Journal of Water and Environmental Sciences (Jun 2019)
ANCIENNES FORMES DE TERRAIN FOSSILES EN TANT QU’EVIDENCE D’UNE VARIABILITE DU NIVEAU DE LA MER A L’ECHELLE DU MILLENAIRE
Abstract
1] Geomorphology has frequently be solicited to give evidence of quaternary sea level variations, are they from orbital, suborbital, or millennial-scale variability. In fact, it has been frequently used, for this research, of a geomorphological analysis, bearing on relict subaerial and/or buried landforms, whether they are from continental or from marine origin. But, into loose marine detritic sediments, as a non-consolidatedconglomerate or a sand or a silt, such ancient subaerial landforms are frequently destroyed. However, it is possible to use ancient buried landforms, thathave been protected by their continental or marine cover. About the same way that, thanks to archeological excavations, when the researched objects are not present upon the soil surface.[2] The more common markers for sea level oscillations (Fig. 1) are a relict subaerial depositional platform («terrace tread») and the relict related superficial sea-cliff (“terrace riser”), that is associated to it, in the same topography of a marine terrace (from “terra = earth = a marine deposit, not a wave-cut platform).[3] So, the most commonly used marker for the highstand palaeoshorelines, at the end of a sea level rising, is represented by the inner margin, or inner edge, of the ancient subaerial depositional platform, at its junction pointwith the related relict superficial sea-cliff.[4] But it is also possible to observe some ancient buried landforms, that are covered by marine terrace deposit.Even if relict superficial landforms have been destroyedinto loosedetritic marine sediments, because of later natural erosion, including differential erosion; or by agricultural arrangements (Fig.2).[5] Among such ancient buried landforms, relict wave-cut platforms and related relict buried sea-cliffs are representedin good places (Fig. 3).[6] After Lajoie (1986, Fig. 6.2), the ancient wave-cut platform and the related “relict sea-cliff” are not covered by marine sediments, but only by a colluvial deposit. In reality, it appears that such relict landforms are effectively buried under marine transgressive sediments, after the «concepts and principles of sequence stratigraphy». So, a possible evolution is not to be neglected:a differential erosion, having largely exhumed an original ancient buried wave-cut platform from its loose detrititic marine sediments cover (Fig. 4).[7] Then, for Lajoie (1986, Fig. 6.2 and 6.6), the inner edge of any «wave-cut-platform», at its junction point with the related «relict sea cliff», is denominated « shoreline angle» and iscorrelated with an eustatic peak, that is recorded on a «sea-level fluctuations curve», modified from Chappell (1983), and derived from the benthic isotopic curve V28-238, of Shackleton and Opdyke, 1973. So, the somewhat long marine abrasion process will be completely held during an eustatic maximum, curiously, just at the upper turning point of a sea level curve. In reality, into a first part of the marine sediment deposition.[8] In fact, the marine abrasion process, that is accountable for the shaping of an active wave-cut platform and the correlative retreat of a living sea cliff, has operated only during the initial phase of a sea level rise. The cause of its ending is the conversion of waves from translatory to oscillatory form; as they pass from shallow to deep water, for a result from accelerating rate of sea level rise (Fig. 5).[9] In these conditions, only translatory waves are erosive on the sea floor, not the oscillatory ones. As marine abrasion process come to its end, there is no more advance for the wave-cut platform, and no more retreat of the sea-cliff, that becomes flooded at its foot by seawater, as a plunging cliff; and, then, partially buried by marine transgressive sediments. [9] The present elevation (m) of this relict buried cliff foot gives evidence for the past sea level position, for each palaeoshoreline, at the end of the marine abrasion process and of the sea-cliff retreat;taking into account, eventually, the uplift rate. [10] The same translatory waves operate, for the same reasons,at the end of a sea level rise; but on sandy beaches, with the swash beyond the shoreline.[11] The deposition of the first transgressive onlapping sediments was acting as the abrasion zone progressed, more and more rapidly, up-side (Fig. 5D). The later transgressive sediments, that cover the sea-cliff, were aggrading, as it was possible only after the end of the sea-level retreat. [12] In unstable areas, such an evolution may be pertubated by coseimic uplift events. This possibility has been largely evocated, for MIS 3, during see level fallings, between high spaced sea level risings (Chappell et al., 1996). Or during the only assumed long holocene sea level rise (Ferranti et al., 2008). Rather than a more frequent sealevel instability. [13] The end of the marine abrasion process occursbetween the beginning of a sea level rising and itsmaximum rate of sea level change, of water depth and of icebergsdischarges, that stands about at the middle point of the transgression. For a rate of sea level rise at about 20 to 30 m/kyr, after Rohling et al. (2008, Fig. 2c) during a little part of MISS 5.5; or Jung et Kroon (2010, Fig. 2) during MIS 3.[14] So, the end of the marine abrasion process and this of the retreat of an active sea-cliff haveoccured in the past, as the rate of sea level change was only about 10 à 15 m/ka. A decisive value that may be reached by 2100, after Jones (2013) and the Nationalclimate assessment (2014).[15] Taking the past into account, it is to be feared, for thefuture, near 2100, not so much an unlimited sea cliff retreat, than a partial flooding of it and a total flooding of the adjacent beaches.[16] There are also, among such interesting ancient buried landforms, some valleys (Fig. 6) or lateral gullies (Guérémy, 2013, Fig. 13), that have been incised during a so proclamated sea level fall, and then partly filled, during a subsequent so proved sea level rise. [17] There are, too, some relict buried irregular profiled landforms, that are due to some concentrated continental water flow, between the valleys, into loose detritic marine materials (Guérémy, 2013, Fig. 7 ; Guérémy and Debruyser, 2017, Fig. 3). This work occursduring a subsequent emergence, after a sea level falling, and it is a proof for that. Such buried landforms may be encrusted (Fig. 7), or alterated (Guérémy, 2013, Fig. 46c), before they have been covered by colluvial deposits. This “ravinement surface” is a geomorphic unconformity, without any tectonic deformation. [18] The resulting relict superficial landform is an ancient colluvial glacis (Fig. 7, Fig. 8). It may be thickly encrusted on the surface (Fig. 7). It is progressively connected with a relict regularized subaerial sea-cliff, without any abrupt angle between them, so that it will be difficult to choice a precise measure point for its elevation (Fig. 8). Such a relict colluvial glacis is not to be mistaken with a marine terrace (Guérémy and Debruyser, 2017, Fig. 5).[19] There are also some marine geomorphostratigraphic units (MGU), that are entirely delimited byrelict buried marine landforms: a relict buried wave-cut platform, and its relict buried seacliff, as frequently, at its lower boundary ; but a latter relict wave-cut platform, at its upper boundary, without any ancient subaerial landforms ; because the second sea level position was higher than the former one (Fig. 10, Fig. 11).[20] So, geomorphology indicates a succession of resolvable or distinct marine geomorphostratigraphic units, each of them supporting a corresponding oscillation in sea level. Some of them are completely delimited by relict buried landforms;whether theybe represented, more often thannot, by a „ravinement surface“ at their upper boundary (the roof), or due to marine abrasion process, at their floor (the lower boundary).[21] Their counting, by such a geomorphological analysis, permits to give evidence of each glacio-eustatic oscillation, as they are recorded on each high-resolutionsegment of the KL 11 sea level curve, in spite ofits error margin (12 m),above all for low magnitude oscillations. So is it, for example between 40 and 45 kyr and two major sea level oscillations (Fig. 12), that are answersto GI 8 and GI 12 (on NGRIP).[22] A similar response is valid, by utilization of the thermic record of NEEM (NEEM Community Members, 2013), also for the last interglacial, although the KL 11 high-resolution sea-levelcurve does not completely cover MISS 5.5 (Fig.13).[23] Better than a high resolution eustatic curve, that may be handicapped for second order sea-level oscillations,because of its errormargin, a geomorphological analysis, including unambiguous relict buried landforms (Fig. 14), may be a proof for a millennial scale sea-level variability.
