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J. Wat. Env. Sci. Vol. 3, Numéro spécial RQM9 (2019), 432-452 ______________________________

ISSN : 2509-0445

http://revues.imist.ma/?journal=jwes

Copyright © 2016 - 2018

ANCIENNES FORMES DE TERRAIN FOSSILES EN TANT YU'EVIDENCE D'UNE VARIABILITE DU

E/sh>DZ>[,>>hD/>>E/Z

RELICT BURIED LANDFORMS AS AN EVIDENCE FOR PAST MILLENNIAL-SCALE SEA LEVEL

VARIABILITY

P. GUEREMY

Professeur honoraire de Géographie Physique à Université de Reims Champagne Ardenne, France. 12 Bd des

Clèches, 51420 Cernay-lès-Reims, France.

Corresponding author Email: pgueremy@orange.fr

Received: March 11, 2019, Accepted: May 23, 2019, Online: June 17, 2019 Qu'est-ce que la géomorphologie peut avoir à dire de plus, par une analyse de quelques anciennes formes de terrain superficielles, ou/et, principalement fossiles, au sujet d'une ǀariabilitĠ du niveau de la mer résolution de la courbe eustatique, dérivée de la courbe isotopique planctonique KL 11 (Mer Rouge), en réponse à des épisodes de réchauffement du climat, figurant, sous l'appellation de DO sur les courbes isotopiques glaciaires GRIP et GISP2 ou de GI sur

NGRIP ?

Mots clés : géomorphologie, variations du niveau de la mer, anciennes formes de terrain fossile. [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-consolidated conglomerate or a sand or a silt, such ancient subaerial landforms are frequently destroyed. However, it is possible to use ancient buried landforms, that have 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 с 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 point with 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 destroyed into loose detritic 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 represented in 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

RESEARCH ARTICLE

434
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 is correlated 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 sea level instability. [13] The end of the marine abrasion process occurs between the beginning of a sea level rising and its maximum rate of sea level change, of water depth and of icebergs discharges, 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 have occured 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 National climate assessment (2014). [15] Taking the past into account, it is to be feared, for the future, 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 occurs during 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 coǀered by colluǀial deposits. This ͞raǀinement 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 by relict 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 J. Wat. Env. Sci. Vol. 3, Numéro spécial RQM 9 (2019) P. Guérémy 435
delimited by relict buried landforms; whether they be represented, more often than not, by a "raǀinement 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- resolution segment of the KL 11 sea level curve, in spite of its 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 answers to 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-level curve 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 error margin, a geomorphological analysis, including unambiguous relict buried landforms (Fig. 14), may be a proof for a millennial scale sea-level variability. Keywords: geomorphology, sea level variations, relict buried landforms.

1. INTRODUCTION

[1] La géomorphologie a souvent été sollicitée, afin de mettre en évidence les variations du niveau suborbitale, voire à celle du millénaire. Pour cela, il a été fait appel à une analyse géomorphologique d'anciennes formes de terrain superficielles et/ou fossiles, d'origine continentale ou marine. Mais un sable ou un silt, il peut se faire que les anciennes formes de terrain superficielles aient été formes de terrain fossiles qui ont été protégées continentale ou marine ; un peu comme il en est objets recherchés sont introuvables à la surface du sol. Apports continentaux du fleuve Tewai à la sédimentation marine Tewai river conglomeratic contribution to marine sedimentation

Couches progradantes

Prograding layers, foreset beds

050 km

0 5 kmC/145150 E

5 10 S

Huon Peninsula

Plaque Australienne/Australian Plate

Nord N

Bobongara: 3.21-3.13 m/ka (2)

T (1) Chappell and Shackleton, 1986, Table 1 (2) Chappell, 2002.

Kanzarua : 2.61 m/ka (1) ;

2.62 - 2.70 m/ka (2) ;

Tewal : 3.5 m/ka (1)

436

Fig. 1. Le tĠmoignage d'anciennes formes de terrain superficielles et fossiles. Péninsule de Huon, Nouvelle

Guinée, transects de Tewai et de Bobongara, terrasse IIa, MISS 3.1, GI 8.

Anciennes formes de terrain fossiles : 1. ancienne plate-forme d'abrasion marine fossile ; 2. ancienne falaise

fossile (morte et enterrée) ; Ÿ. ancien pied de falaise fossile ; 1 + 2 = surface de transgression (ST), érodée dans

un continent

Anciennes formes de terrain superficielles : 3. ancienne plate-forme d'accumulation marine ; 4. ancienne

falaise subaérienne (morte sans sépulture) ; ź. ancien pied de falaise superficiel ; 3 + 4 = terrasse marine.

UGM. Unité géomorphostratigraphique marine distincte, à rapporter à une oscillation glacio-eustatique.

A/ Un profil topographique et une coupe stratigraphique et géomorphologique. Terrasse IIa. Transect de

Tewai, d'aprğs Chappell, 1983, Fig. 2.

B/ Une carte d'une ancienne ligne de riǀage de haut niǀeau, situĠe en bord interne d'une ancienne plate-forme

d'accumulation marine, à son point de jonction, sur un ancien pied de falaise superficiel, avec une ancienne

falaise subaĠrienne. Terrasse IIa. Transect de Bobongara, d'aprğs Chappell et al., 1996, Fig. 11.

C/ Croquis de localisation, d'aprğs Chappell, 1974, Fig. 1.

Fig.1. The evidence of relict superficial and buried landforms. Huon Peninsula, New Guinea, Tewai and

Bobongara transects, terrace IIa, MISS 3.1, GI 8.

Relict buried landforms: 1. ancient buried wave-cut platform ; 2. ancient buried seacliff ; Ÿ. ancient buried cliff

foot ; 1 + 2 = transgressive surface (TS) as excavate

Relict superficial landforms: 4. relict subaerial seacliff, ͞terrace riser"; 3. relict marine depositional platform,

͞terrace tread" ; ź. highstand palaeoshoreline ; 3+4 = marine terrace .quotesdbs_dbs13.pdfusesText_19

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