L371 Chert, bedded

Two rock types, with an irregular contact between them.

1 – Very dark brown, microcrystalline rock. This fizzes with acid. It is a limestone.

2 – Grey and black, intermingled, microcrystalline silica. Brittle and well jointed. Some medium grained white quartz vein material.

1 – A fine to medium grained mosaic of calcite with cross-sections of foraminifera and other shell material.

2 – Microcrystalline silica, quartz forming a uniform mosaic texture, slightly coarser in some places than others. Scattered with grains of calcite and opaques, both typically 0.1 mm across. Criss-crossing calcite veins, some dark red-brown discolouration.

This rock has a complicated history. In essence the chert (2) has replaced the limestone (1). The irregular patches of limestone within the chert are probably relics of pre-existing limestone, but the calcite rhombs are related to the calcite veining and therefore later than the chert. Similarly the opaques, which cut across the structure of the chert, formed later than the chert.

The cubic form of the opaques is suggestive of pyrite, but we cannot be sure.

bedded chert