BURIAL DIAGENESIS OF MIDDLE ORDOVICIAN CARBONATE BUILDUPS (ALABAMA, USA) - DOCUMENTATION OF THE DOMINANCE OF SHALLOW BURIAL CONDITIONS

Middle Ordovician carbonate buildups at two localities (Red Mountain E xpressway = RME; Tidwell Hollow = TWH) record evidence of burial diage nesis. Burial diagenetic phases include (from oldest to youngest): (1) blocky calcite, (2) planar dolomite associated with stylolites, (3) s addle dolomite in primary voids and late veins, and (4) blocky calcite in late veins. All blocky calcite cements post-date Ordovician meteor ic cement based on cross-cutting and superposition relationships and b locky calcite is the most common burial diagenetic phase present in th e Chickamauga buildups. Blocky calcite (not in late veins) formed from post-Ordovician meteoric water based on the multiple lines of evidenc e, which include: (1) variable delta(18)O values (-5.2 to -11.4 parts per thousand) some of which are significantly more negative than value s from Ordovician meteoric cement; (2) non-radiogenic Sr-87/Sr-86 valu es (0.70847 to 0.70869); and (3) low Fe and Mn values near or below de tection limit (400 ppm). Planar dolomite associated with stylolites ha s variable delta(18)O values (-1.4 to -6.1 parts per thousand), is Ca- rich (non-stoichiometric), and has relatively low Mn and Fe values. Th is phase is interpreted as forming during the Mississippian or earlies t Pennsylvanian at a relatively shallow burial depth. Rare saddle dolo mite has delta(18)O = -5.9 to -7.2 parts per thousand. This phase post -dates planar dolomite and appears to record migration of basinal brin es into the platform succession during the Alleghenian orogeny. Late v eins filled with saddle dolomite and blocky calcite have variable delt a(18)O compositions like blocky calcite (-4.8 to -12.8 parts per thous and) and likely formed under elevated temperature conditions if precip itation from delta(18)O-enriched burial brines is assumed.