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The Early Kimmeridgian succession at Kodrąb (Radomsko elevation, central Poland) and its palaeogeographical and palaeotectonic implications

Wierzbowski A., Głowniak E.

Geological Quarterly

2018

Vol. 62, No. 3

509--521

The Early Kimmeridgian succession in the Rogaszyn Quarry at Kodrąb (Radomsko elevation) represents mostly shallow-water carbonate platform deposits that, over wide areas of central Poland were controlled by the activity of the tectonic zone being the prolongation of the Holy Cross lineament. The ammonites collected enable precise recognition of ammonite zones – the Platynota and the Hypselocyclum zones, some subzones and horizons. The precise dating enables detailed correlation of the particular units of the succession with those recognized at the eastern and northern borders of the Wieluń Upland and the NW margin of the Holy Cross Mts. The deposits at Kodrąb from the top of the Planula Zone, through the Platynota Zone, up to the lower part of the Hypselocyclum Zone, reveal markedly smaller thicknesses equaling ~15–20% of those of coeval deposits from the adjoining areas. The differences in sedimentary evolution at Kodrąb during the Early Kimmeridgian resulted mostly from local synsedimentary tectonic movements of fault blocks. Beginning from the late Hypselocyclum Chron, these differences markedly diminished, which resulted from the uniform subsidence of a wider area, and disappearance of the shallow-water deposits of the carbonate platforms, being replaced by deeper-water deposits of the Burzenin Formation.

The Torrecilla Reef Complex (Early Kimmeridgian, N Spain): an example of tectonically forced regression

Benito M., Mas R.

Volumina Jurassica

2006

Vol. 4, no. 1

79-80

Palaeontological, sedimentological, palaeoecological and/or depositional aspects of most Late Jurassic reefs, specifically coral reefs, have been studied in detail (e.g. see references in Flugel & Flugel-Kahler 1992; Leinfelder et al. 1996; Insalaco et al. 1997; Dupraz & Strasser 2002; among others), but only a few papers deal with their depositional architecture and response to tectonics and/or sea-level change. For example, Leinfelder et al. (1994, 1996) conclude that both eustatic and tectonic changes were important in controlling platform geometry and evolutionary trends in Upper Jurassic reefs. The Early Kimmeridgian Torrecilla Reef Complex in the northern Iberian Basin of Spain consists of a fringing reef composed of eight accretionary units (Alonso et al. 1986-1987;Mas et al. 1997; Benito & Mas 2002, 2006). The first four were deposited along a steep margin. They display down-lapping and off-lapping geometries, and are characterized by poor reef-framework development, large volumes of reworked corals and transported sediment, and limited growth of micro-encrusters.In contrast, deposition of the fifth and younger accretionary units occurred on a shallow platform without a pronounced slope where coral reefs grew in a shallow protected environment (Benito & Mas 2006). The main features of these reefs are: an absence of reef-slope facies, a high proportion of preserved framework elements, relatively low volumes of intrareef sediment, high proportions of terrigenous material, and abundant micro-encrusters and microbialites. These reefs were protected from storm waves by long-shore sand bars, which also protected a very shallow lagoon during the last stage of sedimentation. The Early Kimmeridgian was a period of rising global sea level (Haq et al. 1988; Hallam 2001), a trend apparent across other portions of the Iberian Basin. However, geometry and sedimentary evolution of the Torrecilla Reef Complex is consistent with those of off-lapping reefs that develop during sea-level fall. Thus, we conclude that down-stepping geometries and evolution to progressively shallower environments within the Torrecilla Reef Complex occurred as a result of a tectonically forced regression.