Cyclostratigraphy, orbital tuning and inferred productivity for the type Kimmeridge Clay (Late Jurassic), Southern England

Abstract

Three independently measured variables (magnetic susceptibility, photoelectric factor and total gamma-ray) obtained from throughout the type Kimmeridge Clay Fm in Dorset (Southern England) were used to identify regular metre-scale, sedimentary cycles. Spectral analysis demonstrates that for long stratigraphical intervals the cycles are expressed as large-amplitude cycles of 1.87–4.05 m wavelength and smaller-amplitude cycles of around half that wavelength. These cycles are interpreted to record orbital obliquity and precession, respectively. The much larger amplitude of the inferred obliquity cycles compared with the precession cycles may indicate a high-latitude climatic forcing transferred to lower latitudes via sea-level variations. Orbital tuning indicates that the Early Kimmeridgian ( sensu anglico ) lasted at least 3.6 Ma (95 longer-wavelength cycles) and the Late Kimmeridgian at least 3.9 Ma (103 longer-wavelength cycles). The first detailed productivity estimates for the Kimmeridge Clay Fm, on a cycle-by-cycle calculation, indicate that average productivity of the type Kimmeridge Clay (220 g m −2 a −1 ) was less than the average productivity on modern continental shelves. The high average organic carbon content of the type Kimmeridge Clay (3.8% total organic carbon) cannot be attributed to high average productivity. However, the average organic carbon content is consistent with low siliciclastic mineral dilution of organic matter and/or elevated preservation linked to reduced bottom-water oxygenation.

Tethyan-to-boreal correlation in the Kimmeridgian using high-resolution sequence stratigraphy (Vocontian Basin, Swiss Jura, Boulonnais, Dorset)

“…For the same time interval, Hardenbol et al (1998) give duration of 0.51 Ma. This interval corresponds approximately to the Mutabilis ammonite zone, the minimum duration of which was calculated at 0.745 Ma by Weedon et al 2004. The duration given by Weedon et al (2004) for the lower part of the studied interval is therefore comparable to that estimated in the present work.…”

“…The same time interval in the Swiss Jura and the Vocontian Basin includes two medium-scale sequences that correspond to 0.8 Ma as well. This interval coincides approximately with the Eudoxus ammonite zone whose minimum duration was calculated to be 1.486 Ma by Weedon et al (2004) (Fig. 14).…”

“…Hardenbol et al (1998) give 0.620 Ma for the same interval. It corresponds approximately to the Autissiodorensis ammonite zone whose minimum duration is 1.062 Ma according to Weedon et al (2004). The duration of the upper part of the studied interval estimated in this work is intermediate between the duration given by Hardenbol et al (1998) and Weedon et al (2004) (Fig.…”

“…boundaries of ammonite zones or subzones and third-order sequence boundaries), the duration of the different orders of depositional sequences can be inferred. A cyclostratigraphic interpretation thus becomes possible, resulting in a narrow time framework, which is compared with the sequence-stratigraphic interpretation given by Williams et al (2001) for the Kimmeridgian of the Boulonnais (northern France) and Dorset (southern England), and with the cyclostratigraphic interpretation of the Kimmeridge Clay Formation proposed by Weedon et al (2004).…”

“…Comparison between durations given by Weedon et al (2004), Hardenbol et al (1998, and this work Weedon et al (2004) used magnetic susceptibility, photoelectric factor, and total gamma-ray data from the type Kimmeridge Clay Formation in Dorset (southern England) in order to identify sedimentary cycles. Cycles are expressed as large-amplitude cycles of 1.87-4.05 m wavelength and smaller-amplitude cycles of around half that wavelength.…”