Pau Arbués¹, Jordi Corregidor², Miguel Garcés¹, Santiago Sánchez-Villanueva¹, Luís Cabrera¹, Mariano Marzo¹
(1) University of Barcelona, Barcelona, Spain
(2) Independent Consultant

Abstract: Anatomy of transitional fluvial-lacustrine parasequences. Los Monegros lacustrine complex. SE Ebro basin (NE Spain)

A Late-Oligocene, up to 700 meters thick, succession of shallow lacustrine deposits (Los Monegros system) alternating with distal facies of alluvial systems (Montsant and Guadalope-Matarranya systems) accumulated in the Mequinensa area during ~6 Myr. Small-scale deltas and wide interdeltaic plains with mudstone- and sandstone-dominated sedimentation occurred in the marginal lacustrine zones. Marginal nearshore and shallow offshore lacustrine carbonate deposits with minor mudstone-sandstone and coal deposits were deposited in lacustrine zones sheltered from the fluvial influences. Successive spreads and retreats of the lacustrine environment are recorded in several orders of cyclicity, from low frequency, 3rd order (1-2 Myr) to very high frequency, 5th order (25 kyr) sequences.The latter are nested in the former, resulting in a complex cyclic arrangement. Our study deals with a 40 m thick, mostly transgressive (lacustrine-spreading) sequence, deposited in a time span of about 350 kyr. Five major facies assemblages constitute the building blocks of this sequence: 1) sandstone-dominated (channel fill and proximal levee, crevasse-lobe and mouth bar deposits); 2) heterolithic (distal toesets of levee, crevasse-lobe, and mouth bar sandbodies); 3) reddish and mottled, mudstone-dominated (poorly drained flood-plain and marginal lacustrine deposits); 4) graysh, mudstone-dominated (shallow lacustrine deposits); 5) limestone-dominated (biogenic or bioinduced sedimentation in shallow lacustrine zones). The bases of limestone beds constitute key surfaces (flooding surfaces) bounding 14 parasequences. Typically, each parasequence, up to 5 m thick, depicts an overall regressive-progradational trend. We document and discuss the origin of the extreme lateral and vertical variability in geometry, architecture and internal structure shown by these parasequences.

AAPG Search and Discovery Article #90914©2000 AAPG Annual Convention, New Orleans, Louisiana