Jurassic Limestone-Marl Sequences in Northern Spain: Detecting Diagenetic Signals Using Rare Earth and Minor Trace Elements

Mesozoic rhythmically bedded limestone and marl sequences have been traditionally interpreted to be the result of eustatic sea-level changes related to Milankovitch cycles. An alternative interpretation supports the idea that these calcareous alternations are a product of diagenetic bedding. Stratigraphic analysis is typically used to determine the extent of diagenesis and to detect any primary depositional signal in limestone-marl rhythmites. Rare earth and minor-trace element geochemistry is an overlooked method to disentangle the presence of a primary signal from a diagenetic one. Both rare earth and minor trace elements can substitute for more common elements like calcium during active deposition; however, unlike calcium, the heavier trace and rare earth elements will remain immobile during diagenesis. Data will be presented from the application of this geochemical approach, in conjunction with stratigraphic analysis, to Jurassic limestone-marl sequences along the coast of Asturias in northern Spain. These calcareous rhythmites are part of the Rodiles Formation (Pliensbachian, Lower Jurassic), a carbonate ramp deposited in a shallow epicontinental sea, and have previously been interpreted to be the result of diagenetic bedding. As a result, these deposits represent an ideal example to test the validity of using rare earth and minor-trace elements as a method for determining diagenesis in carbonate sequences.  

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California