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Geological Society, London, Special Publications; 2005; v. 252; p. 83-107;
DOI: 10.1144/GSL.SP.2005.252.01.05
© 2005 Geological Society of London

Sedimentary Geology and Sequence Stratigraphy in Continental to Shallow-Marine Deposits

Sedimentology of the tide-dominated Jurassic Lajas Formation, Neuquén Basin, Argentina

Duncan McIlroy1, Stephen Flint2, John A. Howell3 & Nick Timms2

1 Department of Earth Sciences, Memorial University of Newfoundland, St. John’s, Newfoundland, Canada A1B 3X5dmc{at}esd.mun.ca
2 Department of Earth Sciences, University of Liverpool, 4 Brownlow Street, Liverpool L69 3GP, UK
3 Geological Institute, University of Bergen, Allégaten 41, N-5007 Bergen, Norway

Tidal depositional systems are often interpreted as lowstand/transgressive estuarine deposits within sequences that are either wave or river dominated during highstand times. The Middle Jurassic Lajas Formation of the Neuquén Basin, Argentina, comprises 600 m of well-exposed tide-dominated facies deposited within four unconformitybounded sequences, spanning approximately 4.5 Ma. Facies associations include tidedominated deltas, sandy-heterolithic tidal channel fills and extensive progradational tidal-flat successions, which are locally cut by heterolithic tidal channel fills. Despite the narrow bathymetric depositional range and the complex facies variability, flooding surfaces can be defined and mapped along a 48 km-long outcrop belt. These flooding surfaces allow definition of three distinct types of parasequence that exhibit coarsening-upwards, finingupwards and coarsening- to fining-upwards motifs. Sequence boundaries are marked by widespread, but shallow, incision, and the juxtaposition of stacked fluvial/tidal channel fills on a variety of subtidal and intertidal facies. Unconventional grain-size changes at sequence boundaries can occur where basinward facies shifts are marked by juxtaposition of heterolithic-argillaceous intertidal/supratidal mudflat deposits on subtidal sandflat facies. The maintenance of macrotidal conditions through complete base-level cycles is interpreted as being due to the structural topography inherited from rifting, causing the whole sub-basin to behave as a structurally controlled embayment.





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