GEOLOGICA BALCANICA, 32.2-4, Sofia, Decemb. 2002, p. 41-43

Origin of the Early-Middle Jurassic stratigraphic gaps in the Central Balkanids

Ivo Sapunov; ·Veneta Belivanova

Geological Institute, Bulgarian Academy of Sciences, 1113 Sofia, Bulgaria

Introduction

Numerous stratigraphic gaps are known from the Bulgarian Lower and Middle Jurassic sec­tions. The more considerable of them a priori have been accepted as a result of regressions, emersions, erosion and new transgressions.

The most attractive among the Early-Middle Jurassic gaps in this country is the pre­Callovian stratigraphic gap which is very well developed in the Central Balkanids. We made a palaeoecologic (Sapunov) and microfacial (Belivanova) study of the sediments just below and above the gap in limited localities where the gap is a diastem and usually is marked by hardgrounds. Beside this, we made a similar re­search in continuous Bathonian-Callovian sec­tions in the West Balkan Range. The aim of this research was the comparison between the depo­sitional environments of the continuous Ba­thonian -Callovian sediments and the Bathonian and Callovian from the cover and the basement of the pre-Callovian diastem in the Central Balkanids.

The Bathonian -Callovian sequences near the village of Gintsi, Sofia District, West Balkan Range

The Callovian is situated in the first 1 m of the Yavorets Formation. It is represented by dark grey micritic limestones with ammonites. In the very base of the Formation M acrocephalites spp. (Lower Callovian) are found. In the higher levels representatives of Hecticoceras (subgen­era) (Middle- ?Upper Callovian) have been

6 Geologica Balcanica, 2-4/2002

established. In the highest 2 m of the Bov Formation

(clayey limestones and marls) which lies below the Yavorets Formation Clydoniceras cf. discus (index of the highest Upper Bathonian ammo­nite zone) is found. The above-cited ammonite succession shows that riear the Bathonian­Callovian boundary a stratigraphic gap does not exist.

As a result of the microfacies study from the lowermost 10 em of the Yavorets Formation a pelagic wackestone was described. It is built up mainly of abundant filaments from thin-shelled bivalves (ATaHacoB, 1953, 1957; ATaHacoB, CTO}IHOB, 1956; HaqeB, 1957; HaqeB, CaJJYHOB, 1959; Koleva-Rekalova et al., 2002). In the older aulgarian literature (1953-1959) it is well­known as "nematic structure" or "fibrous mi­crostructure" represented by algae. Koleva­Rekalova et al. (2002) considered that these fila­ments belong to Bositra. This microfacies is cor­related with the Standard Microfacies Type 3 -"pelagic mudstone and wackestone" (Wilson, 1975; Fhigel, 1982). Limestones with filaments can be classified as deposits in subtidal (sublitoral) and bathial environments (Fliigel, 1982).

Mud supported texture with rare crinoid frag­ments as well as filaments of thin -shelled bivalves is typical for the clayey limestones/ marls. The deposition of this sediments is pos­sible to proceeds in calm marine environments of moderate depth, without intensive current energy.

These conclusions are in consent with the Bathonian-Callovian faunal spectrum estab­lished in the region of the Gintsi village. Beside

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the above cited ammonite taxa in the Callovian part of the spectrum take part some Pseudo­perisphinctidae and single Phylloceratina. The spectrum shows that during the Bathonian and the Callovian the depositional environments are in the framework of a lower part of the deep sublitoral - depth about 150-200 m.

The pre-Callovian stratigraphic gap in the Teteven District (Central Fore-Balkan)

The cover of the gap

The gap is covered by the grey micritic lime­stones of the Yavorets Formation in which have been collected numerous of ammonites of Hecticoceras (subgenera), single Kosmoceras, Choffatia etc.

The joint presence of Kosmoceras spinosum (Upper Callovian) and Choffatia (Upper Bathonian-Middle Callovian) proves that the lowermost 10 em of the Yavorets Forma­tion belong to condensed Middle-Upper Ca­llovian.

Two microfacies types have been distin­guished in the basement of the Yavorets Forma­tion in the Teteven District (Polaten, Koznitsa River and Kostina River sections): bioclastic­peloidal packstone/grainstone and mudstone/ wackestone with filaments of thin-shelled pele­cipods. The first microfacial type could be cor­related with the SMF Type 2 - "micro bioclastic calcisiltite". Open sea shelf or deep shelf mar­gin are the possible environments of the deposi­tion. The mudstonefwackestones with filaments of thin-shelled bivalves are correlated with the SMF 3 - "pelagic mudstone and wackestone" which deposition is connected with the deep sublitoral environment (deep shelf margin).

These conclusions are in consent with the faunal spectrum established in the Teteven Dis­trict. The ammonites strongly predominate in it. The representatives of Ammonitina repeatedly exceeded Phylloceratina (ammonites of Lyto­ceratina have not been found). The brachiopods and the belemnites take a very limited part in the spectrum. So defined spectrum shows deep sublittoral conditions - depth about !50-200m.

The basement of the gap

It is represented of sandy bioclastic limestones of the Polaten Formation. From the uppermost 10 em of the Formation are found Oecotraustes

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(Paroecotraustes) serrigerus (Upper Bathonian, lower part), 0. (P.) glojanensis (ibid.) etc. These species together with other non cited here taxa prove that the uppermost levels of the Polaten Formation belong to the lower part of the Up­per Bathonian, Prohecticoceras retrocostatum Zone.

In the uppermost part of the Polaten Forma­tion in Teteven District (Polaten, Koznitsa River and KostiQa River sections) one micro­facies type have been distinguished: bioclastic­lithoclastic packstone/rudstone. To some extent it can be attributed to the SMF 4 - "bioclastic­lithoclastic packstone". It may be supposed that the features of this texture direct probably to­wards the more shallow parts of the deep sublit­toral environment.

This conclusions are in consent with the fau­nal spectrum established in the Teteven Dis­trict. Beside the ammonites, which predominate in the spectrum present brachiopods. The am­monites belong only to Ammonitina. In com­parison with the faunal spectrum from the low­ermost levels of the Yavorets Formation, here are presented some elements which indicate more shallow conditions - greater quantity of the benthos, and absence of Phylloceratina. These arguments give us grounds to cQnsider that the sedimentation of the uppermost levels of the Polaten Formation proceeds in shallower conditions of the deep sulittoral - the depth was about 100-150 m.

The pre-Callovian stratigraphic gap in the Troyan, Kazanluk and Gabrovo Districts (Central Balkan Range)

Because of the more or less intensive submarine washout, from the west to the east the duration of the pre-Callovian stratigraphic gap almost gradually increased. Its lower boundary is found in more lower Middle Jurassic levels (Troyan District), in Lower Sinemurian levels (Gabrovo District), and even in Upper Triassic levels (izolated localities in Kazanluk District).

The cover of the gap

Like in the Teteven District, the cover is repre­sented by the micritic limestones of the Yavorets Formation. In the lowermost 10-20 em of the Formation numerous ammonites as well as single benthic representatives have ·been found. The joint presence of the Upper Callo-

vian Kosmoceras compressum together with the middle Callovian Hecticoceras (Zieteniceras) tuberculatum and H. (Rossieniceras) metom­pha/um acuticosta prove that the lowermost part of the Yavorets Formation belongs to con­densed Middle and Upper Callovian, like in the Teteven District.

One microfacies type have been distinguished in the basement of the Yavorets Formation in Gabrovo District. It is described as pelagic mudstone. Mud supported texture with single allochems of thin -shelled pelecypods as well as undefined micritic fragments (peloids?) are characteristic for this sediments. This microfa­cies type could be correlated with the SMF Type 3 "pelagic mudstone and wackestone". It formation is connected with quite low energy environment in the conditions of the deep shelf margin.

These conclusions are in consent with the faunal spectrum established in the Troyan, Kazanluk and Gabrovo Districts. Here the am­monites strongly predominate among the fau­nas. Ammonitina repeatedly exceeded Phyllo­ceratina (Lytoceratina are not found). Single bivalves, brachiopods and belemnites are pre­sented too. This spectrum is connected with the lower part of the deep sulittoral environment -depth about 150-200 m.

Conclusions

1. The results of our palaeoecologic and micro­facial study of the basement and the cover of the pre-Callovian stratigraphic gap prove that the gap has been realized in submarine condi­tions, especially in the deep sublittoral. They throw away the former ideas which connected this gap with a Bathonian regression, a Late Bathonian-Early Callovian emersion and ero­sion, and a Middle-Late Callovian transgres­sion. The others known inner Early-Middle Ju­rassic stratigraphic gaps (all of them more insig­nificant in comparison with the pre-Callovian gap) have been realized in submarine condi­tions.

2. The bathirnetric conditions which have ex­isted during the sedimentation of the cover and the basement of the pre-Callovian gap are simi­lar to the conditions which have existed during the continuous Bathonian-Callovian sedimen­tation.

3. The transition from submarine stratigra­phic gap to a normal sedimentation (in which the biostratigraphic superposition can be fol-

lowed) passes through a stage of very slow ini­tial sedimentation (represented by conde_nsation of the Middle and Upper Callovian). -

4. As a result of the study of the pre-Callovian stratigraphic gap, it outlines a general natural development according to which four stages are differentiated: normal sedimentation ~ slow sedimentation ~ submarine stratigraphic gap (result of pause in the sedimentation - dias­tem) ~ submarine stratigraphic gap (result of washout). In some occasions the four stages can be followed (as in the example with the pre­Callovian stratigraphic gap). In others only two or three stages have been realized.

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