Multiple Parallel Microstylolites and Early Diagenetic Pressure Solution in Chalk: ABSTRACT

Abstract

Pressure-solution phenomena, including both early-stage microstylolites and late-stage macrostylolites, are locally common in chalk. It is now well known that Upper Cretaceous chalks of northwestern Europe exhibit a wide range of stylolitic development in association with nodular fabrics and hardgrounds, generally in Thalassinoides-rich facies. It is not widely recognized, however, that virtually uncemented chalk commonly exhibits extensive microstylolitization of remarkably unique character. Multiple parallel horizontal microstylolites are well-developed in homogeneously white non-nodular pure-calcitic chalks of Denmark and elsewhere. They can be seen, however, only when samples are treated with contrast-enhancing methods, such as the application of light oil, to increas the visibility of small-scale primary and secondary structures in the sediment. The microstylolites are nearly planar and some are slightly wavy, but almost none are zigzag or sharply spiked. They occur in fairly evenly spaced sets that render a finely laminated appearance to the rock upon close inspection. Typically, each dark lamina is only about 10-50 µm thick, and they are spaced approximately 100-500 µm apart. The lamination planes obviously are diagenetic and not primary, because they are imprinted on top of a totally bioturbated ichnofabric; the tiny dissolution seams cut straight through some burrows and are diverted around others. The relationship of these planar microstylolites to healed hairline microfractures and to various trace fossils, especially Zoophycos, suggests that the microstylolites are very early diagenetic in origin, postdating he burrows but predating or coinciding with the microfractures. End_of_Article - Last_Page 252------------