Peripheral bulge—a causal mechanism for the Lower/Middle Ordovician unconformity along the western margin of the Northern Appalachians

Jacobi, Robert D.

doi:10.1016/0012-821x(81)90131-x

Cited by 185 publications

(75 citation statements)

References 34 publications

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“…In that situation, passage of a forebulge (a regional flexure marking the initiation of an orogenic phase) has been postulated as a causeof the unconformity (Jacobi 1981;Knight etal. 1991).…”

Section: Discussionmentioning

confidence: 99%

Anatomy of an unconformity on mid‐Oligocene Amuri Limestone, Canterbury, New Zealand

Lewis

1992

New Zealand Journal of Geology and Geophysics

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Data from 50 field localities are used to interpret a middle Oligocene (c. 28-32 Ma) regional unconformity cut into the top of the pelagic Amuri Limestone in the Canterbury Basin of New Zealand. The Amuri Limestone comprises coccolith-rich foraminiferal biomicrite deposited in an outer shelf to upper bathyal setting on flat-lying calcareous mudstones and glauconitic sandstones. Prior to deposition of the next succeeding unit, substantial erosion and dissolution variably reduced original thicknesses of up to 70 m and in places totally removed the limestone; locally, differences range from 0 to 10+ m over less than 1 km.The character of the unconformity varies dramatically over short distances, and the immediately overlying sediments vary substantially in character (and age). Trace fossils indicate local differences in substrate consistency at the time the unconformity surface was buried; at their most complex, they reflect omission plus post-omission burrows and borings. Large solution pits are present at a few localities and small-scale dissolution features are more common; both are inferred to require local emergence into a meteoric groundwater regime. In contrast, no evidence for emergence exists in other nearby exposures of the unconformity surface. The unconformity is everywhere overlain by marine sediments; limestone clasts in the immediately overlying sediment show varied degrees of glauconitisation and phosphatisation, indicative of different times of residence time on the seafloor prior to burial.Eustatic fall and rise of about 30 m may have accompanied development of the unconformity but alone cannot explain the varied unconformity features. The range of features indicates uplift, folding, and differential erosion during the late Whaingaroan Stage of the Landon Series, then resubsidence of the unconformity during the Duntroonian Stage of the Landon Series (all within the Chattian Stage). The mild tectonic event is inferred to mark the initiation of a new plate boundary and the beginning of late Cenozoic regression in the New Zealand region.

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Section: Discussionmentioning

confidence: 99%

Anatomy of an unconformity on mid‐Oligocene Amuri Limestone, Canterbury, New Zealand

Lewis

1992

New Zealand Journal of Geology and Geophysics

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“…Lithology Aguathuna *70 m thick; dolomitized peritidal carbonates; burrow mottled dolostone, dolomicrite, and stromatolitic dolostones; rare thin shale beds; skeletal peloidal, oolitic and oncolitic grainstones; peloidal wackestones and packstones; microbial (stromatolitic) lime mudstones Catoche Up to 160 m thick; mainly limestones in the lower part (*120 m) and dolostones in the upper part (*40 m); bedded gray carbonates; bioturbated at times; skeletal garinstone to peloidal wackestone and a packstone; microbial lime mudstones Boat Harbour Up to 170 m thick; lower member (*44 m) of partially dolomitized grainstones, wackestones, thrombolites, and laminated microbial mats; mainly microbial lime mustone and stromatolitic mounds but rarely grainstones in the middle member (*70 m) between the lower disconformity and the overlying Boat Harbour disconformity; peloidal grainstone to microbial lime mudstones in the upper member (*52 m, Barbace Cove Member) Watts Bight *70 m thick; partially dolomitized microbial lime mudstone in the lower part (*33 m); burrowed grainstone in the middle part (*25 m); microbial lime mudstone in the upper part (*11 m) rapid sweeping of a tectonic peripheral bulge on the margin in earliest Middle Ordovician (Jacobi 1981;. The migration of that lithospheric high led to compression, block faulting, uplift, and erosion of the St. George carbonate platform and the development of the regional St. George Unconformity, which leaves the first physical imprint of the transition from a passive margin to a foreland basin (Mussman and Read 1986;Knight et al , 2007Lavoie 1994;Cooper et al 2001).…”

Section: Formationmentioning

confidence: 99%

High-resolution isotope stratigraphy of the Lower Ordovician St. George Group of western Newfoundland, Canada: implications for global correlation

Azmy

Lavoie

2009

Can. J. Earth Sci.

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Abstract:The Lower Ordovician St. George Group of western Newfoundland consists mainly of shallow-marine-platform carbonates (*500 m thick). It is formed, from bottom to top, of the Watts Bight, Boat Harbour, Catoche, and Aguathuna formations. The top boundary of the group is marked by the regional St. George Unconformity. Outcrops and a few cores from western Newfoundland were sampled at high resolution and the extracted micritic materials were investigated for their petrographic and geochemical criteria to evaluate their degree of preservation. The d 13 C and d 18 O values of wellpreserved micrite microsamples range from -4.2% to 0% (VPDB) and from -11.3% to -2.9% (VPDB), respectively. The d 13 Ccarb profile of the St. George Group carbonates reveals several negative shifts, which vary between *2% and 3% and are generally associated with unconformities-disconformities or thin shale interbeds, thus reflecting the effect of or link with significant sea-level changes. The St. George Unconformity is associated with a negative d 13 Ccarb shift (*2%) on the profile and correlated with major lowstand (around the end of Arenig) on the local sea-level reconstruction and also on those from the Baltic region and central Australia, thus suggesting that the St. George Group Unconformity might have likely had an eustatic component that contributed to the development-enhancement of the paleomargin. Other similar d 13 C carb shifts have been recorded on the St. George profile, but it is hard to evaluate their global extension due to the low resolution of the documented global Lower Ordovician (Tremadoc -middle Arenig) d 13 C carb profile.Résumé : Le Groupe de St. George (Ordovicien inférieur) de l'ouest de Terre-Neuve comprend surtout des carbonates de plate-forme marine peu profonde (épaisseur *500 m). De la base au sommet, il comprend les formations suivantes : Watts Bight, Boat Harbour, Catoche et Aguathuna. La limite supérieure du groupe est marquée par la discordance régionale de St. George. Des affleurements et quelques carottes provenant de l'ouest de Terre-Neuve ont été échantillonnés à haute rés-olution et les roches micritiques extraites ont été étudiées pour leurs critères pétrographiques et géochimiques afin d'évaluer leur degré de préservation. Les valeurs d 13 C et d 18 O de micro-échantillons de micrites bien préservées variaient respectivement entre -4,2 % à 0 % « VPDB » et entre -11,3 % à -2,9 % (VPDB). Le profil d 13 C carb des carbonates du Groupe de St. George montre plusieurs changements à des valeurs négatives; ces changements varient entre *2 % à 3 % et ils sont généralement associés à des discordances ou à de minces interlits de shale, reflétant ainsi l'effet des changements importants de niveau de la mer ou ayant un lien avec ces changements. La discordance de St. George est associée à un changement négatif d 13 C carb (*2 %) du profil et elle est corrélée à un important bas niveau dans une reconstruction locale du niveau de la mer (vers la fin de l'Arenigien) ainsi qu'à des bas niveaux dans des régio...

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“…The carbonate-dominated passive margin of the Middle to Late Cambrian consisted of a narrow high-energy carbonate platform (Port au Port Group) that evolved into an Early to earliest Middle Ordovician wide, low-energy carbonate platform (St. George Group) (James et al 1989). The passive margin period ended with onset of significant sea-floor subduction, which led to the migration of a tectonic peripheral bulge on the margin in earliest Middle Ordovician (Jacobi 1981;Knight et al 1991). The migration of the tectonic peripheral bulge caused compression, block faulting, uplift, and erosion of the St. George carbonate platform.…”

Section: Geologic Settingmentioning

confidence: 99%

Dolomitization of the Lower Ordovician Aguathuna Formation carbonates, Port au Port Peninsula, western Newfoundland, Canada: implications for a hydrocarbon reservoir

et al. 2008

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Abstract:The Lower Ordovician Aguathuna Formation (*100 m thick) is formed of shallow-marine carbonates, which constitute the uppermost part of the St. George Group of western Newfoundland. Sedimentation was paused by a major subaerial exposure (St. George Unconformity), which likely developed a significant pore system in the underlying carbonates by meteoric dissolution. The sequence has been affected by multiphase dolomitization that caused complex changes in the rock porosity. The Aguathuna dolomites are classified into three main generations ranging in crystal size between *4 mm and 2 mm. The occurrence of fabric-retentive dolomicrites implies that dolomitization likely started during the early stages of diagenesis. Although dolomitization is pervasive in the upper part of the formation and significantly occludes the pores, some intervals in the lower part have higher porosity. The development of lower permeable layers overlain by an impermeable (seal) cap suggests a possible potential diagenetic trap. Unlike sabkha deposits, the Aguathuna carbonates do not have evaporite interlayers. Furthermore, the low Sr contents (*96 ppm) and the 18 O values of earlier dolomites (-3.3% to -6.9% VPDB (Vienna Pee Dee Belemnite)) are also difficult to reconcile with a brine origin. The Sr/Ca molar ratios (0.0067-0.0009), calculated for the earliest dolomitizing fluid, suggest a modified seawater origin, likely mixed sea and meteoric waters. The least radiogenic 87 Sr/ 86 Sr values of the earliest dolomite are consistent with those of early Ordovician seawater, which supports an early-stage diagenesis. Petrography, geochemistry, and fluid inclusions of the late dolomites suggest precipitation at higher temperatures (*73-95 8C) in deeper burial environments from hydrothermal solutions.Résumé : La Formation d'Aguathuna (Ordovicien inférieur), d'une épaisseur *100 m, est composée de carbonates marins d'eau peu profonde; ces carbonates constituent la partie supérieure du Groupe de St. George de l'Ouest de Terre-Neuve. La sédimentation a été arrêtée par une exposition subaérienne majeure (discordance de St. George), ce qui a vraisemblablement conduit au développement d'un important système de pores dans les carbonates sous-jacents par dissolution mété-orique. La séquence a été affectée par plusieurs phases de dolomitisation qui ont causé des changements complexes à la porosité de la roche. Les dolomites d'Aguathuna sont classifiées selon trois générations principales dont la dimension des cristaux varie de *4 mm à 2 mm. La présence de dolomicrites qui retiennent la texture implique que la dolomitisation a probablement débuté durant les premières phases de la diagenèse. Bien que la dolomitisation soit pénétrante dans la partie supérieure de la formation et ait bouché les pores de manière significative, quelques intervalles dans la partie inférieure ont une porosité plus élevée. Le développement de couches inférieures perméables sur lequel repose une couche impermé-able (un scellement) suggère un potentiel piège diagénétique. Cont...

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Peripheral bulge—a causal mechanism for the Lower/Middle Ordovician unconformity along the western margin of the Northern Appalachians

Cited by 185 publications

References 34 publications

Anatomy of an unconformity on mid‐Oligocene Amuri Limestone, Canterbury, New Zealand

Anatomy of an unconformity on mid‐Oligocene Amuri Limestone, Canterbury, New Zealand

High-resolution isotope stratigraphy of the Lower Ordovician St. George Group of western Newfoundland, Canada: implications for global correlation

Dolomitization of the Lower Ordovician Aguathuna Formation carbonates, Port au Port Peninsula, western Newfoundland, Canada: implications for a hydrocarbon reservoir

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