1 Uni ver sity of War saw, In sti tute of Ge ol ogy, ¯wirki i Wigury 93, 02-089 Warszawa, Po land Leonowicz, P., 2015. Storm-in flu enced de po si tion and cyclicity in a shal low-ma rine mudstone suc ces sion -ex am ple from the Mid dle Ju ras sic ore-bear ing clays of the Pol ish Jura (south ern Po land). Geo log i cal Quar terly, 59 (2): 325-344, doi: 10.7306/gq.1221Sedimentological anal y sis of bioturbated mudstones from the Mid dle Ju ras sic ore-bear ing Czêstochowa Clay For ma tion has re vealed com mon rel ics of sed i men tary struc tures, such as thin silt/sand laminae, bed ding-plane ac cu mu la tions of shell debris, small and me dium silt-sand lenses, as well as silt-, sand-and shell-de bris-rich lev els, ac com pa nied by ero sion sur faces and mi nor scours. These fea tures doc u ment pe ri od i cal high-en ergy con di tions of mud de po si tion, in clud ing storm stir ring, event sed i men ta tion and the ac tiv ity of storm-gen er ated bot tom cur rents, which were re spon si ble for sea-floor ero sion and sed i ment sup ply from shal lower parts of the ba sin. The sea bot tom was prev a lently be low the storm wave base, but it rose above it dur ing ex cep tion ally strong storms and in the Early Bathonian zig zag chron when the ba sin ex pe ri enced con sid erable shallowing. The shal low est and high est en ergy con di tions oc curred in the late macrescens subchron, when the bot tom was close to, or above the fair-weather wave base. The es ti mated depth of the sea did not ex ceed sev eral tens of metres, but in some pe ri ods it could be even less than 20 m. Based on the ver ti cal vari a tion of the sand, silt and clay con tents, seven transgressive-re gres sive cy cles have been dis tin guished in the ap prox i mately 75 m thick suc ces sion span ning the Up per Bajocian-Up per Bathonian. Cy cles TR 1 , TR 2+3 , TR 5 and TR 6+7 cor re late with those dis tin guished in the co eval suc ces sion from cen tral Po land, al though the strati graphic po si tion of cy cle bound aries is slightly shifted. These cy cles re cord rel a tive sea level changes that af fected the en tire Pol ish Ba sin. Re gres sions R 2 and R 6 were smaller-scale-events, gen er ated in response to a lo cal tec tonic ac tiv ity and autocyclic shore line progradation.

Anal y sis of the fab ric of lam i nated black mudstones of the Ore-Bear ing Czêstochowa Clay For ma tion (Bathonian, south ern Po land) is used for re con struc tion of sed i men tary pro cesses and con di tions. Small-scale sed i men tary fea tures in di cate that ore-bear ing clays were de pos ited be low storm-wave base in a shal low epicontinental sea, but struc tures re flect ing quiet settling from sus pen sion are not com mon. Most of rec og nized lamina types re cord the ac tiv ity of bot tom cur rents gen er ated by storms. The ben thic fau nal as so ci a tion in di cates that suboxic con di tions pre vailed on the sea-floor, re strict ing infaunal ac tivity to cryptobioturbation. Re cur rent short-lived reoxygenetion events, linked prob a bly to storms, re sulted in the de vel op ment of dysoxic con di tions, re corded by a more di verse trace fos sil as so ci a tion (con sist ing of Chondrites, Trichichnus, pyritized bur rows, Palaeophycus and Protovirgularia) and col o ni za tion of the sea-floor by epifaunal bi valves (Bositra). Truly anoxic con di tions might have been briefly es tab lished, lead ing to mass mor tal ity of Bositra.Key words: black mudstones, par al lel lam i na tion, storm de pos its, ichnofabric, bot tom ox y gen a tion, Ore-Bear ing Czêstochowa Clay For ma tion. INTRODUCTIONMid dle Ju ras sic black mudstones in south ern Po land, referred to as the Ore-Bear ing Czêstochowa Clay For ma tion, have re cently re ceived in creas ing in ter est as re gards their palaeo ec ol ogy. The sed i men tary en vi ron ment of these dark, or ganic-rich rocks that are for the most part strongly bioturbated and rich in di verse ben thic fauna still in spires study. In con trast to the grow ing vol ume of lit er a ture on this de posit (for sum mary see Zatoñ, 2011), sedimentological stud ies re main scarce (Merta and Drewniak, 1998; Gedl et al. 2006a, b, c;Leonowicz, 2012) and un der stand ing of the sed i men tary processes, which largely de ter mined bot tom con di tions, has been poor. This study in te grates microstructural and ichnofabric anal y sis in re con struct ing sed i men tary pro cesses, en vi ron mental en ergy and palaeo ec ol ogy of the sea-floor. It fo cuses on lam i nated mudstones of the Ore-Bear ing Czêstochowa Clay For ma tion, crop ping out in the cen tral part of the Silesian-Cracow Up land in south ern Po land (Fig. 1).The Ore-Bear ing Czêstochowa Clay For ma tion is composed of dark grey, or ganic-rich, cal car e ous mudstone with sev eral ho ri zons of sid er ite and cal car e ous con cre tions, clayey sid er ite bands and sub or di nate in ter ca la tions of sand stone (Dayczak-Calikowska and Kopik, 1973). The greater part is strongly bioturbated and rich in di verse ben thic fos sils; lam inated, sparsely fossiliferous mudstones are much less common. Geo chem i cal and pet ro graph i cal stud ies point to de po sition on a well-ox y gen ated sea-floor with si mul ta neously dysoxic or even anoxic con di tions pre vail ing within the sed i ment (Marynowski et al., 2007...

Multidisciplinary studies of the Middle-Upper Bathonian ore-bearing clays at Gnaszyn revealed variable palaeoenvironmental conditions during the deposition of this seemingly monotonous sequence. We interpret the conditions in the bottom environment and the photic zone, and also evaluate the influence of the adjacent land areas, based on sedimentology, geochemistry, sporomorphs and palynofacies composition, benthic (foraminifera, gastropods, bivalves, scaphopods, echinoderms), planktonic (calcareous nannoplankton, dinoflagellate cysts), and nektonic (sharks) fossils. The Gnaszyn succession originated relatively close to the shore, within reach of an intense supply of terrestrial fine clastic and organic particles. The latter are mainly of terrestrial origin and range from 1.5 to 2.5 wt.%. The precise water depth is difficult to estimate but most likely ranges from several tens of metres to a few hundred metres. All fossil groups show minor changes throughout the succession. As the climate seems to have been quite stable during this period we consider sea-level fluctuations to have been the main factor responsible for the changes. The terrestrial input, including freshwater and land-derived clastic and organic particles (sporomorphs and cuticles), increased during periods of sea-level lowstand. As a consequence, stress conditions (lower salinity, higher nutrient availability, lower water transparency) in the photic zone caused blooms of opportunistic planktonic taxa. Furthermore, a faster sedimentation rate led to oxygen depletion and deterioration of the living conditions in the bottom environment due to an increased accu-

Leonowicz, P. 2012. Sedimentology and ichnology of Bathonian (Middle Jurassic) ore-bearing clays at Gnaszyn, Krakow-Silesia Homocline, Poland. Acta Geologica Polonica, 62 (3), 281-296. Warszawa. The lithology, sedimentary structures and trace fossil association from the Bathonian organic-rich mudstones from Krakow-Silesia Homocline (SW Poland) are described and a preliminary interpretation of the palaeoenvironment is presented. The ore-bearing clays exposed in the Gnaszyn clay-pit are developed as dark grey claysilt deposits with a few horizons of sideritic concretions. Sedimentological analysis has revealed that these deposits originated in the offshore part of a marine basin, below the fair weather wave base. Relict parallel lamination suggests that the predominant mechanism of deposition was quiet settling from suspension. However, locally preserved storm deposits indicate that the sea bottom was affected by offshore flows, deriving coarser material from the nearshore area. The degree of bioturbation is high but the diversity of the trace-fossil association is low. It consists mainly of Chondrites, Trichichnus, pyritized burrows and less common Palaeophycus. Other traces, including Thalassinoides, Planolites, Taenidium and some undetermined burrows, occur mainly in the lower part of the succession. This ichnoassociation combines features of distal Cruziana and Zoophycos ichnofacies, suggesting poor oxygenation of the bottom sediment. Temporary improvements in bottom oxygenation, linked with more intense mixing of water during major storms, resulted in the more diverse trace fossil suite that is found in some intervals of the succession.

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