Controls on marine–erg margin cycle variability: aeolian–marine interaction in the mid-Cretaceous Iberian Desert System, Spain
Juan Pedro Rodríguez-López, Nieves Meléndez, Poppe L. de Boer, Ana Rosa Soria
Sedimentology 2011 ( http://www.sedimentologists.org/ )
The interaction between aeolian dunes of the Iberian erg and Tethys waters during the mid-Cretaceous led to a variety... more The interaction between aeolian dunes of the Iberian erg and Tethys waters during the mid-Cretaceous led to a variety of sedimentary facies associations such as subtidal deposits, aeolian dunes, playa lakes, coastal lakes with tidal creeks, and marshes and lagoonal embayments with tide-influenced delta deposits. Facies associations are organized in several stacked cycles. Every cycle is defined by a sand-drift surface separating playa lake deposits below from aeolian dune deposits above, followed by a transgressive surface separating aeolian deposits below from shallow marine deposits above. The latter are covered by playa lake deposits and finally topped by the next sand-drift surface. Similar marine–erg margin cycles have been explained previously in terms of high-order relative sea-level variations (normally controlled by glacioeustasy). The studied erg margin developed during the mid-Cretaceous which is considered to be the archetypal example of a polar ice-free greenhouse period. This manuscript presents an alternative explanation for the cycles as caused by climate-induced variations in the aeolian sediment supply. In this scenario, orbitally induced latitudinal shifts of the boundary between climate belts led to alternating periods of increased and decreased precipitation in the highland catchments of the back-erg system (Variscan Iberian Massif). When climate belts shifted to lower latitudes, back-erg highlands were under the influence of the Northern Warm Humid Belt. While arid conditions prevailed in the lowland central-erg, increased precipitation in the highlands and the consequent recharge of ground water led to a rise of the phreatic level in the erg system; that diminished wind erosion and windblown sediment input to the fore-erg, so that the desert margin contracted. With ongoing basin subsidence this favoured the transgression. The increased desert-ground water flux favoured permanent coastal lakes in the fore-erg margin and the local development of vegetation. In places where such lakes (lagoons) were connected to the sea, tidal channels were active, as seen nowadays along the desert coast of Qatar (Persian Gulf). When climate belts moved to higher latitudes, the catchment areas in the back-erg Iberian Massif were under the influence of the Northern Hot Arid belt and a decrease of precipitation led to a drop of the phreatic level, allowing deflation of dunes and exposed wadi channel floors, and the formation of desert pavements and deflation lags in the back-erg area; this favoured the import of large volumes of windblown sand and dust, forcing progradation of the erg and consequently a retreat of the coastline. The sand fraction accumulated in climbing aeolian dunes and dust was trapped in extensive playa lake systems. Another long-term allocyclic control is formed by active extensional tectonics that enhanced the creation of accommodation space, especially in the first cycle. A high ground water table sustained by subsurface water supply from the Variscan Iberian Massif generated coastal fresh/brackish lakes in which tide-influenced meandering channels and salt marshes developed. Erg margin cycles thin upward in response to a decrease of the rate of basin subsidence, suggesting that transgressions led to deep erosion of the underlying aeolian dune sands.
ORBITALLY FORCED SEDIMENTARY RHYTHMS IN THE STRATIGRAPHIC RECORD: IS THERE ROOM FOR TIDAL FORCING?
Poppe L. de Boer and João Trabucho Alexandre
The imprint of orbital cycles, which result from the varying eccentricity of the Earth’s orbit and changes in the... more
The imprint of orbital cycles, which result from the varying eccentricity of the Earth’s orbit and changes in the orientation of its axis have been recognised throughout the Phanerozoic rock record. Variations in insolation and their effect on climate are generally considered to be the sole transfer mechanism between the orbital variables and cyclic sedimentary successions. Common oceanographic principles, however, show that also the ocean tide responds to variations in the orbital parameters. The ocean tide has not yet been considered to be a valid, additional transfer mechanism for the orbital variations. In geological studies of Milankovitch cycles in sedimentary successions the insolation paradigm offers satisfying explanations, and the role of long-term variations of the ocean tide has not yet been appreciated. Variations in the ocean tide, related to changing eccentricity (at present 0.0165, theoretical maximum 0.0728), affect a variety of oceanographic and sedimentary processes. In addition to the widely accepted paradigm of orbitally forced insolation changes, the tidal transfer of orbital signals may explain certain less well understood aspects of orbitally induced cycles in the stratigraphic record related to ocean mixing, organic productivity and tidal processes in shallow seas and deep water. Variations of the ocean tide in relation to the 18.6 year lunar nodal cycle, which has no insolation counterpart by which they may be diluted, indeed show that these relatively small variations can produce significant effects in sedimentary environments that are sensitive to variations in the strength of the ocean tide. In analogy with the 18.6 year lunar nodal cycle, orbital variations of the tide on Milankovitch timescales are likely to have affected sedimentary systems in the past.
Models and paradigms often drive the analysis and interpretation of data (Miall, 2004; Miall & Miall, 2004). It is common practice that, once astronomical cycles in the fossil sedimentary record have been (statistically) demonstrated, theory-laden observations lead to satisfactory explanations of such cycles in terms of insolation-induced variations of climate, oceanography and sea-level (cf. de Boer & Smith, 1994a; D’Argenio et al., 2004). However, the astronomical variables, eccentricity, obliquity and precession also influence the ocean tide, in phase, and with an identical frequency to that of insolation. These orbitally forced variations of the ocean tide must also produce signals in the sedimentary record.
Sedimentology and development of barrier islands, ebb-tidal deltas, inlets and backbarrier areas of the Dutch Wadden Sea
Albert P. Oost & Poppe L. de Boer (1994)
Senckenbergiana maritima 24, 65-115.
This paper presents an overview of the Dutch Wadden Sea from a sedimentological point of view. After the pioneering... more This paper presents an overview of the Dutch Wadden Sea from a sedimentological point of view. After the pioneering work of scientists new impulses to this kind of research are being given by the need for detailed recent analogues of fossil hydrocarbon-containing rock successions and by the great concern about the future of our coastline in relation to accelerated sea-level rise. After many studies of a descriptive nature in the past, there is now a growing tendency so a more dynamical view to the Wadden Sea system. There is a strong interdependence between various tidal sub-environments within individual inlet systems. Together these sub-environments form so-called Sand Sharing Systems, whose behaviour is largely defined by the tidal prism and the wave climate. Such a dynamical approach may greatly facilitate the research and understanding o fossil barrier-related sediments. Apart from the physical processes the abundant biota plays also an important role in the sedimentological development of the Wadden Sea. The large amount of data on the development of the Wadden Sea in pre-historical and historical times, moreover, allows to test hypotheses about the evolution of the system on the scale of centuries to millennia.
Download (.pdf) Basin dimensions and morphology as controls on amplification of tidal motions (the Early Miocene North Hungarian Bay)
ORSOLYA SZTANÓ, POPPE L. DE BOER
Sedimentology Volume 42, Issue 4, pages 665–682, August 1995
Following the Late Aquitanian sea-level fall, tide-influenced deposition started in the North Hungarian Bay, an... more
Following the Late Aquitanian sea-level fall, tide-influenced deposition started in the North Hungarian Bay, an embayment in the Paratethys open to the north-east. The relatively narrow, funnel shape of the bay supported amplification of tidal movements, resulting in the generation of strong tidal currents. The length and the depth of the North Hungarian Bay and the connecting seaway through East Slovakia fell into the ‘Tidal Amplification Window’and thus fulfilled the conditions needed for resonant amplification of semidiurnal (M2) tides. Tide-influenced deposits were formed at both sides of the North Hungarian Bay. They reflect dominant currents in opposite directions and of different strengths at either side of the basin. This difference was the result of bottom-tide interactions. Cyclonic (anticlockwise) residual currents were induced above the asymmetrical central depression in the bay and were superimposed upon the tidal currents, producing an anticlockwise, time-and-velocity asymmetrical current system.
The North Hungarian Bay and other examples show that amplification of tidal motions and formation of tide-influenced deposits may occur if basin dimensions pass through the ‘Tidal Amplification Window’. This window represents ideal conditions for resonant or amphidromic amplification of tidal currents. It determines an ideal length/depth or width/depth ratio relative to the wavelength of the astronomical tides. Thus signs of strong tidal influence in fossil basin fills can be used to reconstruct the dimensions (length, depth and width) of such basins.
