The Mid-Ordovician oolitic ironstones of North Wales
Oolitic ironstones occur within the Lower Palaeozoic Welsh Basin as isolated deposits found over a wide geographical area. There are two phases of ironstone deposition, a minor Upper Arenig phase and a Mid-Ordovician (Upper Llanvirn to basal Caradoc) phase. Both correlate with eustatic falls of sea level which exposed the Irish Sea Landmass lying immediately to the northwest. This exposure resulted in deep chemical weathering and generation of lateritic soils. Erosion of this material formed the source for the oolitic' ironstones in the Welsh Basin. The ironstones formed above stratigraphic hiatuses on sediment starved shallow water shoals, formed by synsedimentary faulting. These shoals were the favourable sites for the formation of berthierine peloids, which formed the nuclei for ooids. Additionally, they were also the site for the accumulation of berthierine mud, which was closely linked with the development of ferruginous algal mats. Bacterial reduction of organic material associated with ironstones, supplied the necessary reducing conditions for the formation and preservation of berthierine from a kaolinite/iron oxide precursor. Ooids formed by rolling over the muddy surface and mechanically accreting berthierine. Subsequent tidal current reworking of this sediment resulted in the formation of the characteristic lithological features of the ironstones, representing a shallowing-up sequence. Progressive current winnowing led to the formation of a sequence with an upward increasing ooid content and decreasing mud content. The upper facies of the ironstones is an ooid bar deposit worked by tidal currents. Cessation of current reworking allowed faunal colonisation of the bar with significant bioturbation of the sediment, destroying primary sedimentary structures. The presence of some grain-ironstones indicate the original sedimentary state of the upper facies. Tectonic instability during deposition, by synsedimentary faulting, resulted in the formation of disturbed ironstones, and debris flows within the ironstone sequences. Many features of the ironstones are diagenetic in origin, especially the formation of phosphate nodules within the ironstone sequence. These formed just below the sediment/sea water interface, and some nodules were reworked into overlying beds. The source was phosphorus released from adsorption on clays and iron oxides, and also released from organic material. Later siderite development in the ironstones is indicated by the presence of primary cements in grain-ironstones and secondary alterations in pack-ironstones. The generation of diagenetic siderite was dependant upon the amount of organic material within the ironstones, bacterial reduction of which resulted in the formation of bicarbonate and ferrous ions. Sane ironstones were subsequently altered during the Caradoc phase of volcanic activity. The formation of magnetite and stilpnomelane within the ironstones were caused by metasanatic activity associated with dolerite sills and microgranite intrusions. Siderite alteration and base metal sulphides resulted fram late stage hydrothermal activity by some microgranites. Contact metarrorphism by granophyric intrusions led to the extensive replacement of the ironstones by pyrite. Regional metarrorphism resulted in the progressive change of berthierine to chamosite and increased lattice ordering of chamosite.