Formation of dolomite mottling in Middle Triassic ramp carbonates (Southern Hungary)

Abstract

Abstract The Middle Triassic carbonates of the Villany Mountains were deposited on a homoclinal carbonate ramp. Many of the carbonates from the 700 m-thick sequence show partial or complete dolomitization. The present paper describes dolomites that occur in a limestone unit as irregular mottles and as pore- and fracture-filling cements. Replacement-type scattered dolomite rhombs in the mottles having inclusion-rich, very dull luminescent cores and limpid non-luminescent outer zones represent the initial phase of dolomitization. The isotopic composition of these dolomites ( δ 13 C =+2.30‰ VPDB, δ 18 O =−3.60‰ VPDB) is similar to that of the calcitic micrite ( δ 13 C =+2.6‰ VPDB, δ 18 O =−4.00‰ VPDB) indicating that no external fluids were introduced during dolomite formation. The elevated Sr content of the micrites implies that sediment was originally aragonite or high-Mg calcite. Dolomitization took place in the burial realm from a “marine” pore-fluid in a partly closed system. Later fracture-related saddle dolomite reflects elevated formation temperatures and increasing burial. Five calcites were identified. Multiple generations of calcite-filled fractures were formed during burial diagenesis generally having dull or no luminescence ( δ 13 C =+1.80‰ VPDB, δ 18 O =−6.40‰ VPDB). The latest phase calcites are related to karst formation, having a very negative isotopic composition ( δ 13 C =−5.0 to −7.2‰ VPDB and δ18O≈−7.4‰ VPDB). The karst-related processes include dissolution, calcite precipitation and partial replacement of dolomites by complex zoned bright yellow calcite. The timing of dolomitization is uncertain, but the first phase took place in a partly closed system prior to stylolite formation. Late-stage saddle dolomites were precipitated during maximum burial in the Cretaceous. The dissolution of dolomites and karst-related calcite replacement was not earlier than Late Cretaceous.