| Gauja Formation | |
| Type: | Geological formation |
| Age: | Givetian ~ |
| Period: | Givetian |
| Prilithology: | Sandstone |
| Otherlithology: | Siltstone |
| Namedfor: | Gauja River |
| Country: |  Estonia Latvia |
| Subunits: | Sietin & Lode Members |
| Underlies: | Amata Formation |
| Overlies: | Plavinas Formation |
| Thickness: | 80m (260feet) |
The Gauja Formation is a Middle Devonian fossil locality in Estonia and Latvia.[1] It is named after the Gauja River, where it is exposed along the banks.[2]
The Gauja Formation has a maximum thickness of 80m (260feet). It is composed of weakly to moderately cemented layers of fine-grained to very fine-grained sandstone. The layer is predominantly light to yellowish-gray in color, but can be pinkish brown or variegated. It is mostly composed of quartzose arenites. It is overlain by the 30m (100feet) thick Amata Formation and the Plavinas Formation.[3]
The Gauja Formation contains two cyclic members. The lower layers are known as the Sietiņi Member and are composed mostly of sandstone with a thin layer of siltstone at the top.
The upper layers are thicker, and the lower part of it is known as the Lode Member. The Lode Member dates from the Middle Devonian and is composed of light colored sandstone. The area is indicative of a near-shore environment of retreating sea.
The Sietiņi and Lode members have yielded numerous fossils of fishes. Among them are Asterolepis, Bothriolepis, Psammolepis, Cheirolepis, Glyptolepis baltica, Laccognathus panderi, Livoniana multidentata, Strunius, Panderichthys and Megadonichthys kurikae[4] [5] [6] The first fossils of young placoderms ever discovered were found in the Gauja formation. Ranging in size from as small as 10.4 mm to as big as 38 mm (0.4 to 1.4 in) long, these fossils are important for our understanding of ontogeny in extinct fish.
Only large plant remains and miospores are known from this member. Examples of which include Hostinella, Archaeopteris, Retusotriletes rugulatus, and Ancyrospora. Palynological studies indicate that the Middle to Upper Devonian boundary may exist in this member.[7]