Sequences, cycles and hiatuses in the Upper Albian-Cenomanian of the Iberian Ranges (Spain): a cyclostratigraphic approach

Abstract

The Upper Albian-Cenomanian deposits in the Iberian Ranges are composed of a complex alternation of continental (sandstones), coastal (lime sandstones and marls) and marine (carbonates) facies. Facies are arranged mainly in transgressive and deepening-upward sequences and parasequences, but deepening-shallowing-upwards sequences and parasequences also exist. Depositional sequences and parasequences are bounded by subaerial erosive surfaces or ferruginous crusts, and by their correlative conformities. It is evident from the facies alternation that deposition reflects a cyclic process in which is superimposed several orders of cycles of relative sea-level rise and fall. The cycles are preserved as 3rd-order depositional sequences and 4th- and 5th-order discontinuity-bounded parasequences, the latter being the basic blocks of the field record. A cyclostratigraphic model for sedimentation shows that each of the 3rd-order cycles is composed of five 4th-order cycles. A 4th-order cycle is also composed of five 5th-order cycles. This 1:5:5 ratio reflects the maximum number of parasequences observed in the most complete and marine sections. The ratio remains unchanged throughout the basin, except by onlap caused by relative sea-level rises or truncations caused by relative sea-level falls. A detailed correlation of sections separated by more than 200 km shows that thickness differences are caused mainly by parasequence disappearance, not parasequence thinning. Cycles are mainly eustatic in origin, controlled by accommodation space. Increased accommodation generates thick parasequences of wider regional extension; they contain the more marine facies in any section. Only some sequences with a restricted regional distrubution are recorded if the accommodation is low. The average duration of 3rd-order cycles was 1.95 Ma (ranging from 1.33 to Ma), an average 4th-order cycle was 390 ka (266–533 ka) and a 5th-order cycle was 78 ka (53–106 ka).