ABSTRACT

The Lower Fine-grained Quartzite (Middle Dalradian) of Islay, Scotland is subdivided into two facies. Facies 1 consists of massive-bedded, cross-stratified and rippled orthoquartzites. The basal portion is massive, planar bedded, with asymmetrical wavy surfaces on the tops of each bed. The middle and upper portions of Facies I are characterized by sharp-based and sharp-topped trough and planar sets of cross-stratification whose orientation is bimodal-hipolar. Upper sets of cross-stratification are rounded; some sets are drapped with mudstone beds. Both straight-crested and ladder ripples are common. Micro-cross-laminated sets are separated by wavy and bifurcated flaser bedding.

Facies 2 consists of siltstone and mudstone, with accessory fine-grained orthoquartzite. Ubiquitous mudracks, isolated flat and thick lenticular bedding, tidal bedding and burrowing structures are common to this facies.

The two facies are organized into a sharp-based, fining-upward sequence with Faces 1 at the base and Facies 2 at the top. The base of the sequence consists of a sharp contact overlain with massive sandstones (Interval A), which grades into cross-stratified sandstone (Interval B). Next above is a thin set of microcross-laminated orthoquartzite (Interval C) whose orientation is 90° or 180° to the underlying crossstratified set. Some of the cross-bedded and micro-cross-laminated orthoquartzites are draped with thin mudstone (Interval D). The top of the fining-upward sequence consists of Facies 2 mudstones and siltstones (Interval E).

The depositional environments of both Facies 1 and 2 are interpreted to be tide-dominated. The sandstones of Facies 1 were deposited in both a tide-dominated shallow, sub-tidal and intertidal low tidal flat or low tidal sand bar environment, whereas Facies 2 siltstones and mudstones were deposited in a high tidal flat environment. The sharp-based, fining-upward sequences are regressive sequences consisting of basal shallow subtidal sandstones (interval A), intertidal sand flat or sand bar sandstones (Intervals B, C, D) and high tidal flat mudstones (Interval E).

Comparison of the sedimentary features of the Lower Fine-grained Quartzite with sedimentary features from both Recent tidal areas of the North Sea and of the Minas Basin of the Bay of Fundy, and 45 documented intertidal and tide-dominated clastic sedimentary rocks, confirms the tidal origin of these beds. The depositional environments of many orthoquartzites may be similar to the tidal environments of the lower Fine-grained Quartzite.