Abstract The Great American Carbonate Bank (GACB) comprises the carbonates (and related siliciclastics) of the Sauk megasequence, which were deposited on and around the Laurentian continent during Cambrian through earliest Middle Ordovician, forming one of the largest carbonate-dominated platforms of the Phanerozoic. The Sauk megasequence, which ranges upwards of several thousand meters thick along the Bank's margin, consists of distinctive Lithofacies and fauna that are widely recognized throughout Laurentia. A refined biostratigraphic zonation forms the chronostratigraphic framework for correlating disparate outcrops and subsurface data, providing the basis for interpreting depositional patterns and the evolution of the Bank. GACB hydrocarbon fields have produced 4 BBO and 21 TCFG, mostly from reservoirs near the Sauk-Tippecanoe unconformity. The GACB is also a source of economic minerals and construction material and, locally, serves as either an aquifer or repository for injection of waste material. This Memoir comprises works on biostratigraphy, ichnology, stratigraphy, depositional facies, diagenesis, and petroleum and mineral resources of the GACB. It is dedicated to James Lee Wilson who first conceived of this publication and who worked on many aspects of the GACB during his long and illustrious career.

Abstract The great American carbonate bank (GACB) refers to a system of carbonates and related siliciclastics that were deposited on and around the Laurentian continent during the Cambrian, Early Ordovician, and earliest Middle Ordovician. This laterally continuous and diverse sequence has been assigned different sets of formation and group names across the GACB, such as Arbuckle, Beekmantown, Bonanza King, Deadwood, Ellenburger, El Paso, Knox, Prairie du Chien, and Potsdam (to name just a few of the more widely used), but characteristic lith of acies and fauna that are found throughout North America, Greenland, northwestern Scotland, Svalbard, and the pre-Cordillera of Argentina are observed. Fundamental to our understanding of the components of the GACB is the biostratigraphy that ties the disparate parts into a cohesive whole. The importance of biostratigraphy was recognized early on by Jim Wilson, and he worked to integrate it with lithostra-tigraphy, sedimentology, bio facies, cyclic sedimentation, and sequence stratigraphy. Today, biostratigraphy continues to play a key function in devising a correlation framework across the GACB and in unraveling its evolution. Biostratigraph-ic zonation has advanced considerably since the days when Sloss et al. (1949) coined the term “Sauk sequence” and Palmer (1981) subdivided the Sauk into three subdivisions. It is now possible to identify more than 20 biostratigraphic zones within the Sauk succession for the Laurentian continent (with finer subdivision possible on a more local basis) using mostly trilobites and conodonts. This finer biostratigraphic zonation has led to an increased recognition of events that can

Abstract This chapter describes and presents a newly compiled map illustrating the paleogeography of Laurentia during the earliest Ordovician, a time when the great American carbonate bank was at one of its greatest extents and a period for which the most is understood. The map depicts the known or postulated extent of the inner detrital belt, the great American carbonate bank and the more problematic (commonly structurally relocated) outer detrital belt. The period on which the map is based and discussed in the accompanying text is based on the Early Ordovician (early Ibexian) (early Tremadocian) Stonehenge transgression.

Abstract The carbonate strata of the great American carbonate bank (GACB) have been subdivided and correlated with ever-increasing precision and accuracy during the past half century through use of the dominant organisms that evolved on the Laurentian platform through the Cambrian and the Ordovician. Trilobites and conodonts remain the primary groups used for this purpose, although brachiopods, both calcareous and phosphatic, and graptolites are very important in certain facies and intervals. A series of charts show the chronostratigraphic units (series and stages) currently in use for deposits of the GACB and the biostratigraphic units (zones, subzones, and biomeres) whose boundaries delineate them. Older and, in some cases obsolete, stages and faunal units are included in the figures to allow users to relate information from previous publications and/or industry databases to modern units. This chapter also provides a brief discussion on the use of biostratigraphy in the recognition and interregional correlation of supersequence boundaries within the Sauk and Tippecanoe megasequences, and the varied perspectives on the nature of biostratigraphic units and their defining taxa during the past half century. Also included are a concise update on the biomere concept, and an explanation of the biostratigraphic consequences of a profound change in the dynamics of extinction and replacement that occurred on the GACB in the Early Ordovician when the factors responsible for platformwide biomere-type extinctions faded and ultimately disappeared. A final section addresses recent and pending refinements in the genus and species taxonomy of biostrat-igraphically significant fossil groups, the potential they hold for greatly improved correlation, and the obstacles to be overcome for that potential to be realized.

Abstract The Cambrian–Ordovician Sauk megasequence of the great American carbonate bank (GACB) comprises a succession of mixed lithologies, but dominantly carbonate rocks, whose thickness, stratigraphy, and lithofacies distribution reflect the presence of a complex of intrabank platforms and basins, aulacogens, and tectonically active margins that together make up the major part of the paleocontinent Laurentia. The stratigraphy of the Sauk megasequence can be subdivided and correlated across the GACB through the recognition of major unconformities, marine flooding events, and stratigraphic stacking patterns, documented within a robust biostratigraphic framework. The base of the Sauk megasequence is typically defined as the contact of Cambrian or sub-Tippecanoe-megasequence Ordovician rocks with Precambrian, mostly igneous, basement. The Sauk megasequence is overlain (commonly unconformably) by the Middle Ordovician Tippe-canoe megasequence, the age of which varies across the GACB. Where subsequent erosion has occurred, the Sauk megasequence may be overlain by rocks younger than the Tippecanoe megasequence. Palmer’s (1981b) subdivision of the Sauk megasequence into Sauk I, II, and III subsequences (now referred to as supersequences) is widely, but not universally, recognized. Across many areas of the GACB, the Sauk III supersequence of Palmer can be subdivided into two supersequences (defined as “Sauk IIIA” and “Sauk IIIB” in this chapter), based on an unconformity and/or biostratigraphic changes near the Cambrian-Ordovician boundary. Additional significant unconformities and marine flooding events that can be correlated across much of the GACB are summarily described in this chapter. The recognition of correlatable surfaces across the GACB has been challenging because of local syndepositional tectonics and paleotopography, and lithofacies heterogeneity. However, confidence in correlation across the GACB has been greatly enhanced by an increasingly refined biostratigraphic framework.

Abstract Karsting and collapse brecciation in the Lower Ordovician carbonates have been recognized for many years. However, the time of cavern formation and the geochemical hydrology responsible is debatable. In this chapter, I intend to review pertinent literature and evaluate the evidence of the presence of paleocaverns, the time of their formation, the history of cavern collapse to form collapse breccias, and the relationship of collapse breccias to structure. I will not review chemical hydrology issues because a discussion on the geochemical environment is pertinent only after the time of cavern development has been adequately resolved. The most robust data sets come from outcrop studies. Outcrops with extensive exposures reviewed here are the El Paso Group in the Franklin Mountains, west Texas; the Pogonip Group in Nopah Range, southeastern California; and the St. George Group in Newfoundland. The Lower Or-dovician outcrops in central Texas, the Mississippi Valley, Virginia, and the Arbuckle Mountains are also useful. Robust subsurface data sets include the Ellenburger Group of Texas; the Knox Group of Tennessee, Kentucky, and Ohio; and the Arbuckle Group of central Kansas. Core descriptions from the subsurface Arbuckle Group in Oklahoma and Arkansas are also helpful. The most convincing evidence of cavern formation is roof collapse, that is, evidence that breccia blocks are below their stratigraphic positions. The time of cavern formation is more difficult to ascertain and most commonly is based on the source of the infilling sediment by comparing lithologies and, in places, using biostratigraphy. The history of the collapse can be determined only in the most extensive outcrops and mining operations, although modern three-dimensional (3-D) seismic volumes are useful. The relationship of collapse breccias to structure is basically a timing issue and can be resolved only by detailed geologic studies. I conclude from reviewing published data that convincing evidence shows that an extensive cavern system existed in the Lower Ordovician carbonates at the time of the Sauk-Tippecanoe unconformity. In some areas, the unconformity surface is highly irregular and appears to represent karst terrain. Caverns located far below the unconformity were most likely formed in response to internal disconformities. Lower Ordovician fractures and faults can have a controlling influence on the location and geometry of the caverns. Collapse of these caverns produced the collapse breccia and fracturing of the cavern roof. In some instances, cavern collapse has produced structural sags similar to those produced by the expansion related to strike-slip faulting. In extreme cases, collapse of large caverns produced breccia pipes that extended more than 330 m (>1000 ft) into overlying Ordovician, Silurian, and possibly Devonian units.

Abstract The diversity, abundance, distribution, and depth of trace fossils in the Cambrian–Ordovician deposits in Laurentia, from California and Nevada to New York (United States) and Quebec (Canada), are a series of biozones that record the early evolution and radiation of metazoans in shallow-marine environments. The Neoproterozoic–Paleozoic transition (NPT) plays a significant part in understanding the diversity, timing, rate, circumstance of first appearances and subsequent metazoan radiations, and trends in ecospace utilization through the Cambrian–Ordovician as recorded in carbonate and mixed carbonate-siliciclastic deposits. The first burrows with spreiten and complex branching, designated as the Phycodes (Treptichnus) pedum Zone, delineate the base of the Cambrian. This zone overlies the uppermost Neoprotero-zoic Harlaniella podolica Zone and is composed of relatively simple horizontal burrows. The Rusophycus avalonensis Zone, characterized by the occurrence of more complex burrow architectures, overlies the Phycodes (Treptichnus) pedum Zone and represents the last pretrilobite biozone. As recorded by ichnofabric through the Cambrian–Ordovician, trends in the depth and extent of bioturbation illustrate the spatial and temporal change in ecospace utilization. With the onset of the substrate (media) revolution across the NPT, animals adapted, and evolved new innovations to penetrate microbial-mediated sedimentary environments. This change reflects ongoing Cambrian–Ordovician evolution and radiation of metazoans from shallow inner-shelf environments to middle-shelf environments with increasing biogenic reworking through time. Nonetheless, a mixing depth of 6 cm (2.4 in.) was not surpassed until later in the Ordovician. This pattern ismirrored by the first appearances of trace-fossil ichnotaxa in shallow-water environments that later gradually moved offshore to shelf environments. The ichnological patterns are debated, however, asevidence of deep burrowing (i.e.,>6cm[>2.4in.]) has been described from the Cambrian and the Ordovician deposits in the Mackenzie Mountains (western Canada) and the Great Basin (western United States). Evidence for the early evolution of continental ecosystems does not exist in Laurentian deposits until the Late Ordovician, although some evidence for the invasion of land in the Early Cambrian and the Early Ordovician exists.

Abstract This chapter contains hydrocarbon production statistics and maps for the great American carbonate bank reservoirs. Reservoir characteristics depend on depositional setting and subsequent modification by paleokarst processes and fractures. Trap styles include a wide variety of stratigraphic and structural accumulations. Hydrocarbon production occurs primarily at or near the Sauk-Tippecanoe unconformity in association with source rock and seal, but production can occur in strata many hundreds of feet below this unconformity. Strategies for optimizing exploration and production and a wide variety of analogs are discussed. It is hoped that improved understanding of historical production, along with application of new tools and insights, will lead to future discoveries in both old and new areas.

Abstract The Middle Cambrian through Lower Ordovician carbonate rocks of North America host some of the largest economic Mississippi Valley-type (MVT) base-metal sulfide deposits in the world. These rocks also host numerous subeconomic MVT deposits, minor and trace occurrences of mineralization, and hydrocarbon fields. Mississippi Valley-type deposits commonly contain bitumen, pyrobitumen, and/or liquid petroleum, suggesting that MVT mineralization is associated with the generation and migration of hydrocarbons and thus is a normal part of basin evolution. In addition to sulfide and sulfate mineralization, common characteristics of MVT deposits are large-scale dissolution and brecciation of carbonate rocks, precipitation of large volumes of dolomite and calcite cements, epigenetic (hydrothermal) dolomitization, and recrys-tallization of preexisting dolomite. Mineralizing fluids have the effects both of increasing the original porosity by dissolution and brecciation and of occluding porosity because of precipitation of cements. Mississippi Valley-type fluids are not localized but affect sedimentary rocks across large regions. It is likely that most, if not all, Cambrian–Ordovician carbonate rocks in North America have undergone at least some diagenetic alteration because of exposure to these fluids. This conclusion is supported by the observation that subeconomic MVT mineralization has been observed in Cambrian and Lower Ordovician carbonates throughout much of North America. These fluids commonly have affected carbonate petroleum reservoir rocks in regions distal from known ore deposits. Mississippi Valley-type mineralization is believed to result from a complex mixing and/or cooling of saline fluids expelled from sedimentary basins. These fluids have temperatures ranging from 60 to 250°C. Most of the fluids originate from evaporated seawater or water that has dissolved halite and that has interacted with sedimentary rocks and, possibly, basement rocks. Several geochemical and hydrogeological mechanisms have been proposed for MVT deposits. However, the precise mechanisms driving fluid flow and deposition are not yet completely understood. Major tectonic events associated with MVT mineralization of the great American carbonate bank strata include the Acadian orogeny (Late Devonian–Early Mississippian) for early mineralization in the Appalachian Mountain region, the Alleghanian-Ouachita orogeny (Pennsylvanian–Permian) for mineralization in the Appalachian and midcontinent regions, and the Laramide orogeny (Late Cretaceous–early Tertiary) for the Cordilleran region.

Abstract Decimeter-scale sampling of the Cambrian and the lowermost Ordovician (Sauk megasequence) rocks of the Llano uplift, Texas, has produced a finely resolved biostratigraphic framework based primarily on trilobites and conodonts. Systematically collected trilobites of the Llano Uplift allow recognition of 13 biozones that extend from the Bolaspidella Biozone (Cambrian System, Marjuman Stage) through the Symphysurina Zone (Ordovician System, upper Skullrockian Stage). Systematic collection of conodonts has produced specimens assignable to 13 zones that range from the Proconodontus tenuiserratus Zone (Cambrian System, Sunwaptan Stage) through the Rossodus manitouensis Zone (Ordovician System, upper Skullrockian Stage). The base of the Ordovician System in the Llano uplift, as elsewhere, has been identified by the lowest occurrence of the conodont Iapetognathus fluctivagus and is closely approximated by the lowest occurrences of the cosmopolitan trilobite Juyjuyaspis and the Laurentian trilobite Symphysurina “bulbosa.” Although the overlying Ordovician strata of the Ellenburger Group have not been systematically sampled, scattered trilobite collections do establish the approximate positions of the base of the Stairsian Stage (based on Paraplethopeltis ) and the base of the Jeffersonian Stage (based on Rananasus and Jeffersonia ) in the Tanyard and Honeycut Formations, respectively.

Abstract The Arbuckle Group of the midcontinent comprises the mid-southern part of the great American carbonate bank (GACB) and consists mostly of carbonates with a few laterally consistent sandstones. The Arbuckle Group is found in the Anadarko, Ardmore, and Arkoma Basins and surrounding environs in the Texas panhandle, Oklahoma, and Arkansas. These basins represented a significant downwarp associated with early rifting in the area now located in the southern one half of both the states of Oklahoma and Arkansas. Similar to other parts of the GACB, the thick widespread Cambrian–Ordovician Arbuckle Group was deposited as mostly restricted shallow-water marine carbonates. The Arbuckle is a cyclic carbonate dominated by intertidal and shallow subtidal facies. In some areas, supratidal or deeper subtidal facies are observed. The depositional model is represented byan extensive, dominantly regressive, tidal flat with persistent peritidal facies across much of the GACB. These peritidal cycles shallow upward with significant variation in thickness from as thin as 4 ft (1.2 m) to more than 110 ft (>33.5 m) thick. Large-scale regional changes in relative sea level may have had a large influence on the type of cycles and sequences that formed during Arbuckle deposition. Arbuckle strata, especially within third-order sequence boundaries, are correlatable across the basin. Within the sequence boundaries, cycles can be further grouped into packages of sequences that are composed mostly of either intertidally or subtidally dominated cycles. Detailed local to regional correlation of the facies bundles can be made with gamma-ray and resistivity logs; however, facies are commonly obscured by a strong diagenetic overprint that makes detailed correlation difficult. Reservoirs in the Arbuckle are complex, and porosity is controlled by original depositional fabric, diagenesis, paleokarst, and fracture overprint. Upper subtidal and lower intertidal facies typically have the depositional fabric most conducive to reservoir development. Dia-genetic changes are a continuum that begins with early diagenesis, including hypersaline or evaporative conditions as well as vadose and phreatic conditions, and followed by deep phreatic to late thermal diagenesis. Evidence that porosity formed during multiple diagenetic phases exists. Dolomitization and precipitation events are also evidenced at various levels of the profile. Dolomite is the most abundant mineral and can be subdivided into early (syn-genetic to penecontemporaneous) hypersaline dolomite, shallow burial mixed-water (phreatic) dolomite, and deeper burial to thermal (baroque and xenotopic) dolomite. The super-Sauk unconformity is recognized as evidence of a eustatic sea level drop and has been used to mark the boundary between the Sauk and Tippecanoe depositional mega-sequences. The Arbuckle Group contains multiple unconformities at major sequence boundaries. Paleokarst is especially prevalent beneath the super-Sauk unconformity, especially along major sequence boundaries with related unconformity surfaces. Paleokarstic features in the Arbuckle Group have been identified in outcrop in the Arbuckle Mountains of southern Oklahoma and in the southern Ozark uplift in northeastern Oklahoma. Numerous cores and logs indicate collapse breccias that are interpreted to have formed in response to karst conditions. The Arbuckle Group is an important petroleum reservoir in the midcontinent, and has great potential especially for natural gas. Exploration is enhanced by understanding the complex relationships of depositional processes, stratigraphic relationships, paragenesis, and structural overprints. Reservoir development is typically along sequence boundaries, especially where facies have strong diagenetic overprints from dolomitization and dissolution associated with paleokarstic events. No major source rocks exist below or within the Arbuckle Group, so the best reservoirs are structurally related with strong fracture overprints and juxtaposed with source rocks or are along migration pathways.

Abstract Exposures of Ordovician rocks of the Sauk megasequence in Missouri and northern Arkansas comprise Ibexian and lower Whiterockian carbonates with interspersed sandstones. Subjacent Cambrian strata are exposed in Missouri but confined to the subsurface in Arkansas. The Sauk-Tippecanoe boundary in this region is at the base of the St. Peter Sandstone. Ulrich and associates divided the Arkansas section into formations early in the 20th century, principally based on sparse collections of fossil invertebrates. In contrast, the distribution of invertebrate faunas and modern studies of conodonts will be emphasized throughout this chapter. Early workers considered many of the stratigraphic units to be separated by unconformities, but modern analysis calls into question the unconformable nature of some of their boundaries. The physical similarity of the several dolomites and sandstones, complex facies relations, and lack of continuous exposures make identification of individual formations difficult in isolated outcrops. The oldest formation that crops out in the region is the Jefferson City Dolomite, which may be present in outcrops along incised river valleys near the Missouri-Arkansas border. Rare fossil gastropods, bivalves, brachiopods, conodonts, and trilobites permit correlation of the Cotter through Powell Dolomites with Ibexian strata elsewhere in Laurentia. Conodonts in the Black Rock Limestone Member of the Smithville Formation and the upper part of the Powell Dolomite confirm regional relationships that have been suggested for these units; those of the Black Rock Limestone Member are consistent with deposition under more open marine conditions than existed when older and younger units were forming. Brachiopods and conodonts from the overlying Everton Formation assist in interpreting complex facies within that formation and its correlation to equivalent rocks elsewhere. The youngest cono-donts in the Everton Formation provide an age limit for the Sauk-Tippecanoe unconformity near the southern extremity of the great American carbonate bank. The correlation to coeval strata in the Ouachita Mountains of central Arkansas and in the Arbuckle Mountains of Oklahoma and to rocks penetrated in wells drilled in the Reelfoot rift basin has been improved greatly in recent years by integration of biostratigraphic data with lithologic information.

Abstract The Knox Group in the Black Warrior Basin comprises the southeastern part of the great American carbonate bank (GACB) and consists mostly of carbonates. The Black Warrior Basin is a Carboniferous foreland downwarp developed over a passive margin of early Paleozoic age. Similar to other parts of the GACB, the thick widespread Cambrian–Ordovician Knox Group was deposited as mostly shallow-water, restricted, marine carbonates. The Knox depositional model is that of an extensive regressive tidal flat, made up of shallow subtidal, intertidal, and rare supratidal facies. These facies shallow upward and comprise numerous cycles in the Knox. There exists a tremendous variation in thickness of the cycles that can be as thin as 3 to more than 100 ft (1 to >30 m) thick. These cycles can be further grouped into packages of sequences that are mostly composed of intertidally dominated or subtidally dominated cycles. Large-scale regional changes in relative sea level may have a large influence on the type of cycles and sequences that formed during the Knox. Knox strata, especially within third-order sequence boundaries, are correlatable across the basin. Detailed local to regional correlation of the facies bundles can be made with gamma-ray and resistivity logs; however, facies are commonly obscured by strong diagenetic overprints that make detailed correlation difficult. Numerous unconformities occur within the Knox Group at major sequence boundaries. The super-Knox unconformity is recognized as evidence of a globally eustatic sea level drop and has been used to mark the boundary between the Sauk and Tippecanoe depositional megasequences. Paleokarst is observed regularly within the Knox carbonates, especially along major sequence boundaries with related unconformity surfaces. Paleokarstic features in the Knox Group have been identified in outcrop in central Alabama, with the Knox containing a sinkhole filled with Middle Ordovician strata. Numerous cores contain collapse breccias that are interpreted to have formed in response to karst conditions. With some paleokarst collapse breccias occurring 3000ft (914 m) below the top of the Knox, itislikely that some of these breccias formed in response to intra-Knox unconformities. In the Knox, diagenetic changes are a continuum that begins with early diagenesis, including hypersaline or evaporative, vadose, and phreatic conditions, and followed by deep phreatic to late thermal diagenesis. Evidence exists that porosity formed (some of which may be thought of as karst) during each of these diagenetic phases. Conversely, precipitation events and dolomitization also occurred throughout various levels of the profile. Volumetrically, dolomite is the most abundant mineral. Knox dolomite can be subdivided into early (syngenetic to penecontemporaneous) hypersaline dolomite, shallow burial mixed-water (phreatic) dolomite, and deeper burial to thermal (baroque and xenotopic) dolomite. Reservoir development is typically along sequence boundaries, especially where facies have strong diagenetic overprints from dolomitization and dissolution associated with paleokarstic events. The best reservoirs are structurally related, with strong fracture overprints.

Abstract Analysis of well core and cuttings from the Black Warrior Basin in Mississippi reveals the presence of a Middle Ordovician (Whiterockian) erosional unconformity interpreted to be equivalent to the well-known Knox-Beekmantown unconformity in eastern North America. The unconformity occurs at the top of a peritidal dolostone unit known informally as the upper dolostone, whose stratigraphic placement has been the subject of a long-standing controversy. The unconformity, which represents the Sauk-Tippecanoe megasequence boundary on the North American craton, was previously thought to be short-lived or altogether absent in the Black Warrior Basin. The unconformity is characterized by subunconformity solution pipes, solution-collapse breccias, internal sedimentation, and erosional truncation of the underlying dolostone unit. This erosional surface is veneered with sand- to pebble-size, rounded and angular lithoclasts of the underlying dolostone, and rounded and angular quartz sand and silt. Extensive secondary porosity developed in the upper dolostone below the unconformity. Although much of this porosity was later occluded by internal sedimentation and pore-filling dolomite and calcite cement, porous zones remain in the upper dolostone. Based on conodont biostratigraphy from four cores and from a previous study on cuttings from a nearby well, the unconformity is middle Whiterockian in age and likely spans most or all of the Histiodella holodentata Biozone.

Abstract The southern Appalachian part of the Cambrian–Ordovician passive margin succession of the great American carbonate bank extends from the Lower Cambrian to the lower Middle Ordovician, is as much as 3.5 km (2.2 mi) thick, and has long-term subsidence rates exceeding 5 cm (2 in.)/k.y. Subsiding depocenters separated by arches controlled sediment thickness. The succession consists of five supersequences, each of which contains several third-order sequences, and numerous meter-scale parasequences. Siliciclastic-prone supersequence 1 (Lower Cambrian Chilhowee Group fluvial rift clastics grading up into shelf siliciclastics) underlies the passive margin carbonates. Supersequence 2 consists of the Lower Cambrian Shady Dolomite–Rome-Waynesboro Formations. This is a shallowing-upward ramp succession of thinly bedded to nodular lime mudstones up into carbonate mud-mound facies, overlain by lowstand quartzose carbonates, and then a rimmed shelf succession capped by highly cyclic regressive carbonates and red beds (Rome-Waynesboro Formations). Foreslope facies include megabreccias, grainstone, and thin-bedded carbonate turbidites and deep-water rhythmites. Supersequence 3 rests on a major unconformity and consists of a Middle Cambrian differentiated rimmed shelf carbonate with highly cyclic facies (Elbrook Formation) extending in from the rim and passing via an oolitic ramp into a large structurally controlled intrashelf basin (Conasauga Shale). Filling of the intrashelf basin caused widespread deposition of thin quartz sandstones at the base of supersequence 4, overlain by widespread cyclic carbonates (Upper Cambrian lower Knox Group Copper Ridge Dolomite in the south; Conococheague Formation in the north). Supersequence 5 (Lower Ordovician upper Knox in the south; Lower to Middle Ordovician Beekmantown Group in the north) has a basal quartz sandstone-prone unit, over-lainbycyclic ramp carbonates, that grade downdip into thrombolite grainstone and then storm-deposited deep-ramp carbonates. Passive margin deposition was terminated by arc-continent collision when the shelf was uplifted over a peripheral bulge while global sea levels were falling, resulting in the major 0- to 10-m.y. Knox–Beekmantown unconformity. The supersequences and sequences appear to relate to regionally traceable eustatic sea level cycles on which were superimposed high-frequency Milankovitch sea level cycles that formed the parasequences under global greenhouse conditions.

Abstract In the central Appalachians, carbonate deposition of the great American carbonate bank began during the Early Cambrian with the creation of initial ramp facies of the Vintage Formation and lower members of the Tomstown Formation. Vertical stacking of bioturbated subtidal ramp deposits (Bolivar Heights Member) and dolomitized microbial boundstone (Fort Duncan Member) preceded the initiation of platform sedimentation and creation of a sand shoal facies (Benevola Member) that was followed by the development of peritidal cyclicity (Dargan Member). Initiation of peritidal deposition coincided with the development of a rimmed platform that would persist throughout much of the Cambrian and Early Ordovician. At the end of deposition of the Waynesboro Formation, the platform became subaerially exposed because of the Hawke Bay regression, bringing the Sauk I supersequence to an end. In the Conestoga Valley of eastern Pennsylvania, Early Cambrian ramp deposition was succeeded by deposition of platform-margin and periplatform facies of the Kinzers Formation. The basal Sauk II transgression during the early Middle Cambrian submerged the platform and reinitiated the peritidal cyclicity that had characterized the pre-Hawke Bay deposition. This thick stack of meter-scale cycles is preserved as the Pleasant Hill and Warrior Formations of the Nittany arch, the Elbrook Formation of the Great Valley, and the Zooks Corner Formation of the Conestoga Valley. Deposition of peritidal cycles was interrupted during deposition of the Glossopleura and Bathyriscus-Elrathina Biozones by third-order deepening episodes that submerged the platform with subtidal facies. Regressive facies of the Sauk II supersequence produced platform-wide restrictions and the deposition of the lower sandy member of the Gatesburg Formation, the Big Spring Station Member of the Conococheague Formation, and the Snitz Creek Formation. Resubmergence of the platform was initiated during the late Steptoean ( Elvinia Zone) with the expansion of extensive subtidal thrombolitic boundstone facies. Vertical stacking of no fewer than four of these thrombolite-dominated intervalsrecords third-order deepening episodesseparatedbyintervening shallowing episodes that produced peritidal ribbony and laminated mudcracked dolostone. The maximum deepening of the Sauk III transgression produced the Stonehenge Formation in two separate and distinct third-order submergences. Circulation restriction during the Sauk III regression produced a thick stack of meter-scale cycles of the Rockdale Run Formation (northern Virginia to southern Pennsylvania), the upper Nittany Dolomite, the Epler Formation, and the lower Bellefonte Dolomite of the Nittany arch (central Pennsylvania). This regressive phase was interrupted by a third-order deepening event that produced the oolitic member of the lower Rockdale Run and the Woodsboro Member of the Grove Formation in the Frederick Valley. Restricted circulation continued into the Whiterockian, with deposition of the upper Rockdale Run and the Pinesburg Station Dolomite in the Great Valley and the middle and upper parts of the Bellefonte Dolomite in the Nittany Arch region. This deposition was continuous from the Ibexian into the Whiterockian; the succession lacks significant unconformities and there are no missing biozones through this interval, the top of which marks the end of the Sauk megasequence. During deposition of the Tippecanoe megasequence, the peritidal shelf cycles were reestablished during deposition of the St. Paul Group. The vertical stacking of lithologies in the Row Park and New Market Limestones represents transgressive and regressive facies of a third-order deepening event. This submergence reached its maximum deepening within the lower Row Park Limestone and extended into the Nittany arch region with deposition of the equivalent Loysburg Formation. Shallow tidal-flat deposits were bordered to the south and east by deep-water ramp deposits of the Lincolnshire Formation. The St. Paul Group is succeeded upsection by ramp facies of the Chambersburg and the Edinburg Formations in the Great Valley, whereas shallow-shelf sedimentation continued in the Nittany arch area with the deposition of the Hatter Limestone and the Snyder and Linden Hall Formations. Carbonate deposition on the great American carbonate bank was brought to an end when it was buried beneath clastic flysch deposits of the Martinsburg Formation. Foundering of the bank was diachronous, as the flysch sediments prograded from east to west.

Abstract Cambrian–Ordovician shelf-margin deposits of the great American carbonate bank (eastern North America) experienced significant regional dolomitization and/or metamorphism, but the Middle Cambrian Ledger Formation in south-central Pennsylvania contains a shelf-margin facies complex that includes exceptionally well-preserved microbialite sheet reefs riddled with centimeter- to meter-scale submarine cavities. The reefs and associated sands, composed of reef-related allochems, interfinger with ooid shoals, forming a high-energy shelf-margin facies association located near the seaward margin of the Middle Cambrian Laurentian platform. The Ledger Formation’s ooid shoal complex, exposed in the Magnesita Refractories quarry in York County, Pennsylvania, is pervasively dolomitized. Forthcoming research documents multiple stages of dolomitization and dedolomitization in the ooid dolostone; therefore, the ooid dolo-stone is not discussed here. In contrast to the ooid dolostone, most of the Ledger reef facies remains limestone. This has facilitated detailed interpretation of the reef depositional and diagenetic history, including new information presented here. Previous publications describe the Ledger reef geologic setting, mechanisms for generating the cavities, and petrographic and geochemical analyses of radiaxial fibrous and herringbone calcite fibrous submarine cements within the cavities. This chapter provides new information on the microbial reef sheet facies, describes a previously undocumented type of cryptic microbial morphology (endolite), and interprets a 1-m (3.3-ft)-thick intraclastic grainstone bed. Modern reefs in high-energy settings adapt by building robust coral frameworks that can withstand normal current activity and wave action. In The Middle Cambrian, coral framebuilders were absent, so to exploit high-energy ecological niches, organosedimentary constructers, primarily cyanobacteria (±algae and bacteria), had to develop a similarly robust morphology. We propose that low-growing, thick, cohesive microbial sheets, such as documented here from the Ledger Formation, provided minimal wave resistance and, therefore, outcompeted stromatolites and thrombolites to form subtidal wave-resistant structures in such high-energy settings. Similartomodern reefs, these microbial sheets contain cavities across arange of scales from millimeter-size fenestrae to meter-size stromatactis-type voids capable of sheltering and supporting delicate shrubs of Epiphyton -like dendrites and cryptic endolites, as detailed later in this chapter. Microbial processes dominated all ecological niches, forming the substrate, colonizing cryptic spaces, and coating and encrusting other microbes. The reef microbialite consists of weakly bedded sheets composed of shrubs and stubby strands of calcified Epiphyton - and Angulocellularia -like elements. Centimeter-scale domal stromatolites, thrombolites, oncolites, dendrolites, and oval multiple-layered organosedimen-tary cryptic structures, termed “endolites,” form lenses and distinctive structures. Petro-graphically, the microbialite is expressed as clots, stringers, arborescent garlands, and dendritic shrubs. Stromatactislike and fenestral cavities within the microbialite formed primarily through processes of gas and water escape, although syndepositional slumping and channel undercutting produced other types of cavities and void spaces. Grainstone, composed of microbial clasts and fragments, accumulated as cross-bedded intrareef channel sands. Large stromatactislike cavities were stabilized with multiple generations of microdolomite-bearing calcite radiaxial fibrous and herringbone calcite cements and intercalated internal sediment. Cement morphology, internal sediment associations, stable isotopes, and trace element geochemistry suggest that the cements precipitated from marine fluids as magnesium calcite and subsequently stabilized to calcite during diagenesis. The Ledger microbial assemblage closely resembles living cryptic, mat, and domal cyano-bacterial forms reported from the Tikehau Atoll, French Polynesia. Detailed descriptions of the cyanobacteria involved in creating the modern structures provide useful analogies for enigmatic Middle Cambrian fossil morphologies.

Abstract The Cambrian–Ordovician carbonate platform units on the New York promontory of eastern Laurentia reflect the south tropical location of the area. The slow subsidence of the region through much of the Cambrian–Ordovician meant that strong eustatic rises and falls defined unconformity-bound carbonate formations. These depositional sequences aid in paleoocean-ographic reconstruction as they correlate with organic-rich dysoxic–anoxic mudstones on the Laurentian continental slope. Eustatic rise increased insolation as epeiric seas covered the platform and produced climate maximums with reduced deep-water circulation. The oldest carbonate platform unit (Forestdale Marble and equivalents, upper Lower Cambrian) overlies rift facies deposited with the Rodinia breakup and origin of the Iapetus Ocean and marks the transition to a passive margin. Drowning of the Forestdale platform occurred, and the overlying anoxic black mudstone (Moosalamoo Phyllite) abruptly shoals up into tidalite sandstone (Cheshire Formation). This depositional history records a decreased rate of sea level rise as the Cheshire Formation continued to onlap middle Proterozoic basement. Super-Cheshire Cambrian carbonate platform units in the northern Appalachian are mostly hydrothermally dolomitized, record eustatic highs (Dunham, Winooski, and Little Falls Formations), and correlate with black mudstone macroscale units on the slope (Browns Pond and Hatch Hill dysoxic– anoxic intervals). The latest Early Cambrian Hawke Bay regression ended carbonate platform deposition of the Dunham Formation, led to quartz arenite or red shaly dolostone of flap or shoaling deposits on the platform, and was coeval with oxic green mudstone on the continental slope (Hawke Bay oxic interval in Taconian allochthons). Subsequent Middle Cambrian eustatic rise is recorded by dolostone (Winooski and upper Stissing), but carbonate deposition was again suppressed as quartz sand swept toward the shelf margin (Danby Formation) coincident with cratonic transgression by the upper Potsdam Formation (uppermost Middle Cambrian–lower Upper Cambrian). Post-Potsdam depositionwas carbonate dominated through the middle Late Ordovician and included the Beekmantown, Chazy, Black River, and Trenton Groups. The Cambrian-Ordovician boundary is an unconformity between platform carbonates (Little Falls and Tribes Hill Formations of the Beekmantown Group). The Lower Ordovician–lower Upper Ordovician is a series of unconformity-bound platform depositional sequences (Tribes Hill, Rochdale, Fort Cassin, and Providence Island Formations of the upper Saukmegasequence and Chazy Group of the lower Tippecanoe megasequence). The Ordovician depositional sequences coincide with eustatic highs and show a repeated depositional motif (lower transgressive sandstone, upper highstand carbonate). The Ordovician eustatic highs also correlate with thin (as much as 10 m [33 ft] thick) macroscale dysoxic–anoxic black mudstones on the slope. The black mudstones alternate with oxic greenish mudstones, locally with debris flows with giant carbonate blocks on the upper slope (Levis conglomerates), which indicate platform-margin caving during eustatic falls. Ordovician green mudstones are composed of mesoscale redox-carbonate mudstone cycles (Logan cycles) on the upper slope. A major development was the abrupt formation of the latest Early Cambrian–Early Ordovician Franklin Basin in northwestern Vermont. The dysoxic–anoxic Franklin Basin resulted from fault-driven foundering of part of the carbonate platform that overlay the failed arm of the Ediacaran triple junction. This faulting is coeval with the oldest (late Early Cambrian) onlap in the Ottawa-Bonnechere aulocogen. Late Ordovician collision with the Ammonusuc arc ended carbonate platform deposition in the New York promontory region, as sands and muds eroded from the Taconic orogen filled a fore-arc basin and extinguished carbonate deposition across eastern Laurentia.

Abstract The northern Appalachian sedimentary deposit consists of strata that are related to tectonic movements in two significant ways: (1) they are controlled by large-scale tectonic movements and (2) strata that were horizontal when deposited are no longer horizontal and can be used to delineate and measure the extent of structural deformation. Among the most influential alumni of Rensselaer Polytechnic Institute of Troy, New York, was James Hall (1811–1898), the “Father of the Geosyncline.” Hall (1859) was the originator of the geosynclinal concept ( Sharpe, 1998 ). The concept of a geosyncline was inspired by the geologic relationships that were worked out for the northern Appalachian Mountains. Hall observed that, where the Paleozoic marine strata in the interior of North America are thin (thicknesses of only a few hundreds or a few thousands of meters), they are flat lying. By contrast, in the Appalachians, thicknesses of equivalent strata amount to thousands of meters and the strata are not horizontal. Hall hypothesized that the substance of the strata within a trough, where they would be extra thick, provided the mechanism for folding them. Both Hall and James Dwight Dana emphasized an important inference about the Appalachian area that had subsided, throughout the thousands of meters of vertical sinking, the depth of the marine waters had remained shallow (Figure 1 ). In other words, subsidence had been more or less exactly matched by accumulation of sediment. The original idea that a part of the sea floor might subside and yet sediment could