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Abstract

A virtually complete succession of shallow-marine, epicontinental Lower Jurassic strata crops out in excellent exposures along the North Yorkshire coast (U.K.). Borehole control of the inland continuation of these deposits, together with the availability of a detailed ammonite biostratigraphy, allows for the construction of a cross-section correlating the relatively thick succession of the Cleveland Basin (exposed in the coastal sections) with the more condensed succession on the adjacent Market Weighton High (encountered in boreholes). This study is concerned with only the Hettangian to Pliensbachian part of the Liassic succession, corresponding to the informal 'lower Lias' and 'middle Lias' divisions of traditional usage.

Detailed examination of sedimentological features within a biostratigraphic framework revealed a systematic set of changes occurring both in the basin and on the high in medium-scale depositional sequences (10 to 60 m thickness; duration of 1 to 3 ammonite biozones). These sequences show an overall shallowing-upward trend from mudstone, deposited below storm wave-base, through quartz siltstone and sandstone or shelly mudstone, deposited above storm wave-base in the shoreface domain, to Fe-enriched deposits (limonite, glauconite, chamosite, ferruginous-ooliths) at the top of the sequence. The mudstones were deposited relatively rapidly and extend over both basin and high. The shelly mudstones, siltstones and sandstones represent much lower overall accumulation rates. The Fe-enriched deposits represent periods of condensation and are best developed on and immediately around the topographic Market Weighton High.

Three orders of depositional sequences have been identified: (1) 3 large-scale, 2nd-order sequences, that record major sea-level rises in the planorbis/liasicus zones, semicostatum zone and jamesoni zone and are bounded by major erosional phases in the bucklandi and margaritatus zones and a basin-wide shallowing in the raricostatum zone; (2) 6 medium-scale, 3rd-order sequences that are bounded by phases of reduced, Fe-rich sedimentation or erosional/nondepositional surfaces and that have a duration in the order of one ammonite subzone; and (3) several types of small-scale, high-frequency, 4th and 5th order cycles.

The transgressions of the 2nd-order depositional sequences (early Hettangian, early Sinemurian, early Pliensbachian times) are recognized worldwide and are thus most probably of eustatic origin. Tectonic uplift of the Market Weighton High influenced the expression of the medium-scale, 3rd-order sequences. Detailed comparison with other British and European sites is required to further clarify this aspect. Long-term climatic influence was also important in setting the scene for the abundant supply of iron, which became concentrated in the marine environment. At a higher frequency, climate influenced sediment distribution within the shallow epicontinental basin by relatively short-term, orbitally induced variations in the weather conditions.

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