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A short Geological History of the Loire valley
112
A
SHORT
GEOLOGICAL HISTORY LOIRE VALLEY. B,-
OF
THE
G. F. DOLLFUS.
[Received Ncoember 27th, 1928.J
[This is an expansion at an address to the Association on the termination at the visit to Angers, in August, 1928, certain allusions to which will be noticed. Translated by A. Morley Dacies.) I. Geographical generalities: nature of the sediments. II. Primary and Secondary Periods. III. Eocene Period: first great extension of the granitic sediments. IV. Oligocene Period: great development of the lacustrine regime. V. Miocene Period: second extension of granitic alluvia captured by the sea; multiple movements of the ground; final retreat of the sea. I.
GEOGRAPHICAL GENERALITIES.
THEandLoire is a great river, a thousand kilometres in length, its regime is torrential, its maximum volume reaching three hundred times its minimum. It is the outflow channel to the sea from the northern slope of the crystalline Central Massif of France. Its course may be divided into three sections : The Up per L 0 ire is joined by the Allier near Nevers, and their principal elements above the confluence are as follows : Gradient Flow Altitude (Km.) (cubic metres at source. Length. per second). 1562 m. 430 km. 1.05 180 Loire Allier 1423 m. 4IO km. 1.84 120 The altitude and length differ little, but the gradient of the Allier is much steeper, while its mean flow expressed III cubic metres per second is much less. The Mid d l e L air e includes the region from Nevers to Gien, where the direction is still from south to north and the flow little changed. The Lower L air e extends from Gien to the sea: the direction is westward and the flow increases considerably, for all the great tributaries of the Loire are in this section and on the left bank: Cher, Indre, Creuse (with its tributary Vienne) and the Thoulet which joins at Saumur, all furnishing
GEOLOGIC.\L HISTORY OF THE LOIRE
VALLE Y .
113
m ore or less granit ic alluvia.
On th e right bank there is onl y the Maine, which we saw at Angers, formed by the union of the Loir , Sarthe and Mayenne, only giving medium gravelly and silt y alluvia. Th en, at Nantes, the Erdre, which is an old sea-fiord , a river of very great depth, of very irregular width , and den oting by the th ickness of it s alluvia a thick accumulat ion lik e th at of the Loire at St. Nazaire. We will not linger over the geographical details of this fine ba sin . the navigation of which has been almost aba ndoned, thou gh it might furni sh an important transport route. ~.-\T U R E OF THE D EPOSITS.-vVe can all the more easily follow the hist ory of th e Loire since the nature of its sediments is charac t eristic. Th e starting-point is in th e granitic rocks of th e Massif Central-Auvergne an d Lim ousin-these materials are hard, unmist akeable and in contras t with the materials supplied by the mi ddle and lower cours es of th e river. The sands carried by the Loire decrease steadily in grade as we descen d its course: th e very coarse pebbles of th e Bee d' Allier are redu ced when Nantes is reached to an extremely fine sand, with some cas ual pebbl e-beds. The volcanic materials, such as basalts, disappear rapidly by , porphyr isation ' in cont act with quartz, 'granulite ' decom poses, Jurassic and Cretaceous flints become rolled and thus disappear little by little, while all the felspat hic materi als become clayey , and specia lists, su ch as M. Bureau , on examining a handful of Loire sand can st ate from what point it has been t aken. All these debri s have not ceased t o feed the Loire and its affiuents during th e Secondary an d Tertiary geological eras ; indeed , their greater or less abundan ce corresponds t o movements of elevation or depression of the Massif Central with maximum and minimum periods of long duration. II. PRIMARY AND SECONDARY PERIODS. Th e ancient rocks, including th e Tri as, form a cont inuous oute r border t o the Loire-Sein e basin broken onlv in the north. They include the massifs of (I ) the Ardenne in t he north ; (2) the Vosges to the eas t, the Morvan also t o th e cast; (3) the lower ramparts of the Massif Central , com prising th e Bourbonnais. the Bois-Chaud, th e Marche, the Limousin , forming a great barrier on the south ; (4) the hill s of th e Vendee on the southwest , which join with (5) the massif of Brittan y, th e advanceposts of which ar e in Anjou, Mayenn e and th e Bocage of Normandy . The circle is com plet ed by (6) the Cot entin peninsula, whi ch is primary, and lies to the north-west. There are only two ga ps in this circle, known as the Strait of Cote d'Or or of L an gr es and th e Strait of Poi tau or of P oit iers , but these depr essions were filled u p by thick Jurassic sedi me nts and played no part duri ng t he Cretaceous and Tertiary.
G . F . DOLLFUS ,
The sediments of th e Loire allow also of an examinat ion of the distribution of materials. We find that those brought down from th e torrential mou ntain ous region absolutely dom inate those derived from the banks and cliffs. We mu st suppose that the flow of the sea and the lakes m ixes all the up stream m aterials and re-distributes them by den sity: the large pebbles form the cordons littoraux and the bottom deposit s of rivers, the sands go a little farther on th e margin of the shores, the light debris derived from clays reach th e open water and go t o form new marls and clays in the deeps. These fact s are well known. The water also contains a high pr oportion of bicarbonate of lime which is pr ecipitated as carbonate in the depths in a quiet zone, in lakes as well as in the sea , by a pro cess, the complete mecha nism of whi ch is not yet kn own , but the results of whi ch are very clear. Constant Prevot, in 1827, called attention to the origin of the sediments of the P ari s Basin and the different nature of the stream-borne material a t various epochs; but other theories followed , and a mysterious int ernal origin was sought for the various rocks before our eyes. To-day, as all th e rocks of the Basin are known, we realise th at the depths could not su pply what they did not contain and that atmosph eric alteration has bee n th e great factor in th e m et am orphism of ro cks . In th e Paris Basin, a very interesting cond ition has much facilit ated th e classification of the beds: namely , th e frequ ent alternation of marine and lacustrine cont inental deposits. The a bunda nce of fossils in all our stra tified form ations has shown us the exist ence of numerous re petitive submergences, of marine or lacustrine deposits which pr ove also that th ere ha ve been a lte rn ations of continental depression and eleva t ion even more numerous than might have been su pposed. In the Primar y era the surface of the Massif Centra l was probably covered by the sea : outliers of Devonian are kn own near R oanne (Loire), and of Carboniferous Lim estone near Vich y (Allier). Bu t afterwards t he country form ed part of a vast continental area of ver y long duration. Denudation has left ev idence of the PermoCarboniferou s in an irregular band that can be traced from the extremity of Brittan y as fa r as the Maconn ais, passing by Ancenis to Chalonn es (where we saw it as Culm) , conti nuing t owards St. Amand, Montlucon , la Machine, and perh ap s joining hands with the Grauwacke of the Vosges and th e ant hraciti c d eposits of the Alps . There still remains above this a narrow band of coarse PermoTriassic grit, in the region of Moulins, but there are only faulted outcrops of which the cont in uat ion is unknown. Wh at is more interesting is that the Lias is perfectly linked wit h th e Trias and it is often difficult t o trace a boundary betw een these forma ti ons whi ch are so dist inct elsewhere. During the Jurassic and
GEOLOGICAL HISTORY OF THE LOIRE VALLEY.
lIS
Lower Cretaceous we cannot point out any gravelly deposit nor transport of sand northwards from the Massif Central; the shore-lines of these secondary deposits are little known, their extent ill-determined, the marine extension being so great. With the base of the Middle Cretaceous (Albian and Cenomanian) the scene changes. We have a continuous shore-line known even in depth, margining very deep deposits. The Massif Central supplied the base of the Albian with characteristic fossiliferous and glauconitic gravels, which have yielded unmistakeable materials from Cosnes on the Loire to Havre. Many deep wells, often gushing, have struck this conglomeratic base of the Gault and its associated glauconitic sands, with characteristic fossils, fragments of wood, pyrites, etc.,; thus proving that the Cretaceous sands crossed by the Loire are continuous under the Paris Basin and contribute to its water-supply, coming by this subterranean route from the Chailles of Nivernais. The ancient course of the Cher has supplied the quartzose sands of the borings of Vierzon, Chartres, Romorantin, etc,; that of the Creuse has brought the similar glauconitic sands found in the borings of He Bouchard and of la Haye-Descartes of Tours. The northern destination of these waters was the English Channel, and the shore moved up-stream, the central floor sank, and the sea gave us successively the very sandy Cenomanian, the Turonian and the Senonian, particularly chalky, formations which in advancing over the Central Plateau masked all arrival of granitic sediments. We know many deep borings which have reached the greensands, aligned along the western margin of the Basin: Vendome, Chateaudun, Brou, Bonneval, Pont-Gouin, Courville, Chartres (Mainvilliers), Evreux, Louviers, Elbeuf, Rouen, etc. Everywhere are the same materials, the same minerals, the same fossils. The eastern shore is much less well-known, but it may be found at SUlly-sur-Loire, Moncresson, Plenoche, St. Sauveuren-Puysaye. However, we have no information on the deep structure of the Beauce, or on the east of the Basin of the upper courses of the Seine, Marne, Oise and Aisne. There are good reasons for supposing that the Massif Central was covered transgressively by the beds of the Middle and Upper Cretaceous; at first greatly depressed it gradually rose to a great height, for the sea at length withdrew progressively towards the north. The fluviatile regime was established, first high up, as the sea withdrew, yet we only find proof of it long after its establishment, when, with continued elevation, the removal by denudation of the secondary deposits exposed the granitic nucleus, and hard quartzose sediments descended to the plain to cover for the second time the various parts of the Basin.
I . E O CE NE <:
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;7l FIG.
7.- RELATION
OF
~
PARIS BASIN DEPOSITS TO
ANCIENT
tI
o
MASSIFS
t'" t'" "'1
IN EOCENE PERIOD.
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in
n4':'"nC'Nasujs _
[A.M.D. after G.F.D.].
GEOLOGICAL HI STORY OF THE LOIRE VALLEY.
II]
lII.
EOCENE PERIOD: FIRST GREAT EXTENSION OF GRANITIC SEDIMENTS. Th e Lower E ocene was for th e Massif Central a period of intense denudation in the sout h and of deposition in th e north. The sands and grits of thi s period on reaching th e sea produced the Thanetian sediments, and in th e continent al region the vast d eposit s of the Sparnacian which formed a great delta. It is impossible to descr ibe all the deposits of the Sparnacian : they cover great stretches of T our aine where Dujardin figured them nearly a hundred yea rs ago; they extend in Maine and Perche in the form of sands and grits with Sa balites ; in the region of l.oing, in the environs of M.ontargis, and as far as Auxerre, they are masses of coarse, hard, siliceous puddingst ones, kn own as the Poudingue de Nem ours . Th e Sarthe, the P erche, the Chartres country were covered by those gritty, mi caceous and titaniferous materials known as Poudingue de Breuillet . Th ey are found also in the north of France, in the east as far as Reims, westwards to Evreux, as well as in Belgium and even in England. We will not linger over the formation of the Calcair e Grossier or that of the" sables moyens " of the Paris district. Th e sea , coming from the north, from Belgium and Flanders, did not penetrate far into the basin. We have shown that the Lutetian spread transgressively in stages t owards the south , carrying in this direction a littoral cordon of feebly conglomeratic cha ract er, formed of glauconitic sands, of chalk-flints and of fragments of organi cally-formed lime st ones. The influence of southern affiuents is not proved . Various intercalations of lacustrine limestone, such as the calcaire de St. Ouen , show the influence of affiuents charged with lime flowing into a tranquil expanse; the intercalation of qu artzose sands, such as those of Beauchamp and Monceau, sh ows, however, that th e connection with the granitic regions was not broken; but it is important t o note that these sandy sediments may possibly be du e to redistribution of earlier deposits, and not first hand pr oduct s. IV.
OLIGOCENE PERIOD: GREAT DEVELOPMENT OF THE LACUSTRINE REGIME. We take the Oligocene as beginning, in the P aris Basin, with the Gypseous marls cont aining Palceotherium magnum and Anoplotherium, and as ending (following Tournouer) with the calcaire de Beauce. It has been divided into three sta gesSannoisian, Stampian, Firmitian-and we observe in it important mineralogical and palseontological changes. Sannoisian. At the base are marls and clays (of which we have already discussed the origin), which pass into a lacustrine limestone, the Calcaire de Brie, the poor fauna of which (N ystia ducha steli , Potamides moniliier, Enielodon magnum) is very
IT .OLIG O CE NE .
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FIG.
S.-REI.ATlON OF
o
PARIS BASIN DEPOSITS MASSIFS
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IN OLIGOCENE PERIOD.
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TO
ANCIENT
ot"'
t""' "1
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[A.M.D. afte» G,F.D;-,
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GEOLOGICAL HISTORY OF THE LOIRE VALLEY.
II9
widespread, from the north of Belgium to the valleys of the Loire and Allier, the sediments descending from south to north. Stampian. In the middle part occurs the interesting deposit of the Fontainebleau sands. The gulf-entrance was depressed and the sea invaded Belgium and the Paris Basin, without reaching the valleys of the Massif Central which continued to fill up with sediments having practically the same fauna. A large part of the shore-line of the Stampian sands is known, with its cordons of pebbles and its dunes, the summits of which were cemented into sandstone. It is incontestable that this enormous mass of quartzose sands can only have come from a powerful mountainous massif, just as the vast sands of the Landes, in the south-west, originated in the denudation of the Pyrenees. Perhaps the contemporaneous Rupelian clays were derived from the more distant transport of the lighter materials formed from the decomposition of felspar. Firmitian (Chattian). The end of the Stampian is marked by a new earth-movement: the sea retired to the north and another lake was established in the Seine and Loire basins. This was the age of the ealeaire de Beauee characterised by Helix ramondi and Anthracoiherium magnum (the extension and importance of which for general geology we have insisted on), Potamides lamarcki, Cvelostoma (Erieia) antiqua, Cyrena semistriata, etc. Aquitanian. Lacustrine conditions long prevailed and continued after a very marked faunal change. The ealeaire de l'Orleanais followed the ealeaire de Beauee, practically over the same area, but with an altogether different fauna: Helix aurelianensis, Anchiiherium aurelianense, Mastodon, Dinotherium, We will admit that the marls and limestones of this age may also have originated in the Paris region, from materials from the east formed by the slow dissolution of the calcareous border of the Jurassic and Cretaceous masses. The lakes of Auvergne certainly played the part of settling-tanks, stopping the march of alluvia towards the north. V.
MIOCENE PERIOD: SECOND EXTENSION OF GRANITIC ALLUVIA CAPTURED BY THE SEA, MULTIPLE EARTH-MOVEMENTS, FINAL RETREAT OF THE SEA. An important change ended this long lacustrine period, comprising the Upper Oligocene and Lower Miocene. The crystalline massif once more rose, and, as the great lakes of the Allier and the Loire were now filled up, the great granitic alluvia spread out from the borders of the Plateau Centra1. Here we are only considering the northern" slope," but in other directions, west and south, the situation was the same: the
PROC. GEOL.
Assoc.•
VOL,
XLII.,
PART 2, 1931;
9
TIl .MI O CEN E.
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~
N
o
I
d
F IG
9 . -H.EL AT ION OF L OI R E
P
D E P OSITS TO
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PAR IS AN D BAS IN A N CIEN T
:M A SSIFS
IN
:::l
o
M IO CE N E PERIO D.
I"" I"" 'T1
c::
(fl
Fo r Orleannais read
Orleanais
.'
[Jj§ill
[A.M.D. ujter G.F.1J.j.
GEOLOGICAL HISTORY OF THE LOIRE VALLEY.
121
«leposit of a vast aureole of quartzose sands and kaolinitic clays with varied debris, more or less well-sorted. Let us add that, since their deposition, these ancient alluvia have undergone profound alterations which have modified their aspect. All the calcareous constituents have disappeared, the whole volume has diminished, the stratification has been obscured; a new grouping has followed, the quartzose sands have separated from the felspars, the micas have taken another road. We are faced by confused deposits that the earlier geologists found inexplicable, and that many others have considered as eruptive. However, in some privileged places the original characters have been preserved in the deposit known as the sables de l'Orleannais. These are calcareous, stratified and fossiliferous and are only a different facies of the sables de la Sologne, which, in contrast are clayey, confused and unfossiliferous. We know some other places where there are patches that have remained normal, and others again where the alteration has been accentuated and the grey colour has changed to red. I have described the granitic sands of the neighbourhood of Paris under the name of the Sables de Lozere, and I have followed them step by step from their origin in Auvergne through the Nivernais, the Bourbonnais, the Loiret and through the Paris district on the plateaux of the left bank of the Seine, between Seine and Eure to the region of Rouen and to the cliff-tops of the Seine estuary at Havre, the characters remaining constant. However, at the base are found rolled fragments of the rocks met with on the way: flints from various Chalk horizons, chert (meuliere) and siliceous pebbles derived from various Tertiary beds. Stanislas Meunier has given a list of various mineral fragments found in a pocket in the neighbourhood of Paris. I give here a sketch (fig. 9) of the distribution of the granitic sands which extend to the Lower Miocene, tracing the course of the ancient Loire. It was during the course of this continental accumulation that the event occurred which determined the change of course of the Loire. The central part of the Loire Basin foundered, sinking so far as to allow the entry of the sea (the North Atlantic of the Middle Miocene) as far as the Orleanais, This sea captured the Loire and its tributaries and intercepted the quartzose sediments which, travelling in quite another direction, were found on its path. The sea-gulf of Touraine includes on its shore-line all the elements of the alluvia of the Massif Central. We recognise that this marine invasion did not extend far, certainly not beyond Orleans, if it reached that town; that the vertical amplitude was moderate, since the greatest altitude of the faluns does not exceed 130 metres; and that its duration was not very long, since the marine molluscan fauna remained
122
G. F. DOLLFUS,
the same in all the deposit s of t he Blesois, Touraine and Anjo u. With a m oderate thickn ess, the depth at which the bryozoan faluns were form ed did not exce ed 60 metres and was ofte n mu ch less. This marine capture com plet ely stopped the descent of the granitic sands t owards P aris, and probably th is depression of the Loire was acco m panied by a slight elevation of the adj acent region s, sinc e the northward extension of the faluns is limited by an anticlinal fold , not very pronoun ced though very exte nsive, indicated a century ago by d' Archi ac under the name of the anticline of Merlerault, starting from Normandy , crossing the Beauce, and forming, since Miocene times, the watershed between the Seine and Loire basins. Nevertheless, throughout the region of the lower Loire the Miocene depression has pred ominated over the series of N.W.-S.E . folds which advan ced upon the Paris Basin. Helvetian. It is well t o sa y a word on what is understood by the term " Faluns " : t hey are calcareous sands found on certain plateaux, principally in the neighbourhood of Blois, and form erly employed as calcareo us agricultural fertilisers. Grains of clear or milky quartz, ofte n st ill angular, preponderate; next come fragments of flint derived from the clay-with-flints, shellfragments particularly of Bryozoa , and bones identified with t hose of the Sables de l'Orleanais . These whi te or greyish marine deposit s are ofte n altered : losing their calcareous matter , their strat ificatio n, their fossils, they become more or less clayey and reddish . They some ti mes descend into deep pipes in the lim estones: at P oitiers they fill" natural pits descen ding t o t he level of the Clain. The gra velly and sa ndy facies whi ch is richly fossiliferous has been named P o n til e v i an [from P ont-le-Voy] ; t he calcareous detritic facies, some ti mes hardened , has t ak en the nam e S a v i g n e a n . Th e as pect is essent ially littoral and the molluscan fauna indicat es a warm sea and has close affinities with that of th e prese nt coas t of Senegal. The visit t o Doue-la-Fontaine gave us a good id ea of th e form ation as a wh ole. The bryozoan deposits indicat e where th e sea was some what deeper. There is no trace of volcanic deb ris, the gra nd eruptive ph enomenon of the Massif Central had not yet begun; the basaltic alluvia did not form until after th e retreat of the H elvetian sea , followin g th e upheaval which put an end t o it. It may be observed that this Helvetian marine incursion is not restricted to Tourain e : at the same epoch the Bordeau x basin was also depressed an d the sea entered it. The ph enomenon was essentially the same, and inserted between identical continental formations ; we may t abulate the situation thus :-
GEOLOGICAL HISTORY OF THE LOIRE VALLEY. MOVEMENTS.
STAGES.
TOURAINE.
123
BORDELAIS.
Slow elevation Tortonian Maximum depression Helvetian Elevation Depression Elevation Depression Elevation
Faluns d' A njou Faluns de Saubrigues Faluns Faluns de Salles et de Touraine Leognan Aurellanlan Calcaire lacustre Calcaire gris de l'Orleanais de f'Agenais Aquitanian Molasse du Faluns de La Brede Gtitinais and Bazas Firmitian Calcaire lacustre Calcaire blanc de Beauce de l'Agenais Stampian Sables de Fontaine- Calcaire it Asteries bleau marnes de Gaas Sannoisian Calcaire lacustre Calcaire lacustre de de Brie C asfi 110 t et du Perigord
Perhaps the depression of the English Channel, similar to those of Gironde and Touraine, may be attributed to the same Helvetian epoch; a slight trace being left in the presence of faluns in the Cotentin. There is here a phenomenon of elevation and depression of a curious regularity, evidenced by successive fossil faunas, identical stage by stage. There are almost regular spasms, of comparable value in gulfs in the same orientation on the border of the same continental plateau. Charles Mayer of Zurich, who formerly was a great student of Tertiary seas, divided them into ' full seas' or ' seas of extension,' and ' low seas' or seas ' of regression': there was much truth in this idea. Tortonian. The sea of the faluns did not perhaps maintain itself for long at the points of its greatest extension, or in the central region of Contres, where we find them carried to their greatest altitude (133 metres). Always fed with granitic sands, it withdrew little by little towards the west, towards its place of origin. It remained longer in Anjou and Brittany with a slightly modified molluscan fauna. Redonian. The really tropical elements which link the fauna of the faluns to that of the Senegal coast gradually disappeared, and the deposits in the neighbourhood of Rennes that we have designated as Red 0 ri ian contain a more temperate fauna. They only occupy the entry to the gulf, (a channel across Brittany), though gaining in extent on the shore of la Vendee. These very interesting argillaceous sands may be classed with the Tortonian, in the Upper Miocene. The reason why we have given them a distinct stage-name is that their fauna is much less warm-water than that of the typical Tortonian of Saubrigues in the south-west, while it is still more remote from the typical Tortonian fauna of Italy. We do not think that this Redonian gulf lasted very long: the ground rose steadily, and in the Pliocene the sea probably kept within its present limits. At the end of the Miocene, a date difficult to fix, the sea was in slow and regular retreat.
1 24
GEO LOG ICAL H ISTORY OF TIlE LOIR E ' ·ALL E Y .
We mu st su ppose that th e rivers a nd th eir alluv ia followed t his re t reat a t their lowest points, occupyi ng marine cha nnels as they extended their courses. H ith erto t he stages of racin ement of the peneplain during th e Pli ocene have only been ve ry sum marily recogni sed. It was on a vast peneplain, however , th at the gra vels (now seen on the plateaux), and the loam s of th e slopes that have great importan ce in the northern region of Brittan y wer e deposited. These loam s are so similar on bo t h sides of the va lley that they appear t o have bee n deposit ed be fore and during its excav at ion. Moreover , th e Loire shows us a th ick acc um ulatio n in its be d that could only have come about through a progressive sinking o f th e region . This movement of depression still conti nues, and t he ex tensive digg ings made at St. Nazaire in the deposits of the est uary have shown, below deposits of the older historic peri od , beds with p rehist oric implements occurring a t a great depth . Th e study of the movem ents of the ground is not com plete. We h ave seen them beginnin g in remot e epochs and cont inuing through many vici ssitudes t ha t man has been abl e t o recognise. We must accept with a profound ph ilosophy these geographical and biological cha nges , and consider it a great victo ry t o have been able to unravel t he principal t hrea ds of thi s long history, as we have tried t o do.
A
SHORT
GEOLOGICAL HISTORY LOIRE VALLEY. B,-
OF
THE
G. F. DOLLFUS.
[Received Ncoember 27th, 1928.J
[This is an expansion at an address to the Association on the termination at the visit to Angers, in August, 1928, certain allusions to which will be noticed. Translated by A. Morley Dacies.) I. Geographical generalities: nature of the sediments. II. Primary and Secondary Periods. III. Eocene Period: first great extension of the granitic sediments. IV. Oligocene Period: great development of the lacustrine regime. V. Miocene Period: second extension of granitic alluvia captured by the sea; multiple movements of the ground; final retreat of the sea. I.
GEOGRAPHICAL GENERALITIES.
THEandLoire is a great river, a thousand kilometres in length, its regime is torrential, its maximum volume reaching three hundred times its minimum. It is the outflow channel to the sea from the northern slope of the crystalline Central Massif of France. Its course may be divided into three sections : The Up per L 0 ire is joined by the Allier near Nevers, and their principal elements above the confluence are as follows : Gradient Flow Altitude (Km.) (cubic metres at source. Length. per second). 1562 m. 430 km. 1.05 180 Loire Allier 1423 m. 4IO km. 1.84 120 The altitude and length differ little, but the gradient of the Allier is much steeper, while its mean flow expressed III cubic metres per second is much less. The Mid d l e L air e includes the region from Nevers to Gien, where the direction is still from south to north and the flow little changed. The Lower L air e extends from Gien to the sea: the direction is westward and the flow increases considerably, for all the great tributaries of the Loire are in this section and on the left bank: Cher, Indre, Creuse (with its tributary Vienne) and the Thoulet which joins at Saumur, all furnishing
GEOLOGIC.\L HISTORY OF THE LOIRE
VALLE Y .
113
m ore or less granit ic alluvia.
On th e right bank there is onl y the Maine, which we saw at Angers, formed by the union of the Loir , Sarthe and Mayenne, only giving medium gravelly and silt y alluvia. Th en, at Nantes, the Erdre, which is an old sea-fiord , a river of very great depth, of very irregular width , and den oting by the th ickness of it s alluvia a thick accumulat ion lik e th at of the Loire at St. Nazaire. We will not linger over the geographical details of this fine ba sin . the navigation of which has been almost aba ndoned, thou gh it might furni sh an important transport route. ~.-\T U R E OF THE D EPOSITS.-vVe can all the more easily follow the hist ory of th e Loire since the nature of its sediments is charac t eristic. Th e starting-point is in th e granitic rocks of th e Massif Central-Auvergne an d Lim ousin-these materials are hard, unmist akeable and in contras t with the materials supplied by the mi ddle and lower cours es of th e river. The sands carried by the Loire decrease steadily in grade as we descen d its course: th e very coarse pebbles of th e Bee d' Allier are redu ced when Nantes is reached to an extremely fine sand, with some cas ual pebbl e-beds. The volcanic materials, such as basalts, disappear rapidly by , porphyr isation ' in cont act with quartz, 'granulite ' decom poses, Jurassic and Cretaceous flints become rolled and thus disappear little by little, while all the felspat hic materi als become clayey , and specia lists, su ch as M. Bureau , on examining a handful of Loire sand can st ate from what point it has been t aken. All these debri s have not ceased t o feed the Loire and its affiuents during th e Secondary an d Tertiary geological eras ; indeed , their greater or less abundan ce corresponds t o movements of elevation or depression of the Massif Central with maximum and minimum periods of long duration. II. PRIMARY AND SECONDARY PERIODS. Th e ancient rocks, including th e Tri as, form a cont inuous oute r border t o the Loire-Sein e basin broken onlv in the north. They include the massifs of (I ) the Ardenne in t he north ; (2) the Vosges to the eas t, the Morvan also t o th e cast; (3) the lower ramparts of the Massif Central , com prising th e Bourbonnais. the Bois-Chaud, th e Marche, the Limousin , forming a great barrier on the south ; (4) the hill s of th e Vendee on the southwest , which join with (5) the massif of Brittan y, th e advanceposts of which ar e in Anjou, Mayenn e and th e Bocage of Normandy . The circle is com plet ed by (6) the Cot entin peninsula, whi ch is primary, and lies to the north-west. There are only two ga ps in this circle, known as the Strait of Cote d'Or or of L an gr es and th e Strait of Poi tau or of P oit iers , but these depr essions were filled u p by thick Jurassic sedi me nts and played no part duri ng t he Cretaceous and Tertiary.
G . F . DOLLFUS ,
The sediments of th e Loire allow also of an examinat ion of the distribution of materials. We find that those brought down from th e torrential mou ntain ous region absolutely dom inate those derived from the banks and cliffs. We mu st suppose that the flow of the sea and the lakes m ixes all the up stream m aterials and re-distributes them by den sity: the large pebbles form the cordons littoraux and the bottom deposit s of rivers, the sands go a little farther on th e margin of the shores, the light debris derived from clays reach th e open water and go t o form new marls and clays in the deeps. These fact s are well known. The water also contains a high pr oportion of bicarbonate of lime which is pr ecipitated as carbonate in the depths in a quiet zone, in lakes as well as in the sea , by a pro cess, the complete mecha nism of whi ch is not yet kn own , but the results of whi ch are very clear. Constant Prevot, in 1827, called attention to the origin of the sediments of the P ari s Basin and the different nature of the stream-borne material a t various epochs; but other theories followed , and a mysterious int ernal origin was sought for the various rocks before our eyes. To-day, as all th e rocks of the Basin are known, we realise th at the depths could not su pply what they did not contain and that atmosph eric alteration has bee n th e great factor in th e m et am orphism of ro cks . In th e Paris Basin, a very interesting cond ition has much facilit ated th e classification of the beds: namely , th e frequ ent alternation of marine and lacustrine cont inental deposits. The a bunda nce of fossils in all our stra tified form ations has shown us the exist ence of numerous re petitive submergences, of marine or lacustrine deposits which pr ove also that th ere ha ve been a lte rn ations of continental depression and eleva t ion even more numerous than might have been su pposed. In the Primar y era the surface of the Massif Centra l was probably covered by the sea : outliers of Devonian are kn own near R oanne (Loire), and of Carboniferous Lim estone near Vich y (Allier). Bu t afterwards t he country form ed part of a vast continental area of ver y long duration. Denudation has left ev idence of the PermoCarboniferou s in an irregular band that can be traced from the extremity of Brittan y as fa r as the Maconn ais, passing by Ancenis to Chalonn es (where we saw it as Culm) , conti nuing t owards St. Amand, Montlucon , la Machine, and perh ap s joining hands with the Grauwacke of the Vosges and th e ant hraciti c d eposits of the Alps . There still remains above this a narrow band of coarse PermoTriassic grit, in the region of Moulins, but there are only faulted outcrops of which the cont in uat ion is unknown. Wh at is more interesting is that the Lias is perfectly linked wit h th e Trias and it is often difficult t o trace a boundary betw een these forma ti ons whi ch are so dist inct elsewhere. During the Jurassic and
GEOLOGICAL HISTORY OF THE LOIRE VALLEY.
lIS
Lower Cretaceous we cannot point out any gravelly deposit nor transport of sand northwards from the Massif Central; the shore-lines of these secondary deposits are little known, their extent ill-determined, the marine extension being so great. With the base of the Middle Cretaceous (Albian and Cenomanian) the scene changes. We have a continuous shore-line known even in depth, margining very deep deposits. The Massif Central supplied the base of the Albian with characteristic fossiliferous and glauconitic gravels, which have yielded unmistakeable materials from Cosnes on the Loire to Havre. Many deep wells, often gushing, have struck this conglomeratic base of the Gault and its associated glauconitic sands, with characteristic fossils, fragments of wood, pyrites, etc.,; thus proving that the Cretaceous sands crossed by the Loire are continuous under the Paris Basin and contribute to its water-supply, coming by this subterranean route from the Chailles of Nivernais. The ancient course of the Cher has supplied the quartzose sands of the borings of Vierzon, Chartres, Romorantin, etc,; that of the Creuse has brought the similar glauconitic sands found in the borings of He Bouchard and of la Haye-Descartes of Tours. The northern destination of these waters was the English Channel, and the shore moved up-stream, the central floor sank, and the sea gave us successively the very sandy Cenomanian, the Turonian and the Senonian, particularly chalky, formations which in advancing over the Central Plateau masked all arrival of granitic sediments. We know many deep borings which have reached the greensands, aligned along the western margin of the Basin: Vendome, Chateaudun, Brou, Bonneval, Pont-Gouin, Courville, Chartres (Mainvilliers), Evreux, Louviers, Elbeuf, Rouen, etc. Everywhere are the same materials, the same minerals, the same fossils. The eastern shore is much less well-known, but it may be found at SUlly-sur-Loire, Moncresson, Plenoche, St. Sauveuren-Puysaye. However, we have no information on the deep structure of the Beauce, or on the east of the Basin of the upper courses of the Seine, Marne, Oise and Aisne. There are good reasons for supposing that the Massif Central was covered transgressively by the beds of the Middle and Upper Cretaceous; at first greatly depressed it gradually rose to a great height, for the sea at length withdrew progressively towards the north. The fluviatile regime was established, first high up, as the sea withdrew, yet we only find proof of it long after its establishment, when, with continued elevation, the removal by denudation of the secondary deposits exposed the granitic nucleus, and hard quartzose sediments descended to the plain to cover for the second time the various parts of the Basin.
I . E O CE NE <:
H H
0\
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;7l FIG.
7.- RELATION
OF
~
PARIS BASIN DEPOSITS TO
ANCIENT
tI
o
MASSIFS
t'" t'" "'1
IN EOCENE PERIOD.
G
in
n4':'"nC'Nasujs _
[A.M.D. after G.F.D.].
GEOLOGICAL HI STORY OF THE LOIRE VALLEY.
II]
lII.
EOCENE PERIOD: FIRST GREAT EXTENSION OF GRANITIC SEDIMENTS. Th e Lower E ocene was for th e Massif Central a period of intense denudation in the sout h and of deposition in th e north. The sands and grits of thi s period on reaching th e sea produced the Thanetian sediments, and in th e continent al region the vast d eposit s of the Sparnacian which formed a great delta. It is impossible to descr ibe all the deposits of the Sparnacian : they cover great stretches of T our aine where Dujardin figured them nearly a hundred yea rs ago; they extend in Maine and Perche in the form of sands and grits with Sa balites ; in the region of l.oing, in the environs of M.ontargis, and as far as Auxerre, they are masses of coarse, hard, siliceous puddingst ones, kn own as the Poudingue de Nem ours . Th e Sarthe, the P erche, the Chartres country were covered by those gritty, mi caceous and titaniferous materials known as Poudingue de Breuillet . Th ey are found also in the north of France, in the east as far as Reims, westwards to Evreux, as well as in Belgium and even in England. We will not linger over the formation of the Calcair e Grossier or that of the" sables moyens " of the Paris district. Th e sea , coming from the north, from Belgium and Flanders, did not penetrate far into the basin. We have shown that the Lutetian spread transgressively in stages t owards the south , carrying in this direction a littoral cordon of feebly conglomeratic cha ract er, formed of glauconitic sands, of chalk-flints and of fragments of organi cally-formed lime st ones. The influence of southern affiuents is not proved . Various intercalations of lacustrine limestone, such as the calcaire de St. Ouen , show the influence of affiuents charged with lime flowing into a tranquil expanse; the intercalation of qu artzose sands, such as those of Beauchamp and Monceau, sh ows, however, that th e connection with the granitic regions was not broken; but it is important t o note that these sandy sediments may possibly be du e to redistribution of earlier deposits, and not first hand pr oduct s. IV.
OLIGOCENE PERIOD: GREAT DEVELOPMENT OF THE LACUSTRINE REGIME. We take the Oligocene as beginning, in the P aris Basin, with the Gypseous marls cont aining Palceotherium magnum and Anoplotherium, and as ending (following Tournouer) with the calcaire de Beauce. It has been divided into three sta gesSannoisian, Stampian, Firmitian-and we observe in it important mineralogical and palseontological changes. Sannoisian. At the base are marls and clays (of which we have already discussed the origin), which pass into a lacustrine limestone, the Calcaire de Brie, the poor fauna of which (N ystia ducha steli , Potamides moniliier, Enielodon magnum) is very
IT .OLIG O CE NE .
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FIG.
S.-REI.ATlON OF
o
PARIS BASIN DEPOSITS MASSIFS
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IN OLIGOCENE PERIOD.
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TO
ANCIENT
ot"'
t""' "1
For Orleannais read Orleanais
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Li mit . o f Ch ll!: l':J.t t. n c.~ L,.;a. Io< c..s
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GEOLOGICAL HISTORY OF THE LOIRE VALLEY.
II9
widespread, from the north of Belgium to the valleys of the Loire and Allier, the sediments descending from south to north. Stampian. In the middle part occurs the interesting deposit of the Fontainebleau sands. The gulf-entrance was depressed and the sea invaded Belgium and the Paris Basin, without reaching the valleys of the Massif Central which continued to fill up with sediments having practically the same fauna. A large part of the shore-line of the Stampian sands is known, with its cordons of pebbles and its dunes, the summits of which were cemented into sandstone. It is incontestable that this enormous mass of quartzose sands can only have come from a powerful mountainous massif, just as the vast sands of the Landes, in the south-west, originated in the denudation of the Pyrenees. Perhaps the contemporaneous Rupelian clays were derived from the more distant transport of the lighter materials formed from the decomposition of felspar. Firmitian (Chattian). The end of the Stampian is marked by a new earth-movement: the sea retired to the north and another lake was established in the Seine and Loire basins. This was the age of the ealeaire de Beauee characterised by Helix ramondi and Anthracoiherium magnum (the extension and importance of which for general geology we have insisted on), Potamides lamarcki, Cvelostoma (Erieia) antiqua, Cyrena semistriata, etc. Aquitanian. Lacustrine conditions long prevailed and continued after a very marked faunal change. The ealeaire de l'Orleanais followed the ealeaire de Beauee, practically over the same area, but with an altogether different fauna: Helix aurelianensis, Anchiiherium aurelianense, Mastodon, Dinotherium, We will admit that the marls and limestones of this age may also have originated in the Paris region, from materials from the east formed by the slow dissolution of the calcareous border of the Jurassic and Cretaceous masses. The lakes of Auvergne certainly played the part of settling-tanks, stopping the march of alluvia towards the north. V.
MIOCENE PERIOD: SECOND EXTENSION OF GRANITIC ALLUVIA CAPTURED BY THE SEA, MULTIPLE EARTH-MOVEMENTS, FINAL RETREAT OF THE SEA. An important change ended this long lacustrine period, comprising the Upper Oligocene and Lower Miocene. The crystalline massif once more rose, and, as the great lakes of the Allier and the Loire were now filled up, the great granitic alluvia spread out from the borders of the Plateau Centra1. Here we are only considering the northern" slope," but in other directions, west and south, the situation was the same: the
PROC. GEOL.
Assoc.•
VOL,
XLII.,
PART 2, 1931;
9
TIl .MI O CEN E.
-
~
N
o
I
d
F IG
9 . -H.EL AT ION OF L OI R E
P
D E P OSITS TO
:'1
PAR IS AN D BAS IN A N CIEN T
:M A SSIFS
IN
:::l
o
M IO CE N E PERIO D.
I"" I"" 'T1
c::
(fl
Fo r Orleannais read
Orleanais
.'
[Jj§ill
[A.M.D. ujter G.F.1J.j.
GEOLOGICAL HISTORY OF THE LOIRE VALLEY.
121
«leposit of a vast aureole of quartzose sands and kaolinitic clays with varied debris, more or less well-sorted. Let us add that, since their deposition, these ancient alluvia have undergone profound alterations which have modified their aspect. All the calcareous constituents have disappeared, the whole volume has diminished, the stratification has been obscured; a new grouping has followed, the quartzose sands have separated from the felspars, the micas have taken another road. We are faced by confused deposits that the earlier geologists found inexplicable, and that many others have considered as eruptive. However, in some privileged places the original characters have been preserved in the deposit known as the sables de l'Orleannais. These are calcareous, stratified and fossiliferous and are only a different facies of the sables de la Sologne, which, in contrast are clayey, confused and unfossiliferous. We know some other places where there are patches that have remained normal, and others again where the alteration has been accentuated and the grey colour has changed to red. I have described the granitic sands of the neighbourhood of Paris under the name of the Sables de Lozere, and I have followed them step by step from their origin in Auvergne through the Nivernais, the Bourbonnais, the Loiret and through the Paris district on the plateaux of the left bank of the Seine, between Seine and Eure to the region of Rouen and to the cliff-tops of the Seine estuary at Havre, the characters remaining constant. However, at the base are found rolled fragments of the rocks met with on the way: flints from various Chalk horizons, chert (meuliere) and siliceous pebbles derived from various Tertiary beds. Stanislas Meunier has given a list of various mineral fragments found in a pocket in the neighbourhood of Paris. I give here a sketch (fig. 9) of the distribution of the granitic sands which extend to the Lower Miocene, tracing the course of the ancient Loire. It was during the course of this continental accumulation that the event occurred which determined the change of course of the Loire. The central part of the Loire Basin foundered, sinking so far as to allow the entry of the sea (the North Atlantic of the Middle Miocene) as far as the Orleanais, This sea captured the Loire and its tributaries and intercepted the quartzose sediments which, travelling in quite another direction, were found on its path. The sea-gulf of Touraine includes on its shore-line all the elements of the alluvia of the Massif Central. We recognise that this marine invasion did not extend far, certainly not beyond Orleans, if it reached that town; that the vertical amplitude was moderate, since the greatest altitude of the faluns does not exceed 130 metres; and that its duration was not very long, since the marine molluscan fauna remained
122
G. F. DOLLFUS,
the same in all the deposit s of t he Blesois, Touraine and Anjo u. With a m oderate thickn ess, the depth at which the bryozoan faluns were form ed did not exce ed 60 metres and was ofte n mu ch less. This marine capture com plet ely stopped the descent of the granitic sands t owards P aris, and probably th is depression of the Loire was acco m panied by a slight elevation of the adj acent region s, sinc e the northward extension of the faluns is limited by an anticlinal fold , not very pronoun ced though very exte nsive, indicated a century ago by d' Archi ac under the name of the anticline of Merlerault, starting from Normandy , crossing the Beauce, and forming, since Miocene times, the watershed between the Seine and Loire basins. Nevertheless, throughout the region of the lower Loire the Miocene depression has pred ominated over the series of N.W.-S.E . folds which advan ced upon the Paris Basin. Helvetian. It is well t o sa y a word on what is understood by the term " Faluns " : t hey are calcareous sands found on certain plateaux, principally in the neighbourhood of Blois, and form erly employed as calcareo us agricultural fertilisers. Grains of clear or milky quartz, ofte n st ill angular, preponderate; next come fragments of flint derived from the clay-with-flints, shellfragments particularly of Bryozoa , and bones identified with t hose of the Sables de l'Orleanais . These whi te or greyish marine deposit s are ofte n altered : losing their calcareous matter , their strat ificatio n, their fossils, they become more or less clayey and reddish . They some ti mes descend into deep pipes in the lim estones: at P oitiers they fill" natural pits descen ding t o t he level of the Clain. The gra velly and sa ndy facies whi ch is richly fossiliferous has been named P o n til e v i an [from P ont-le-Voy] ; t he calcareous detritic facies, some ti mes hardened , has t ak en the nam e S a v i g n e a n . Th e as pect is essent ially littoral and the molluscan fauna indicat es a warm sea and has close affinities with that of th e prese nt coas t of Senegal. The visit t o Doue-la-Fontaine gave us a good id ea of th e form ation as a wh ole. The bryozoan deposits indicat e where th e sea was some what deeper. There is no trace of volcanic deb ris, the gra nd eruptive ph enomenon of the Massif Central had not yet begun; the basaltic alluvia did not form until after th e retreat of the H elvetian sea , followin g th e upheaval which put an end t o it. It may be observed that this Helvetian marine incursion is not restricted to Tourain e : at the same epoch the Bordeau x basin was also depressed an d the sea entered it. The ph enomenon was essentially the same, and inserted between identical continental formations ; we may t abulate the situation thus :-
GEOLOGICAL HISTORY OF THE LOIRE VALLEY. MOVEMENTS.
STAGES.
TOURAINE.
123
BORDELAIS.
Slow elevation Tortonian Maximum depression Helvetian Elevation Depression Elevation Depression Elevation
Faluns d' A njou Faluns de Saubrigues Faluns Faluns de Salles et de Touraine Leognan Aurellanlan Calcaire lacustre Calcaire gris de l'Orleanais de f'Agenais Aquitanian Molasse du Faluns de La Brede Gtitinais and Bazas Firmitian Calcaire lacustre Calcaire blanc de Beauce de l'Agenais Stampian Sables de Fontaine- Calcaire it Asteries bleau marnes de Gaas Sannoisian Calcaire lacustre Calcaire lacustre de de Brie C asfi 110 t et du Perigord
Perhaps the depression of the English Channel, similar to those of Gironde and Touraine, may be attributed to the same Helvetian epoch; a slight trace being left in the presence of faluns in the Cotentin. There is here a phenomenon of elevation and depression of a curious regularity, evidenced by successive fossil faunas, identical stage by stage. There are almost regular spasms, of comparable value in gulfs in the same orientation on the border of the same continental plateau. Charles Mayer of Zurich, who formerly was a great student of Tertiary seas, divided them into ' full seas' or ' seas of extension,' and ' low seas' or seas ' of regression': there was much truth in this idea. Tortonian. The sea of the faluns did not perhaps maintain itself for long at the points of its greatest extension, or in the central region of Contres, where we find them carried to their greatest altitude (133 metres). Always fed with granitic sands, it withdrew little by little towards the west, towards its place of origin. It remained longer in Anjou and Brittany with a slightly modified molluscan fauna. Redonian. The really tropical elements which link the fauna of the faluns to that of the Senegal coast gradually disappeared, and the deposits in the neighbourhood of Rennes that we have designated as Red 0 ri ian contain a more temperate fauna. They only occupy the entry to the gulf, (a channel across Brittany), though gaining in extent on the shore of la Vendee. These very interesting argillaceous sands may be classed with the Tortonian, in the Upper Miocene. The reason why we have given them a distinct stage-name is that their fauna is much less warm-water than that of the typical Tortonian of Saubrigues in the south-west, while it is still more remote from the typical Tortonian fauna of Italy. We do not think that this Redonian gulf lasted very long: the ground rose steadily, and in the Pliocene the sea probably kept within its present limits. At the end of the Miocene, a date difficult to fix, the sea was in slow and regular retreat.
1 24
GEO LOG ICAL H ISTORY OF TIlE LOIR E ' ·ALL E Y .
We mu st su ppose that th e rivers a nd th eir alluv ia followed t his re t reat a t their lowest points, occupyi ng marine cha nnels as they extended their courses. H ith erto t he stages of racin ement of the peneplain during th e Pli ocene have only been ve ry sum marily recogni sed. It was on a vast peneplain, however , th at the gra vels (now seen on the plateaux), and the loam s of th e slopes that have great importan ce in the northern region of Brittan y wer e deposited. These loam s are so similar on bo t h sides of the va lley that they appear t o have bee n deposit ed be fore and during its excav at ion. Moreover , th e Loire shows us a th ick acc um ulatio n in its be d that could only have come about through a progressive sinking o f th e region . This movement of depression still conti nues, and t he ex tensive digg ings made at St. Nazaire in the deposits of the est uary have shown, below deposits of the older historic peri od , beds with p rehist oric implements occurring a t a great depth . Th e study of the movem ents of the ground is not com plete. We h ave seen them beginnin g in remot e epochs and cont inuing through many vici ssitudes t ha t man has been abl e t o recognise. We must accept with a profound ph ilosophy these geographical and biological cha nges , and consider it a great victo ry t o have been able to unravel t he principal t hrea ds of thi s long history, as we have tried t o do.