Journal of Conference Abstracts

Origin of Bajocian Hardgrounds from Condensed Sedimentary Sequence of Dorset

Julia Semil (beerbear@twarda.pan.pl)¹, Michal Gruszczynski (beerbear@twarda.pan.pl)², Max Coleman (m.l.coleman@reading.ac.uk)³ & Michael Isaacs³

¹ Zwirki i Wigury 93, 02-089 Warsaw, Poland

² Twarda 51/55, 00-818 Warsaw, Poland

³ Uniwersity of Reading, Reading RG6 6AB

The condensed sedimentary sequence of the Inferior Oolite (Aalenian-Bajocian) occurring at Burton Bradstock and Horn Park, near Beaminster revealed discontinuity surfaces of hardground characteristics (Callomon & Cope, 1995). The detailed isotope and elemental investigations have shown interesting regularities. Above hardground surfaces, the carbon isotope values oscilate slightly below +2 per mil, then drop suddenly to 0 per mil at the hardground surfaces. Beneath the hardground surfaces, the carbon isotope values are about +2 per mil again, and slightly shift towards more positive values (from +1.7 to +2 per mil). The oxygen isotope values follow the trend of the carbon equivalent, varying around -2 per mil above the hardground surfaces, then dropping to -4 per mil at the hardground surfaces, and being heavier again, shifting from -2 to -1 per mil beneath the hardground surfaces. The carbon and oxygen isotope values for the fillings of the burrows and borings are even more negative than those values just above the hardground surfaces. All this suggests two possble reasons for hardground formations: inflow of fresh water or change of the chemistry of ambient seawater. Our prefered explanation for the break in calcium carbonate precitipation and hardground formation is the excess of carbonate alkalinity (lowering amount of free calcium ions) and/or increased partial pressure of carbon dioxide, because of invasion of geochemically different seawater. To confirm this, strontium isotope composition has been measured across the hardground surfaces. The strontium isotope values have not shown any influence of runoff in the examined hardgrounds.

Callomon JH & Cope JCW, Field Geology of the British Jurrasic; The Geological Society, 51-103, (1995).

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