Synopsis

Two fish-bearing levels are present in the Lower Old Red Sandstone of the island of Kerrera, near Oban. On the north side of Port Dubh [NM 791267] 11 m of basal conglomerate and cross- bedded sandstone rests unconformably on deformed Easdale Slates. The succeeding 17 m of strata are predominantly fine-grained sandstones and red mudstones with desiccation cracks, but contain two levels that include grey, laminated, fossiliferous lithologies. These are termed the Upper and Lower Port Dubh fish beds. Thick conglomerates complete the sequence. Lacustrine shales at Orasaig [NM7980 2675] yield organic remains interpreted as algal/bacterial mats, and delicate impressions of algal thalli. Fish remains are rare at this locality. The fauna of the Port Dubh fish beds comprises agnathan fish (the cornuate osteostracan Cephalaspis, a non-cornuate osteostracan, Gylenaspis n. n., and an anaspid), arthropods (a millipede, and a ‘eurypterid’), and rare plant debris. The animal remains are disarticulated, and have been transported. Arthropod trackways (Siskemia, Diplichnites and Beaconichnus) and fish traces (Undichna unisulca) are closely associated with the fish beds. Short vertical burrows are present, rarely associated with radial bedding-parallel grooves. The fish beds probably formed in shallow temporary lakes created during burial of the irregular regional unconformity surface, and were eventually covered by fluvial conglomerates. Contemporaneous volcanic activity close to the locality resulted in the intrusion of andesites into wet sediment, and the dominance of volcanic debris in sandstones and conglomerates. The general features of the fauna support previous palynological evidence that the rocks are latest Silurian to earliest Devonian in age.

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