Contrasting Styles of Diagenesis and Porosity Evolution Recorded by Depositional Sequences of Fault-bounded Isolated Carbonate Platforms (Albian of Northern Spain)

Idoia Rosales and Isabel P. Montanez

The Albian carbonate succession of the Castro Urdiales area of Northern Spain developed as multiple small-scale residual platforms which resulted from break-up and drowning of a larger Aptian-Lower Albian carbonate ramp. This study evaluates the development and diagenetic history of three residual platforms (Arenillas, Punta Pena and Candina) which developed on elevated blocks separated by deeper-water intraplatform troughs. Syndepositional block faulting resulted in differential subsidence of the three platforms, and governed the deposition of sequences, sequence boundary development, and the early diagenetic history of these platform carbonates. The Arenillas platform is about 300 m thick and 2 km wide. It consists of six depositional sequences separated by type 1 sequence boundaries characterized by very well developed paleokarstic features, and extensive dissolution and meteoric cementation of host carbonates. Meter-size solution cavities and small-scale paleokarstic structures are developed below sequence boundaries and re filled by meteoric and/or marine cements and marine sediments deposited during the subsequent transgression. Much depositional and early secondary porosity was preserved due to extensive dissolution and meteoric cementation which limited compaction during burial. In contrast, the Candina platform is greater than 2 km wide and 280 m thick, and records near continuous sedimentation throughout the Albian. Sequence boundaries are poorly developed within Candina platform carbonates; meteoric cements are sparse and most initial porosity was lost during burial compaction. The Punta Pena platform records an intermediate subsidence history, and is 7 km wide and 400 m thick. It contains at least 3 sequence boundaries associated with moderately developed paleokarst features. This study character zes the highly complex and varied diagenetic history and porosity evolution associated with sequence development in these three carbonate platforms from a tectonically active setting. Detailed petrographic and geochemical study reveals contrasting styles of diagenesis and systematic changes in cement distribution and porosity evolution below sequence boundaries within each platform.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California