring Blackstone Group and Milford Granite. The two main rock types are interbedded with chlorite phyllite which contains minerals such as muscovite, quartz, zoisite, magnetite and chloritoid.
The Mattapan Volcanic Complex is a suite of Proterozoic felsic volcanic rocks in the U.S. state of Massachusetts. It is situated at the edge of the Boston Basin into the Blue Hills and lying unconformably atop the Dedham Granite. The complex is overlain by the Roxbury Conglomerate. In terms of its petrology, the rock types and compositions in the Mattapan Volcanic Complex are very similar to the Lynn Volcanic Complex and include ancient rhyolite and rhyodacite flows, some of which have porphyry characteristics, welded ash flows, tuff, breccia, extrusion domes and breccia pipes.
The geology of Tanzania began to form in the Precambrian, in the Archean and Proterozoic eons, in some cases more than 2.5 billion years ago. Igneous and metamorphic crystalline basement rock forms the Archean Tanzania Craton, which is surrounded by the Proterozoic Ubendian belt, Mozambique Belt and Karagwe-Ankole Belt. The region experienced downwarping of the crust during the Paleozoic and Mesozoic, as the massive Karoo Supergroup deposited. Within the past 100 million years, Tanzania has experienced marine sedimentary rock deposition along the coast and rift formation inland, which has produced large rift lakes. Tanzania has extensive, but poorly explored and exploited natural resources, including coal, gold, diamonds, graphite and clays.
The geological history of Zambia begins in the Proterozoic eon of the Precambrian. The igneous and metamorphic basement rocks tend to be highly metamorphosed and may have formed earlier in the Archean, but heat and pressure has destroyed evidence of earlier conditions. Major sedimentary and metamorphic groups formed in the mid-Proterozoic, followed by a series of glaciations in the Neoproterozoic and much of the Paleozoic which deposited glacial conglomerate as well as other sediments to form the Katanga Supergroup and rift-related Karoo Supergroup. Basalt eruptions blanketed the Karoo Supergroup in the Mesozoic and Zambia shifted to coal and sandstone formation. Geologically recent windblown sands from the Kalahari Desert and alluvial deposits near rivers play an important role in the modern surficial geology of Zambia. The country has extensive natural resources, particularly copper, but also cobalt, emeralds, other gemstones, uranium and coal.
The geology of Senegal formed beginning more than two billion years ago. The Archean greenschist Birimian rocks common throughout West Africa are the oldest in the country, intruded by Proterozoic granites. Basins formed in the interior during the Paleozoic and filled with sedimentary rocks, including tillite from a glaciation. With the rifting apart of the supercontinent Pangaea in the Mesozoic, the large Senegal Basin filled with thick sequences of marine and terrestrial sediments. Sea levels declined in the Eocene forming large phosphate deposits. Senegal is blanketed in thick layers of terrestrial sediments formed in the Quaternary. The country has extensive natural resources, including gold, diamonds, and iron.
The geology of Thailand includes deep crystalline metamorphic basement rocks, overlain by extensive sandstone, limestone, turbidites and some volcanic rocks. The region experienced complicated tectonics during the Paleozoic, long-running shallow water conditions and then renewed uplift and erosion in the past several million years ago.
The Vasquez Formation (Tvz) is a geologic formation cropping out at the eponymous Vasquez Rocks in southern California. The formation dates to the Late Oligocene to Early Miocene.
