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Original Article |
School of Earth and Environmental Sciences, University of Portsmouth, Burnaby Building, Burnaby Road, Portsmouth PO1 3QL, UK (e-mail: jim.hendry@port.ac.uk)
Syn- and post-lithification cemented fractures are abundant in concretions formed during shallow burial of Hauterivian turbidites in the Outer Moray Firth. Early fractures are irregular, bifurcating, and contain entrained sand grains. They are confined to the concretion bodies, but commonly splay into narrower fracture swarms at the margins. Multiple, cross-cutting fills of calcite microspar have similar isotopic composition to adjacent concretionary cements, suggesting that rupture and cementation cycles occurred as host concretions lithified. Expulsion of cementing pore fluids from the fractures produced cement protrusions at concretion margins. A hydraulic fracture mechanism resulting from localized permeability heterogeneity and episodic seismic activity is proposed. Rapid seismic loading created high pore fluid pressures for fracture dilation in lithifying concretions and, through cavitation and CO2 degassing, a trigger for rapid homogeneous nucleation of calcite. Many of the early structures were subsequently reactivated as brittle tensile fractures, and host a variety of mesogenetic cements. Reactivation probably occurred in response to stress amplification during rapid Palaeocene burial. By this time lithification of the concretions had dramatically changed their rheology and failure style. Minor permeability remained in the concretion cements adjacent to reactivated fractures, permitting invasion of intercrystalline voids by late diagenetic fluids.
