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In order to predict the style and impact of post-depositional modification of carbonate successions, well-studied and accessible outcrop analogues are invaluable. The Lower Carboniferous (Dinantian) carbonate platforms of the Pennine Basin of northern England have a long history of investigation. As such, they offer the potential to evaluate the mechanisms and timing of fluid flux during extensional tectonism, post-rift basinal subsidence and inversion. This study concentrates upon the diagenetic evolution of the late Dinantian of the southern margin of the Askrigg Platform of North Yorkshire and a comparison with published data from the age-equivalent Derbyshire Platform. A pattern of consistent, diagenetic modification during early diagenesis is evident, but key differences occur in the burial realm. On both the southern margin of the Askrigg Platform and the Derbyshire Platform, patterns of dolomitization, hydrocarbon emplacement and mineralization can be determined on the platform that reflect the diagenetic evolution of the adjacent basins. However, within the study area of the Askrigg Platform, there is only local evidence for a fault/fracture control on the migration of Mg-enriched, hydrocarbon-bearing fluids. In contrast, on the Derbyshire Platform, burial diagenesis is intimately associated with NW–SE- and NE–SW-trending faults and fractures. Data suggest that pervasive cementation in the marine and meteoric realm occluded matrix porosity in both areas, such that fluid migration was almost entirely fracture controlled. With the localization of structural deformation along the Craven Fault Zone, and a low abundance and density of open fault/fracture networks, circulation of fluids on to the southern margin of the Askrigg Platform was inhibited, however. Furthermore, the presence of local aquifers in the Craven Basin may have led to fluid expulsion from the basin during early burial.
In order to predict the style and impact of post-depositional modification of carbonate successions, well-studied and accessible outcrop analogues are invaluable. The Lower Carboniferous (Dinantian) carbonate platforms of the Pennine Basin of northern England have a long history of investigation. As such, they offer the potential to evaluate the mechanisms and timing of fluid flux during extensional tectonism, post-rift basinal subsidence and inversion. This study concentrates upon the diagenetic evolution of the late Dinantian of the southern margin of the Askrigg Platform of North Yorkshire and a comparison with published data from the age-equivalent Derbyshire Platform. A pattern of consistent, diagenetic modification during early diagenesis is evident, but key differences occur in the burial realm. On both the southern margin of the Askrigg Platform and the Derbyshire Platform, patterns of dolomitization, hydrocarbon emplacement and mineralization can be determined on the platform that reflect the diagenetic evolution of the adjacent basins. However, within the study area of the Askrigg Platform, there is only local evidence for a fault/fracture control on the migration of Mg-enriched, hydrocarbon-bearing fluids. In contrast, on the Derbyshire Platform, burial diagenesis is intimately associated with NW–SE- and NE–SW-trending faults and fractures. Data suggest that pervasive cementation in the marine and meteoric realm occluded matrix porosity in both areas, such that fluid migration was almost entirely fracture controlled. With the localization of structural deformation along the Craven Fault Zone, and a low abundance and density of open fault/fracture networks, circulation of fluids on to the southern margin of the Askrigg Platform was inhibited, however. Furthermore, the presence of local aquifers in the Craven Basin may have led to fluid expulsion from the basin during early burial.
The diagenetic evolution of thick, cool-water Pliocene limestones that formed within a forearc basin to accretionary wedge setting in eastern North Island, New Zealand, can be usefully tracked by applying at the thin-section scale the concepts of stratal patterns (onlap, offlap, discontinuity surfaces) in sedimentary sequences. The petrographic approach, supported by geochemical data, involves recognizing genetically related packages of zoned cements under cathodoluminescent (CL) light, named cement suites, which are bounded in thin section by (correlative) diagenetic discontinuities, including dissolution surfaces, renucleation surfaces and/or fractures. Based initially on detailed petrographic study of the early Pliocene Kairakau Limestone formation, a bryozoan-epifaunal bivalve-barnacle grainstone to rudstone, this procedure identifies five distinctive cement suites labelled K1–K5 separated, respectively, by discontinuity surfaces d1 to d4, and referred to collectively as the Kairakau diagenetic motif. Suites K1 and K2 have a pre-compaction origin and are inferred to have formed in a sedimentary system paced by high-frequency glacio-eustasy cycles, and reflecting deposition in transgressive (TST), highstand (HST) and regressive (RST) systems tracts, followed by initial shallow burial. Cement suite K1 is developed only locally, typically immediately above (early TST) and sometimes below (late RST) sequence boundaries. It consists of neomorphosed marine turbid cements growing upon the abraded surface of skeletons, and is bounded above by a dissolution surface (d1). Pre-compaction cement suite K2 has this dissolution surface at its base and a fracture surface (d2) at its top. K2 cements formed from oxidizing waters, either under shallow marine burial or, more likely, mixed marine-meteoric influences; they are inferred to relate mainly to the HST-RST portion of a depositional cycle. Post-compaction cement suites K3–K5 comprise pore-filling and fracture-hosted cements that formed during the burial of depositional sequences by overlying sequences, and during subsequent uplift. Suite K3 comprises ferroan calcite cements precipitated from compaction-driven reduced fluids that are terminated against fracture event d3, whereas suites K4 and K5 are interpreted as telogenetic cement phases that formed from meteoric, dominantly oxidizing, waters during uplift and exhumation of the whole succession and are separated by fracture and dissolution surface d4. Significantly, the same Kairakau diagenetic motif is developed in all the other Pliocene limestone occurrences in the study area. In an attempt to explain the emplacement of the successive cementing aquifers within limestones of different ages and separated by thick siliciclastic deposits, a cement stratigraphic model for the Pliocene succession concludes the paper, utilizing the concept of onlap and downlap cementational trends within the pre- and post-compaction cement suites of the eastern North Island carbonates. Ideal pre-compaction onlap-downlap diagenetic suites K1 and K2 mimic the evolution of the depositional environment from marine to subaerial forced mainly by short-term high-frequency (104 – 105 a) relative sea-level changes, whereas their post-compaction counterparts (suites K3–K5) record burial followed by exhumation of the sediment pile forced by subsidence and tectonic mechanisms of longer duration (105–107 a).
It is generally accepted that carbon isotope variations in seawater were muted between c. 2.06 Ga, after the end of the Lomagundi carbon isotope excursion (LCIE), and c. 1.3 Ga. Evidence is presented here that c. 30 myr after the end of the LCIE, the biogeochemical cycle of carbon experienced a short-term ( c. 2 myr), high-amplitude (up to +8.4‰ V-PDB) perturbation, recorded in the Horseshoe rift basin, Western Australia. The basin was initiated at c. 2.03 Ga with deposition of fluvial and shallow-marine sandstones, followed by the eruption of flood basalt, and culminated with the deposition of platform carbonates, and accompanying volcaniclastic and siliciclastic sediments (Wooly Dolomite). The Horseshoe rift basin during deposition of the Wooly Dolomite was fault-compartmentalized but connected to an ocean. Six depositional sequences make up the Wooly Dolomite. Sequence 1 records establishment of a carbonate platform conformably on basalt and coevally with volcaniclastic sedimentation. All other sequences have dominant carbonate-platform deposits and are unconformity-bounded. Sequence 3 contains a c. 57 m thick section with 13 C-enriched carbonates bracketed between carbonates with close to 0‰ carbon isotope values. Further high-resolution chemostratigraphic studies may reveal a more complex pattern of carbon isotope variations during the ‘boring billion years’, but without precise geochronology similar short-term carbon isotope excursions in carbonate successions could be incorrectly correlated to the LCIE. Supplementary material: Table 2 including chemical and isotopic data, sample locations and their position in Figure 8 is available at https://doi.org/10.6084/m9.figshare.c.2868055 .
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