Zemes un vides zinātnes Earth and Environment Sciences - Latvijas ...
Zemes un vides zinātnes Earth and Environment Sciences - Latvijas ...
Zemes un vides zinātnes Earth and Environment Sciences - Latvijas ...
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102 ADVANCES IN PALAEOICHTHYOLOGY<br />
have been eroded. The Devonian deposits are also spread throughout the territory of<br />
Latvia, in the Gulf of Riga <strong>and</strong> in the aquatorium of the Baltic Sea. Although not<br />
outcropping, the Lower Devonian deposits are to be fo<strong>un</strong>d in the whole territory of<br />
Latvia, excluding the Viļaka tectonic rampart <strong>and</strong> the territory east of it. The Middle<br />
Devonian rocks are widely represented throughout the whole territory of Latvia <strong>and</strong><br />
exposed in many sites in picturesque outcrops of the Old Red S<strong>and</strong>stone facies rocks.<br />
The distribution of the Upper Devonian deposits corresponds to that of the Middle<br />
Devonian rocks, lacking only in areas where they have been destroyed by glacier. The<br />
Upper Devonian is represented by the Frasnian <strong>and</strong> the Famennian stages. Frasnian<br />
<strong>and</strong> Famennian deposits are rather various by origin <strong>and</strong> composition. At the very<br />
beginning of the Frasnian a new record of mixed carbonate <strong>and</strong> siliciclastic sedimentation<br />
started. Diversity of deposits is particularly characteristic for the Famennian, which is<br />
composed of s<strong>and</strong>stones, dolostones, dolomite marls, siltstones, clays, limestones <strong>and</strong><br />
other rocks intercalating in a complicated pattern (Savvaitova 1977). The Famennian<br />
basin was situated in the Latvian-Lithuanian Depression, which in the west <strong>and</strong> southwest<br />
was periodically connected to the open sea. Hence, Famennian deposits are spread<br />
only in south-western Latvia <strong>and</strong> north-western Lithuania. During maximum<br />
transgressions the Baltic paleobasin was probably connected also with the sea of Middle<br />
Russia through the Pripyat’ Deep (Savvaitova 1977).<br />
The Ketleri Formation comprises s<strong>and</strong>s, s<strong>and</strong>stones, clays, <strong>and</strong> dolomitic marls<br />
which overlie the carbonate deposits of the Žagare Formation. In Latvia the thickness<br />
of the Ketleri Formation reaches 45 m, while in Lithuania it is 41 to 56 meters thick.<br />
The formation is divided into three members separated from each other by the erosional<br />
surfaces (Lyarskaya <strong>and</strong> Savvaitova 1974). The lowermost, Nīgr<strong>and</strong>e Member is<br />
characterized by 8 to 12 meters thick clayey carbonate deposits which overlie an eroded<br />
surface of carbonate rocks of the Žagare Formation. The lower part of the member is<br />
composed of greenish grey <strong>and</strong> brownish dolomite marls with a low content of silt.<br />
Among them several thin beds of greenish grey microcrystalline dolomite <strong>and</strong> clay<br />
occur.<br />
Tiny poorly preserved fish remains have been fo<strong>un</strong>d in the dolomite marls at the<br />
base of the member. P. Liepinš, E. Mark-Kurik <strong>and</strong> V. Talimaa have determined them as<br />
belonging to the taxa: Chelyophorus sp., Bothriolepis ornata Eichwald, Devononchus<br />
sp., Holoptychius cf. nobilissimus Agassiz, Eusthenopteron sp., Osteolepididae gen.<br />
indet., Dipnoi indet., Palaeoniscidae gen. indet. (Liepiņš 1959; Savvaitova <strong>and</strong> Žeiba<br />
1981). However, the identification of a species may be inaccurate <strong>and</strong> causes some<br />
doubt. For example, the two species Bothriolepis ornata <strong>un</strong> B. ciecere Lyarskaja resemble<br />
each other in respect to the structure of the head shield <strong>and</strong> ornamentation, but they<br />
differ remarkably in the proportions of the tr<strong>un</strong>k armour plates <strong>and</strong> character of dorsolateral<br />
<strong>and</strong> ventro-lateral ridges (Lukševičs 2001). Quite often it is impossible to<br />
determine the species by small fragmentary remains. L. Lyarskaya mentions<br />
Devononchus tenuispinus (Gross) among the fish remains from the Nigr<strong>and</strong>e Member<br />
(Lyarskaya <strong>and</strong> Lukševičs 1992).<br />
The amo<strong>un</strong>t of carbonate in rocks decreases towards the upper part of the member,<br />
<strong>and</strong> dolomite marls are replaced by clay <strong>and</strong> clayey siltstone, in some cases by finegrained<br />
s<strong>and</strong>y material. Spores of Stenozonotriletes sp. sp., S. triangularis Naum.,<br />
S. simplex Naum., Euryzonotriletes sp., Trematozonotriletes intermedium (Waltz)