Two different quartz cement morphologies have been identified in originally smectite-rich Late Cretaceous mudstones. 1) Extremely fine-grained micro-pore-filling quartz (sub-micron to 1-3 µm) found as spherical discrete grains or as part of short chains/clusters of several inter-connected micro-quartz crystals. This type is also typically inter-grown with clay crystals and found embedded in the fine-grained illitized clay matrix below 2500 m burial (80-85 ºC). 2) Quartz platelets oriented normal to the overburden as laterally extensive sheets with typical irregular or patchy development. The quartz platelets originate as extremely thin flakes that evolve into well-developed patchy continuous quartz cement as identified at 4300 m/150 ºC. The identified quartz cement were most likely sourced by silica released from the dissolution of smectite and precipitation of illite-smectite and illite due to evaluated silica super-saturation in the system. The observed inter-connected micro-quartz crystals are associated with a sudden Vp-velocity increase reflecting an increase in the mudrock stiffening interpreted to be related to pervasive micro-quartz networks and aggregate formation. The quartz platelets will reinforce and further cement together the micro-quartz networks and aggregates during burial.


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