jimologist:Silicified magnesian limestone of the Silkstone formation in Ipswich dates from the Eocen
jimologist:Silicified magnesian limestone of the Silkstone formation in Ipswich dates from the Eocene (56-33.9MA), formed from carbonates in freshwater lakes. It is still quarried for agricultural dolomite in the region, with occurrences of hard, dense chert throughout some outcrops.Microcrystals of silica (SiO2) grow within the original carbonate sediments that eventually become limestone. Dissolved silica is also transported to the site by groundwater movement. Silica nodules and concretions grow from microcrystals and, if numerous enough, can enlarge and merge with each other to form nearly continuous layers of chert within the carbonate sediments. Dolomite from the quarry picture here also contains microscopic palygorskite, a magnesium aluminium phyllosilicate (Mg,Al)2Si4O10(OH)·4(H2O). The palygorskite diminishes the quality of the dolomite, but can be a commodity in the right abundance with the right extraction/production technology. Interbedded basalts are recorded within the Silkstone formation. Geological records from over 50 years ago record a basaltic dyke in the vicinity of the quarry pictured here, but no accounts of basaltic rocks on the property are known. However, I recognised some rocks that could only be evidence very weathered and degraded vesicular basalt. The basaltic lava may also have contributed to the mobilisation and transport of dissolved silica, influencing the formation of the chert. (See also previous post, Oct 2014:https://jimologist.tumblr.com/post/100491326429/limestone-hills-a-chalky-limestone-crowns-the) -- source link
#geology#science#chemistry#metamorphic#dolomite#ipswich#chert#limestone