Arquitectura estratigráfica de alta frecuencia de una secuencia de 3er orden en el margen costero de la Cuenca Ibérica (Provincia de Segovia, España)

Abstract

The cyclicity and the stratigraphic architecture of the Upper Turonian-Lower Coniacian 3rd-order sequence have been analysed in the coastal margin of the Iberian Basin (north of the Central System, province of Segovia). This sequence is composed of coastal terrigenous sediments with some shallow shelf carbonate intercalations. The 3rd-order sequence is composed of several ranges of high-frequency superimposed parasequences. Each of the four parasequence sets (4th-order), can be divided into up to three parasequences (5th-order), which in turn are composed of two minor parasequences (6th-order), recognized only in the carbonate sediments of the sedimentary succession. Thickness and lateral extent of the parasequence sets depend on the number of their internal parasequences. Parasequences and minor parasequences thins and pinch out landwards with onlap and toplap relationships regarding the basal and top parasequence set and parasequence boundaries. The presence in the sedimentary record of 4th- 5th- and 6th-order parasequences originates a complex overlap of Systems Tracts; and thus, depending on the observation scale, different sedimentary trends can be recognized. At a detailed scale, the presence of these Systems Tracts allows to clarify the terrigenous and carbonate facies relationships. Although both facies have been formed in a single depositional episode, they were deposited in different stages of the episode. Siliciclastic facies represent the Shelf Margin Wedge/Transgressive Systems Tracts, being supplied to the Basin during a relative sea level lowstand and located at the coastal margin; later, during the subsequent transgression, they are reworked. On the other hand, carbonate sediments were deposited basinwards, during relative sea level highstands (Highstand System Tract). This depositional model explains the scarce presence of mixed facies. Two other different sedimentary trends are recognized in the entire sequence. First, the thinning upwards trend of parasequence sets, which is related with the overall regressive trend in the upper part of the sequence (due to accommodation loss). Secondly, the progressive upwards decrease of siliciclastic deposits and a correlative increase in carbonate sediments in the parasequence sets, which suggests the existence of a longer transgressive trend (2nd-order), which continues in the overlying Coniacian sequence.