Stratigraphic architecture, palaeogeography and sea-level changes of a third order depositional sequence: The late Turonian–early Coniacian in the northern Iberian Ranges and Central System (Spain)

Abstract

This paper reports a high-resolution stratal, palaeogeographical and quantitative analysis, of a late Turonian–early Coniacian carbonate ramp (the Northern Iberian Ranges) and its siliciclastic coastal margin (the Central System). The analyzed succession comprises a complete 3rd order depositional sequence with a shelf-margin wedge, a transgressive systems tract and a highstand systems tract. Based on the existence of a) subaerial exposure evidences and early diagenetic overprints; b) stratal relationships (onlap, offlap, toplap) regarding underlying and overlying sequences, and c) changes in the vertical facies evolution, the depositional sequence was bounded by two major sequence boundaries. These indicate that a major fall in relative sea level occurred in these boundaries. The comparative study of the internal stratal surfaces and the vertical facies succession allowed the identification of superimposed, higher-order depositional parasequences sets and parasequences (4th to 5th order), which record short-term changes in the relative sea level. Thus, when vertical facies trends in the parasequences are arranged in the framework of a parasequence set, a clearer palaeogeographic picture is obtained, and palaeogeographic maps can more clearly show the evolutionary trend of the basin. Each parasequence set was composed of a basal lowstand/transgressive systems tract in turn composed of fringing siliciclastic shoreface deposits and offshore muds. Older sequences were partially exposed further landwards. The sedimentation of these sands resulted in the progradation of the coastal systems during times of low relative sea level, during which the basal parasequence set boundary was also generated. Subsequent transgression remobilised underlying lowstand sands. The early highstand was composed of carbonate sediments in platform areas and mixed siliciclastic-carbonate sediments landwards, separated by a narrow fringe of muddy-marl offshore sediments. The late highstand showed a progressive shallowing trend, with carbonate littoral and tidal sediments covering broad areas of the basin, and lagoon facies were restricted to the northern outcrops. During late highstand, underlying transgressive systems tract and older highstand deposits were exposed landwards. Sea-level and accommodation curves have been obtained from paleogeographical, sedimentological and sequence stratigraphical data. The mean sea-level curve for the entire 3rd order sequence shows a rise of the sea-level and then an overall stillstand, but accommodation curve shows a positive trend along the sequence. The 3rd order sea-level cycle has a mean amplitude of 52 m and high-frequency cycles have amplitudes of 20–40 m (4th order) and about 1.5–4 m (5th order). The mean final rise in the sea-level (46.6 m) is quite similar to the total accommodation deduced from Fischer plot (45.75 m). The sea-level cycles are considered to be eustatic in origin and driven by orbital forcing in the Milankovitch band based on the estimated 4th order- (400 kyr) and 5th order-cycle lengths (80 kyr).