The Late Precambrian/Lower Cambrian Bonahaven Dolomite of Islay and its stromatolites

Synopsis The Bonahaven Dolomite (?295 m thick) lies in the Dalradian sequence between the Port Askaig Tillite (beneath) and the Jura Quartzite (above) and is best developed on Islay (Inner Hebrides). The sequence in the formation at five coastal outcrops is figured and four members erected. Member 3 (?150 m thick) contains stromatolite structures at ten horizons. These are described in terms of their bed geometry (laterally continuous, discontinuous), lamination (0.5–2 mm thick) and growth structures (stratiform, laterally linked hemispheroids, columnar, irregular). Ptygmatic sandstone crack–structures are ubiquitous in thinly interbedded sandstone/siltstone beds; they are thought to be subaqueous contraction cracks. The apparent palaeoclimatological contradiction posed by a dolomite overlying a tillite is discussed, and it is suggested that late Precambrian dolomites may not have needed a warm climate to form.