Reef diagenesis on tectonically active Siluro-Devonian margin, northern New Brunswick, Canada

Abstract

Documented here is a partial burial history of a foredeep at the active margin of the closing Iapetus Ocean in Late Silurian and Early Devonian times in northern New Brunswick, Canada. Although mainly greywackes and volcanics were deposited in the foredeep at the front of a series of thrust slices, the narrow fringing reefs of the Laplante reef formed around the margins of the Mirimichi Highland and the Elmtree Inlier upthrust blocks in Pridolian times. These reefs are now preserved only as remnant allochtonous blocks within the slope greywackes, but they contained abundant cavities and pore spaces which were filled with cements and sediments reflecting the burial history of these reefs and the evolution of fluids through them. A cathodoluminescence and petrography study of these cements and their chemistry throws considerable light on their timing and relations to tectonism. Marine calcite cements precipitated as fibrous cements in the larger growth cavities while the reefs were growing, and a sequence of CL-zoned early burial bladed calcite cements rapidly followed during the subsequent deposition of the Free Grant Formation as the foredeep subsided. During this subsidence phase significant compression is shown by fracturing (F1) synchronous with cementation, as well as by compression-related pressure-solution. In the earliest Gedinnian further thrusting and sea level lowering exposed the Laplante reefs and karst processes caused significant dissolution and erosion, and subsequently large reef blocks were transported downslope into the basin where reburial, massive injection of overlying siliciclastic sediments, and brecciation due to fracturing took place, as well as extensive pressure solution. All these processes can be related to the significant compression associated with thrusting during the westward movement of the Mirimichi and Elmtree Inlier blocks. This was the major S2 event. During the later Early Gedinnian the foredeep was filled with the siliciclastics and mafic volcanics of the Mitchell Settlement Formation in subaerial and shallow marine environments indicating a general shallowing of the foredeep. Fractures (F3) probably formed by unloading during this uplift phase were first enlarged by criculating corrosive groundwaters of largely freshwater origin, and then filled with ankeritic saddle dolomites which derived their significant amounts of iron from the mafic volcanics. The dolomites probably represent the last syntectonic diagenetic event. Later calcite cements and a fracture lining zoned dolomite, plus minor fracture filling barite show only minor deformation and were probably formed after the main orogenic phase during the Mid Devonian. Subsequently all the dolomites were calcitized to some degree during the general emergence of the area in late Devonian or early Carboniferous times by circulating fresh waters.