Eiszeitalter
u.
Gegenwart
129 — 144
47
14
Hannover
Fig., 1 T a b .
1997
Characteristics and origin o f a Saalian glaciolacustrine to glaciofluvial succession in the Hümmling region, NW Germany JACQUES SCHWAN* & CORNELLS KASSE* G e e s t p l a t t e n , G l a c i o l a c u s t r i n e d e p o s i t s , Saalian g l a c i a t i o n
A b s t r a c t : In t h e l o w l a n d s o f n o r t h w e s t G e r m a n y , g l a c i o
d i e sich im fernen V o r l a n d d e s I n l a n d e i s e s b e f a n d . D u r c h
fluvial p l a t e a u s ( G e r m a n : G e e s t p l a t t e n ) , a n d p u s h m o r a i n e s
d i e s e n V o r g a n g w u r d e d a s G e l ä n d e e i n g e e b n e t u n d vor
built u p b y t h e S a a l i a n i c e s h e e t are s u r r o u n d e d b y flat a n d
bereitet
l o w - l y i n g country. In t h e study area in t h e Hümmling, t h e
s c h e n n a c h glazifluvialen
G e e s t p l a t t e c o n s i s t s o f m a i n l y sandy m e l t w a t e r
auf eine allmähliche Änderung
v o n glazilimni-
Ablagerungsverhältnissen.
deposits
with a c a p p i n g o f g r o u n d m o r a i n e o r till residue. T h i s o u t -
Introduction
w a s h plain w a s built u p during t h e Main D r e n t h e A d v a n c e o f t h e Saalian g l a c i a t i o n a n d s u b s e q u e n t l y overridden b y
I n t h e l o w l a n d o f n o r t h w e s t G e r m a n y , p l a t e a u s a n d hills
t h e e x p a n d i n g i c e s h e e t . T h e m e l t w a t e r d e p o s i t s form c o
c o n s i s t i n g o f Saalian glacial d e p o s i t s a n d
arsening-upward
f o r m e d b y t h e Saalian c o n t i n e n t a l i c e s h e e t a r e s u r r o u n d e d
s e q u e n c e s e x p o s e d in t h r e e sandpits in
t h e s t u d y area.
pushmoraines
b y flat a n d low-lying c o u n t r y . In G e r m a n , t h e s e t w o major
F r o m b a s e to t o p in t h e e x p o s u r e s , t h r e e waterlaid f a d e s
l a n d f o r m types are r e f e r r e d to a s r e s p e c t i v e l y
are distinguished: a b a s a l
ten" a n d "Niederungen" (Figs. 1 a n d 2 ) . T h e latter c a t e g o r y
g l a c i o l a c u s t r i n e f a d e s , a transi
"Geestplat-
tional f a d e s a n d a n u p p e r glaciofluvial f a d e s . Unless e r o s i
r e p r e s e n t s a system o f d e e p b a s i n s a n d v a l l e y s s u p p o s e d l y
o n h a s interfered, t h e u p w a r d c h a n g e from o n e f a d e s t o
left b e h i n d b y t h e S a a l i a n i c e a n d s u b s e q u e n t l y filled with
t h e n e x t is m a r k e d l y g r a d a t i o n a l .
mostly sandy sediment (German: T a l s a n d e ) .
T h e p a p e r gives a r g u m e n t s for t h e distal-lake origin o f t h e
In o u r study area in t h e H ü m m l i n g , t h e G e e s t p l a t t e consists
b a s a l f a d e s . It s u p p o s e d l y f o r m e d a s t h e fill o f drainless
o f c o n s i d e r a b l e t h i c k n e s s e s o f mainly s a n d y m e l t w a t e r d e
d e p r e s s i o n s in t h e distant foreland o f t h e i c e - s h e e t margin.
p o s i t s with a c o v e r o f g r o u n d m o r a i n e
B y this p r o c e s s , t h e terrain w a s levelled a n d prepared f o r
t h r e e e x p o s u r e s , t h e l o c a t i o n o f w h i c h is s h o w n in Fig. 2,
a
t h e s e s e d i m e n t s w e r e a v a i l a b l e for s e d i m e n t o l o g i c analy
gradual
change
from
g l a c i o l a c u s t r i n e t o glaciofluvial
o r till r e s i d u e In
sis. W h e n t h e information from t h e t h r e e sites is c o m b i n e d ,
depositional regime.
t h e picture e m e r g e s o f a c o a r s e n i n g - u p w a r d
glaziolacustriner bis glaziofluviatiler Folgen i m Hümmling-Gebiet, NW Deutschland] Kurzfassung:
Im Tiefland
Nordwestdeutschlands
sind
len Geestplatten u n d Stauchmoränen v o n flachen Niede r u n g e n u m g e b e n . I m U n t e r s u c h u n g s g e b i e t im H ü m m l i n g die
Geestplatte
aus
hauptsächlich
sandigen
Schmelzwasserablagerungen die von Grundmoräne Blockbestreuung überdeckt
oder
sind. D i e s e S a n d e r f l ä c h e ist
v o m Haupt-Drenthe-Vorstoß abgelagert u n d danach b e i m weiteren
Vorrücken
d e s Eises ü b e r f a h r e n
worden. D i e
Schmelzwassersande bilden nach o b e n gröber S e q u e n z e n , die i m U n t e r s u c h u n g s g e b i e t
a d v a n c i n g continental i c e sheet. T h i s h o w e v e r , is a first-sight i m p r e s s i o n o n l y a n d c l o s e r
v o n d e r s a a l e z e i t l i c h e n V e r e i s u n g a u f g e b a u t e n glazifluvia
besteht
fluvioglacial
s e q u e n c e laid d o w n a n d s u b s e q u e n t l y o v e r r i d d e n b y an
[Charakteristica u n d Ursprung saalezeitlicher
werdende
in drei
Gruben
s c a i t i n y s h o w s that t h e fine-sandy b a s a l part o f t h e s e q u e n c e s h o u l d b e o f g l a c i o l a c u s t r i n e r a t h e r t h a n glacioflu vial origin. This interpretation w a s n o t a n o b v i o u s o n e as ( i ) . c l a s s i c a l g l a c i o l a c u s t r i n e features s u c h a s silty o r c l a y e y t e x t u r e a n d varve-like stntcture are a b s e n t in t h e l o w e r part o f t h e s e q u e n c e a n d ( i i ) . t h e u p w a r d transition from fines a n d y l o w e r unit t o u p p e r unit with g r a v e l l y - s a n d c o m p o s i t i o n is gradational
environment.
T h i s p a p e r gives a r g u m e n t s in s u p p o r t o f t h e a b o v e inter pretation and analyzes t h e environmental conditions under which
aufgeschlossen sind.
and, consequently, unsuggestive o f
a n y distinct b r e a k in d e p o s i t i o n a l
glaciolacustrine
t o glaciofluvial s u c c e s s i o n s m a y
form. V o m L i e g e n d e n z u m H a n g e n d e n sind in d e n Aufschlüssen drei v o m W a s s e r a b g e l a g e r t e Fazies e r k e n n b a r : e i n e u n t e re g l a z i l i m n i s c h e F a z i e s , e i n e Ü b e r g a n g s f a z i e s und e i n e o b e r e glazifluviale F a z i e s . Sofern sich k e i n e erosiven Z w i s c h e n f ä l l e e r g e b e n h a b e n , ist die aufwärtsgerichtete
Ver
ä n d e r u n g v o n e i n e r F a z i e s z u r a n d e r e n a u s g e s p r o c h e n all mählich. D i e s e r Artikel bringt A r g u m e n t e dafür, d a ß d i e untere F a zies a b g e l a g e r t w o r d e n
s e i in e i n e r a b f l u ß l o s e n
Senke,
*) A d r e s s o f t h e author: D r . J . SCHWAN* & DR. C. KASSE*,
Geologic Setting Figures 1 and 2 s h o w the generalized g e o m o r p h o l o g y o f the study area a n d its wider surroundings. T h e c o r r e s p o n d i n g stratigraphy is given in Fig. 3 . Unit 1 is believed t o represent a h u g e outwash plain built u p during t h e o l d e r phase o f Saalian glaciation (the Main Drenthe A d v a n c e ) and s u b s e q u e n t l y over
F a c u l t y o f Earth S c i e n c e s , Vrije Universiteit, D e B o e l e l a a n
r i d d e n b y the e x p a n d i n g ice s h e e t (SCHRÖDER 1 9 7 8 ,
1 0 8 5 , 1 0 8 1 HV Amsterdam, T h e Netherlands
CASPERS et al. 1 9 9 5 ) . Less clear is t h e origin o f the
130
J . SCHWAN & C. KASSE
Cuxhaven. 25 k m
Plains and valleys with Weichselian periglacial or Holocene surface deposits Plateaus, hills and hill-ridges consisting of Saalian glacial deposits and including glaciotectonic landforms pushed up by Saalian ice. These features are with or without a thin cover of Weichselian to Holocene aeolian sand End moraines of Rehburger Phase Buried Saalian valleys Prequaternary uplands with or without a cover of Quaternary sediment German - Dutch frontier Fig. 1: G e n e r a l i z e d g e o m o r p h o l o g y o f n o r t h w e s t G e r m a n y a n d t h e n o r t h e a s t e r n N e t h e r l a n d s . B a s e d o n LIEDTKE ( 1 9 7 3 ) , MEYER ( 1 9 8 3 ) , ZAGWIJN et al. ( 1 9 8 5 ) a n d VAN DEN BERG & BEETS ( 1 9 8 7 ) . Abb. LAllgemeine Geomorphologie Nordwestdeutschlands und der nordรถstlichen Niederlande. Nach LIEDTKE (1973), MEYER ( 1 9 8 3 ) , ZAGWIJN et al. ( 1 9 8 5 ) und VAN DEN BERG & BEETS ( 1 9 8 7 ) .
Characteristics and origin of a Saalian glaciolacustrine to glaciofluvial succession in the Hümmling region, NW Germany
131
LEGEND P l a t e a u with undulating t o p o g t a p h y & d i s c o n t i n u o u s c o v e t of a e o l i a n s a n d (Geeslplatte) Z o n e of d u n e s & a e o l i a n s a n d s h e e t s a s s o c i a t e d with rivet E m s G e n t l y s l o p i n g v a l l e y floots L o w - l y i n g & neatly flat plain ( N i e d e t u n g ) Built-up a r e a 28
E x p o s u r e site with height in m a b o v e m e a n s e a level
Fig. 2: Geomorphologie sketchmap of the study area with location of exposure sites. Abb. 2: Grundriß der Geomorphologie des Untersuchungsgebietes mit Lage der Aufschlüsse. d e e p a n d w i d e Talsand-filled basins that surround the Geestplateaus. Various authors ( q u o t e d b y M E Y ER 1 9 8 3 ) have suggested that at least part o f t h e m re sults from either glacial scouring or meltwater ero sion w h e n the ice sheet w a s retreating from t h e sub j e c t region. A similar ambiguity prevails with r e s p e c t to the H u n z e Valley, a buried valley in the subsoil o f the northern Netherlands (Fig. 1). W h e r e a s V A N DEN B E R G & BEETS ( 1 9 8 7 ) interpreted b o t h this feature a n d the fossil V e c h t Valley as a lake-overflow c h a n nel, B o s e n ( 1 9 9 0 ) attributes the origin o f the H u n z e Valley to erosion b y a n a r r o w glacier t o n g u e flowing in south-south eastern direction. Unit 2 comprises ( i ) . a generally thin layer o f aeolian c o v e r s a n d that o c c u r s o n both plateaus a n d adjacent lowlands, and (ii). the Talsand that fills the basins. Essentially, the Talsand deposits in the area o f c o n c e r n consist o f Late Saalian, Eemian a n d W e i c h s e l i a n fluvial b e d s having a capping o f w i n d b o r n e sand. Within this s e d i m e n t type, o r g a n o g e n i c intercala tions are fairly c o m m o n . It has b e e n suggested that b o t h localized fluvio-periglacial activity as well as re gional deposition b y perennial or s e a s o n a l rivers contributed to the buildup o f the subject unit ( B O I G K et al. I 9 6 0 , DUPHORN et al. 1 9 7 3 , MEYER 1 9 8 3 ) . Unit 3 consists o f three subunits, viz. a p o d s o l i c soil m a r k i n g the top o f the Pleistocene plateau-sedi m e n t s , a raised-bog p e a t that covers the T a l s a n d north o f the Geestplateau and, lastly, a e o l i a n d u n e s that o c c u r scattered in the study area. T h e o b s e r v a t i o n sites M e a s u r e d sections, l a c q u e r peels a n d directional da ta w e r e c o m p i l e d in t h e three sandpits B r e d d e n b e r g ,
Lütker Sand and W a t t b e r g (Fig. 2 ) . W i t h respect to t h e s e exposures, t h e following c o m m e n t s are given: 1. SCHRÖDER ( 1 9 7 8 ) has suggested that the site Lütker S a n d might represent an inlier o f Elsterian a g e w h i c h is projecting through the strata o f t h e Saalian outw a s h plain. Natural outcrops o f Elsterian sediment o c c u r only 55 k m w e s t o f the H ü m m l i n g in Drenthe in T h e Netherlands ( T E R W E E 1 9 7 9 , B O S C H 1 9 9 0 ) and this w o u l d s e e m to b e in support o f the a b o v e inter pretation. T h e stratigraphical relationships a s s u m e d by SCHRÖDER ( 1 9 7 8 , his pages 81 - 8 5 ) are b a s e d o n the shallower e x p o s u r e - d e p t h s o f his time. H o w ever, the present condition o f sandpit Lütker S a n d unequivocally s h o w s that the "Sockelsand" is older t h a n the "Sande u n d Kiese" with t h e reverse b e i n g the principal argument in favour o f a n Elsterian age. Consequently, the sediments o f all t h r e e investigated sites are s u p p o s e d t o have b e e n laid d o w n during t h e s a m e period, i. e. during the Main Drenthe Ad v a n c e o f the Saalian Stage. 2. T h e e x p o s u r e s consist o f s e d i m e n t s that w e r e overridden by the advancing i c e s h e e t w h i c h had previously deposited them. T h e first halt o f this i c e s h e e t occurs well t o the south o f the Hümmling and is m a r k e d by the e n d m o r a i n e s o f the Rehburger P h a s e (Fig. 1). In t h e study area itself, glaciotectonic deformation o f t h e meltwater deposits is generally s h a l l o w and mild only. B e c a u s e o f t h e large size o f the exposures, s e c t i o n s that had r e m a i n e d altogether free o f distortion c o u l d b e s e l e c t e d for sedimentologic analysis. A m i n o r feature c o n c e i v a b l y due to the stress exerted b y an overriding i c e s h e e t is the blur ring or partial obliteration o f structure found here a n d there in fine-grained layers. This c o u l d b e a re-
132
J . SCHWAN & C. KASSE
suit o f e x c e s s i v e porewater-pressure and the liquefaction a s s o ciated with it.
E D E R U N G
Facies subdivision In the sediments e x p o sed at sites B r e d d e n berg, Lütker S a n d a n d Wattberg four waterlaid and t w o g l a c i g e n i c fa cies have b e e n distin guished (Figs. 4 - 9 ) .
or
L I T H O L O G Y / S O I L F O R M A T I O N
\—
CO O X h-
LU
The waterlaid facies T h e diagnostic features o f the four waterlaid fa cies A, B , C I a n d C2 are given in T a b l e 1. C o m bination o f the m e a s u red sections s h o w s that the succession o f the waterlaid facies A, B and CI or C2 represents a coarsening-upward s e q u e n c e . Unless erosi o n has interfered, the transition from o n e fa cies to the n e x t is mar kedly gradational. B u t for the fact that in our case a c h a n g e from gla ciolacustrine to g l a c i o fluvial e n v i r o n m e n t is involved, successions o f this type are c o m m o n in G e e s t p l a t e a u s ; in German they are re ferred to as Vorschütt sande, i. e. s a n d s laid d o w n b y an advancing ice sheet (EHLERS &
Ü
GRUBE 1983).
O
F a c i e s A . F a c i e s A is characterized b y b o t h the alternation o f plan e b e d d e d a n d ripplelaminated sets a n d a texture ranging from silt to m e d i u m sand (Fig. 1 0 ) . This facies is supposed to h a v e for m e d b y the discharge of sediment-laden meltwater into a distal
C H R O N O S T R A T I G R A PHY
X Ü
3c 3b 3a
aeolian sand peat podsolic soil
H O L O C E N E
W E I C H S E L I A N
LU z
2b 2a
aeolian sand Talsand E
E M I A N
H CO
LU
1b 1a
glacial till glaciofluvial sand
Fig. 3: Stratigraphy of the Hümmling study area. Abb. 3: Stratigraphie des Untersuchungsgebietes.
S A A L I A N
Characteristics and origin of a Saalian glaciolacustrine to glaciofluvial succession in the HĂźmmling region, NW Germany
SEDIMENTOLOGIC
LEGEND
H e i g h t a b o v e mean s e a level in m Facies types : A B C1 ,C2 D1 , D 2
= = = =
L a c u s t r i n e facies T r a n s i t i o n a l facies Glaciofluvial facies Glacigenic facies
Gradational f a c i e s contact S h a r p or e r o s i v e f a c i e s contact
G r a p h i c log planebed lamination
cross-bedding
planar x - l a m i n a t i o n , t r o u g h x-lamination or climbing-ripple x - l a m i n a t i o n
alternation of t h i n l a y e r s with r i p p l e - f o r e s e t x-lamination and planebed lamination
channel fill ; m i d - c h a n n e l depth < 1m
channel fill ; m i d - c h a n n e l depth > 2 m
single graded bed o r s u c c e s s i o n of s e v e r a l graded b e d s
channel with graded fill
f)
l)
Qâ&#x20AC;&#x201D;TT
residue of w e a t h e r e d till
A
A
\
single-grain s t r i n g of g r a v e l s in matrix of s a n d
A
\
A \
A
ground m o r a i n e
brecciated gravelly s a n d
Fig. 4: L e g e n d to Figs. 6-9.
Abb. 4: Legende zu den Abbildungen 6 bis 9.
133
134
J . SCHWAN & C. KASSE
S E D I M E N T O L O G I C L E G E N D (CON'D)
frost crack
no information
4 = Special features : sjP
isolated particles of gravel size in matrix of s a n d lamination distorted by liquefaction
I
• k
clay pebbles
k •••
thin clay layer
I
I
alternation of thin layers with contrasting texture e.g medium sand / coarse sand or m e d i u m sand / gravel gravelly - sand intercalation with distorted bedding
5 = Texture of waterlaid units : Si
=
silt
fS
=
fine sand
mS =
medium sand
cS
=
coarse sand
gs
=
gravelly sand with gravel - layers in sandy matrix constituting at least 1 0 % by volume
Fig. 5: L e g e n d to Figs. 6-9, c o n t i n u e d .
Abb. 5: Legende zu den Abbildungen 6 bis 9 (Fortsetzung),
Characteristics and origin o f a Saalian glaciolacustrine to glaciofluvial succession in the Hümmling region, NW Germany
135
BREDDENBERG
Si - 28
—D1
_l
A'
fS I
mS cS gS I
I
l_
C2
- 23
///////, '///////
• k
18 Fig. 6: S e c i i m e n t o l o g i c log o f site B r e d d e n b e r g .
Abb. 6: Sedimentologischer Profilschnitt des Aufschlusses Breddenberg. lake, i. e. into a b o d y o f standing water located a w a y from
the
i c e - s h e e t margin
(cf. SMITH a n d
ASHLEY
1985). Accordingly, the s u c c e s s i o n s o f the t w o types o f sets a r e regarded as i n c o m p l e t e , small-scale turbidites in w h i c h only the parallel-laminated division B and t h e current-rippled division C o f the classical s e q u e n c e o f BOUMA ( 1 9 6 2 ) a r e represented. B e t w e e n cycles, t h e thickness ratio o f the t w o c o m p o n e n t s s h o w s c o n s i d e r a b l e fluctuation a n d this suggests that t h e rhythmicity should b e attributed to weatherd e p e n d e n t rather than s e a s o n a l discharge-varia tions. An often applied criterion for the identification o f lake deposits is the p r e s e n c e o f silty and clayey beds. Apart from a f e w thin clay drapes and clay p e b b l e s ,
this characteristic is lacking in facies A. B o t h t h e rel atively elevated glauconite, mica a n d m e t a m o r p h i c mineral contents a n d t h e o c c u r e n c e o f lignite frag ments indicate that facies A (as well as t h e o t h e r wa terlaid facies) derive, t o a large part, from t h e Presaalian Pleistocene a n d Tertiary subsoil o f northwest G e r m a n y (e.g. K R O O K & SCHWAN 1 9 9 4 ) . In t h e proxi
mity o f the G e r m a n - D u t c h border, these s o u r c e beds have a mainly sandy c o m p o s i t i o n ( T E R W E E 1 9 7 9 , B O S C H 1 9 9 0 ) and this s h o u l d b e the r e a s o n w h y in the lacustrine facies A a clayey c o m p o n e n t is prac tically absent. M E Y E R ( 1 9 8 2 ) , EHLERS et al. ( 1 9 8 4 ) and EHLERS ( 1 9 9 4 )
discuss glaciolacustrine strata in sandur deposits that m a y well b e coeval with o u r facies A. T h e y occur
J . SCHWAN & C. KASSE
136
LUTKER
SAND
fS
55
__l
A
A A
A
A
A A
A A
A A
A
A^
A A
A A
50
I
A
A
D2
mS
A
_ 45
C1
_ 40
35
Fig. 7: S e d i m e n t o l o g i c log o f site Lütker S a n d .
Abb. 7: Sedimentologischer Profilschnitt des Aufschlusses Lütker Sand.
cS I
gS
Tl_
s o m e 170 k m northeast o f the Hümmling in a Geestplateau south o f the Elbe-estuary a n d consist o f well-sorted fine sands having an average thickness o f 1 0 m. Sediments o f this type, called B e c k e n s a n d e in German, s u p p o s e d l y w e r e formed during the earliest part o f the Saalian glaciation a n d their paleocurrent data are d o minated by flow in northwe stern to northern directions. Following the just-mentioned authors, these "basin sands" w e r e laid d o w n in huge shal l o w depressions o c c u p y i n g a distal position with respect to the ice-sheet margin or, alter natively, they might have b e e n left b e h i n d b y large braided-riv e r systems. F a c i e s B . Regarding texture a n d structure facies B holds an intermediate position b e t w e e n facies A and C I w h i c h underlie respectively overlie it. W h e n facies B is distinguished from its counterparts o n the basis o f grainsize c o m p o s i t i o n , there is not a unique type o f structure to match. D e p e n d i n g o n locati o n w e find p r e d o m i n a n c e o f either turbiditic couplets o r channel fills in a p l a n e b e d d e d matrix. Since t h e subject facies cannot b e unequivocally tied to a specific depositional envi ronment, it must b e consider e d a transitional facies. Its sig nificance is that it demonstrates the gradualness o f the c h a n g e from glaciolacustrine to glacio fluvial regime. F a c i e s C I a n d C 2 . Facies C I and C2 are t w o fluvioglacial facies having in c o m m o n a gra velly-sand texture. T h e first fa cies consists o f shallow c h a n nel fills with widths o f up to 8 m and depths not e x c e e d i n g 8 0 c m in a matrix o f p l a n e b e d d e d sand (Fig. 1 1 ) . It probably re presents a s h e e t f l o o d deposit associated with an alluvial fan. T h e s e c o n d facies is characteri zed b y planar cross-bedding
Characteristics and origin of a Saalian glaciolacustrine to glaciofluvial succession in the Hümmling region, NW Germany
WATTBERG 1
_ 33
D1
fS _i
A-
mS i
cS i_
137
w h i c h m a y have formed in bars o f a braided river (Miall 1 9 7 7 ) . Fa cies C2 was found at site Bred d e n b e r g (Figs. 6 and 12) where a part o f it suffered deformation or brecciation b y ice push. T h e river c o u r s e w h i c h de posited it appa rently cut d o w n deeply into the substrata as is e v i d e n c e d by an abrupt textural b r e a k at the b a s e o f the facies under consideration.
_ 28 B
d
n g
TT
23
_ 18
The glacigenic facies Facies D I is a re sidue o f scattered erratics on the groundsurface w h i c h has b e e n left over from weathering and w a s h i n g o f facies D 2 , the original groundmoraine. F a c i e s D 2 is a subglacial till with l o a m y textu re, c r u d e stratifi cation a n d an in tercalation o f gra velly sand. In the study area, this facies only o c curs in natural hollows in the groundsurface where is was sheltered from Postsaalian weat hering a n d erosi on.
Directional data
Fig. 8: S e d i m e n t o l o g i c log o f site W a t t b e r g , s e c t i o n 1.
Abb. 8: Sedimentologischer Profilschnitt 1 des Aufschlusses Wattberg.
T h e paleocurrent directions that
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J . SCHWAN & C. KASSK
WATTBERG 2
fS _i
mS I
cS i_
were measu red in the three exposu res are given in Fig. 13. In c l u d e d in this figure are rea dings from t w o n o w dere lict sandpits in the immediate vicinity o f site Lütker Sand. T h e s e data are a conversion of material compiled by SC
_ 28
B
7/>>Z.
-/fez.
~/fe^Z
_ 23
/fez.
7feZ.
A /fez _ 18
/fez
Fig. 9: S e d i m e n t o l o g i c log o f site Wattberg, s e c t i o n 2.
Abb. 9: Sedimentologischer Profilschnitt 2 des Aufschlusses Wattberg.
[I R Ö D H R
(1978) and should com pensate for the lack o f in formation on e x p o s u r e Lüt ker S a n d itself. In the justmentioned sandpit struc tures suitable for measuring paleocurrent directions are hard to find. Shallow chan nel fills in a matrix o f pla ne b e d d e d sand are the prevailing ty pe of sedi m e n t a r y struc ture. With res pect to these features it w a s found that they (i). occur in e q u a l fre q u e n c y in mu tually p e r p e n dicular pit fa ces a n d (ii). are mostly o f the types A a n d B o f Fig. 14 a n d e x c e p tionally o f ty p e C o f the sa-
Characteristics and origin of a Saalian glaciolacustrine to glaciofluvial succession in the Hümmling region, NW Germany
139
m e figure. T h e fill o f the first two types might b e in terpreted as either a lateralaccretion structure or a microdelta. Current flow would have b e e n parallel to the c h a n n e l axis in the first c a s e but perpendicular to it in the s e c o n d case. Thus, there are ambiguities relating b o t h to the planform o f the c h a n n e l s and the direction o f t h e flows that filled t h e m with sedi ment.
Fig. 1 0 : Alternation o f p l a n e b e d d e d a n d r i p p l e - l a m i n a t e d sets in i n c o m p l e t e turbidites o f g l a c i o l a c u s t r i n e facies A. S c a t t e r e d b l a c k m o t t l e s are lignite fragments. L a c q u e r p e e l from e x p o s u r e W a t t b e r g . L e n g t h o f profile = 1 2 5 c m .
Abb. 10: Abwechslung von Parallelschichtung und Rippelschichtung in unvollständig ausge bildeten Turbiditen der glazilimnischen Fazies A. Vereinzelte schwarze Flitter sind Braun kohlstückchen. Lackfilm aus Aufschluß Wattberg. Länge des Profils = 125 cm.
T w o clusters o f paleocurrent directions c a n b e dis tinguished in Fig. 13: o n e in the sector SE to SSE and the other o n e in the sector W to W N W . From t h e bottom hi stogram o f Fig. 13 it might b e inferred that this result merely represents the di spersion inherent to the da ta in question. Alternative ly, the bipartition could b e significant in t h e s e n s e that the clusters c o r r e s p o n d to two different streams o f se diment-laden meltwater forming at the ice-sheet margin and dropping their load in a c l o s e d basin. T h e t w o streams m a y have eit her c o e x i s t e d w h e n a suffi cient size o f the lake per mitted this o r otherwise they s u c c e e d e d e a c h other due to time-dependent c h a n g e s in the position o f the ice-sheet margin. In the first case it must b e assu m e d that the ice-sheet mar gin had a curved o r irregu lar outline rendering possi b l e the formation o f melt water streams with widely differing flow directions within the b o u n d s o f the la ke. From the work o f SCHRÖDER ( 1 9 7 8 ) it is under s t o o d that this requirement w a s met during the Saalian glaciation o f the Hiimmling-area.
140
J . SCHWAN & C. KASSE
Fig. 11: Channel fills of glaciofluvial facies CI in exposure Lütker Sand. Abb. 11: Rinnenfüllungen der glazifluvialen Fazies Cl im Aufschluß Lütker Sand. Discussion Lacustrine deposition is a feature o f wide occur rence in the glacial environment. In low-relief glacioterrestrial settings such as northern Europe and north America in Pleistocene times, huge lakes c a m e into being as a result o f overdeepening b y glacial ero sion, isostatic depression o f the proglacial landsurfac e or the damming o f epicontinental seas b y the advancing ice sheet (GIBBARI > 1 9 8 8 ; DAWSON 1 9 9 2 ; EYLES & EYLES 1 9 9 2 : EHLERS 1 9 9 4 ) . Other, smaller-scale
events conducive to the ponding o f meltwater were the clamming o f river valleys by the ice-sheet and the formation o f tunnel valleys (EHLERS & LINKE 1 9 8 9 ; EHLERS 1 9 9 4 ) a n d glacial basins ( D E GANS et al. 1 9 8 7 ) .
In order to relate our glaciolacustrine to glaciofluvial succession t o o n e o r m o r e o f the above processes, the following hypothesis is proposed to account for its genesis: In the early stages o f Saalian glaciation, the study area occupied a distal position with respect to the ice-sheet margin and its ground surface was at a con siderably lower level than it is now. It may b e e x p e c ted that the fine-grained margin o f the incipient san dur preferentially filled drainless depressions that h a p p e n e d to b e present in the distant foreland. This would result in a general levelling o f the terrain and concomitant gradual c h a n g e from lacustrine to fluvi al regime. Lake deposits s o formed would b e thin, discontinuous a n d restricted to the early phase o f a glaciation event.
T h e p r o c e s s under consideration d e p e n d s o n the p r e s e n c e o f c l o s e d depressions, preferably o f large size, in the foreland o f the continental i c e sheet. It is s u g g e s t e d that, at t h e o n s e t o f Saalian glaciation, this type o f relief w a s represented in northwestern Ger m a n y b y at least t w o different land forms. In t h e first place there must have existed a Presaalian drainage pattern directed towards t h e North S e a and having an overall SE to NW or S to N orienta tion. With the a d v a n c e o f the continental i c e sheet this system b e c a m e b l o c k e d from the north and this p r o v i d e d traps for sediment-laden meltwater. Secondly, the Elsterian ice sheet had left b e h i n d a sys tem o f wide and d e e p tunnel valleys in t h e subject area. It is imaginable that the generally north-south trending topography s o created did survive through Holsteinian and preglacial Saalian times a n d expres s e d itself in the form o f large, elongate depressions. Naturally, their original depth w o u l d n o t h a v e b e e n retained but the present assumption is that Postelsterian deposition did not fully level t h e m either.
Conclusions 1. T h e sandy meltwater deposits e x p o s e d in the study area in northwestern G e r m a n y w e r e laid d o w n during t h e Main Drenthe Advance o f the Saa lian glaciation (= O l d e r Saalian Glaciation) and re present a glaciolacustrine to glaciofluvial s u c c e s s i o n with coarsening-upward trend. 2 . S t a c k s o f i n c o m p l e t e , small-scale turbidites testify
141
Characteristics and origin of a Saalian glaciolacustrine to glaciofluvial succession in the Hümmling region, NW Germany Table 1: Characteristics of waterlaid facies Tabelle 1: Merkmale der vom Wasser abgelagerten Fazies,
A
Silt - m e d i u m sand
A l t e r n a t i o n of thin l a y e r s of c o a r s e , m e d i u m & f i n e s a n d with i s o l a t e d o c c u r r e n c e s of g r a v e l s i z e d particles Either h o m o g e n e o u s t e x t u r e o r multiple g r a d i n g in fine s a n d to silt r a n g e
2 o «3 C
acies contacts radational and harply defined
Fine sand coarse sand
F a c i e s C1: g r a v e l l y c h a n n e l fills in m a t r i x of s a n d
:
B
F a c i e s C2: a l t e r n a t i o n of g r a v e l l y & s a n d y foreset beds
From Iacies A to facies C1/C2 the se diment becomes increa singly heterolithic
Coarse sand gravelly sand. S i z e of g r a v e l s u p to 5 c m
In all facies a coarsening upward trend is oftein found
C1.CZ
Facies contacts
Sorting
Texture
Facies
U_
CD CO
Table 1: Characteristics of waterlaid facies (cont'd) Tabelle 1: Merkmale der vom Wasser abgelagerten Fazies (Fortsetzung). Facies
C1.C2
O t h e r features
D o m i n a n t structure
F a c i e s C 2 : C r o s s - b e d d e d sets with high gravel - content
F a c i e s C 2 is partly b r e c c i a t e d o r g l a c i o t e c t o n i c a l l y distorted
(Fig-12) Facies C1: Frequent occurrence of c h a n n e l fills in m a t r i x of p l a n e b e d d e d s a n d ( F i g . 11) B
F a c i e s B is s t r u c t u r a l l y intermediate b e t w e e n facies A a n d C1
S u c c e s s i o n s of g r a d e d beds
A
A l t e r n a t i o n of p l a n e b e d d i n g a n d s m a l l - ripple l a m i n a t i o n ( F i g . 10). L i m i t e d o c c u r r e n c e of m a i n l y s m a l l c h a n n e l fills
O c c a s i o n a l thin c l a y layers a n d clay pebbles
to the distal-lake origin o f the b a s a l facies o f the s u c c e s s i o n . Despite this m o d e o f deposition, the b a s a l facies hardly contains clay. This is d u e to the p r e d o minantly sandy c o m p o s i t i o n o f t h e Pleistocene a n d Tertiary s o u r c e b e d s o f the m e l t w a t e r deposits. 3. In the meltwater deposits, t w o clusters o f p a l e o current directions c a n b e distinguished. T h e two clus ters correspond to t w o streams o f sediment-laden meltwater forming at the i c e - s h e e t margin and d e positing their load in a c l o s e d basin. T h e two stre a m s either c o e x i s t e d in a large l a k e or otherwise they s u c c e e d e d e a c h other as a result o f c h a n g e s in the position o f the ice-sheet margin. 4. T h e basal facies formed as t h e fill o f drainless d e pressions that h a p p e n e d to b e present in the distant
Scattered dark brown mottles presumably being lignite - f r a g m e n t s . Relatively high m i c a - c o n t e n t , in particular in fine fractions
foreland o f the ice-sheet margin. B y this process, the terrain w a s levelled and p r e p a r e d for a gradual c h a n g e from lacustrine to fluvial depositional regi m e . T h e initial t o p o g r a p h y o f the proglacial landsurface m a y have b e e n shaped b y b o t h damming o f preglacial rivercourses and Elsterian tunnel-valley formation. Acknowledgments W e thank professor Dr. K.-D. M e y e r for providing us with relevant information and Drs. K. Van Gijssel for interesting suggestions.
l a
J . SCHWAN & C. KASSE
Fig. 1 2 : P l a n a r c r o s s - b e d d e d units o f glaciofluvial f a c i e s C2 in u p p e r h a l f a n d sets with variable structure o f i n t e r m e d i a t e facies B in l o w e r h a l f o f l a c q u e r p e e l from e x p o s u re B r e d d e n b e r g . Length o f profile = 125 c m .
Abb. 12: Planparallele, kreuzgeschichtete Einheiten der glazifluvialen Fazies C2 in der oberen Hälfte und Schichte mit veränderlicher Struktur der Obergangsfazies B in der unteren Hälfte eines Lackfilmes aus Aufschluß Breddenberg. Länge des Profils = 125 cm.
Characteristics and origin of a Saalian glaciolacustrine to glaciofluvial succession in the Hümmling region, NW Germany
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84.5°
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—144.5°
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204.5°
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224.5°
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244.5°
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284.5°
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324.5°
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Fig. 1 3 : P a l e o c u r r e n t d i r e c t i o n s m e a s u r e d at four sites. T h e b o t t o m histogram is a c o n v e r s i o n o f data c o m p i l e d b y SCHRÖ DER ( 1 9 7 8 ) . m = m e a n p a l e o c u r r e n t direction.
Abb. 13: Schrägschichtungsmessungen an vier Stellen. Das unterste Histogramm ist hergestellt aus konvertierten Daten von SCHRÖDER (1978). m = Mittelwert der Aufschüttungsrichtung.
1-4 !
J . SCHWAN & C. KASSE
Fig. 1 4 : T h r e e t y p e s o f c h a n n e l fill in facies C I at L ü t k e r S a n d . T y p e s A a n d 13 are c o m m o n a n d t y p e C is rare. I n t y p e s A a n d B flow m a y h a v e b e e n e i t h e r from left t o right ( m i c r o d e l t a ) o r p e r p e n d i c u l a r to t h e p a g e (lateral a c c r e t i o n ) .
Abb. 14: Drei Typen von Rinnenfüllungen in Fazies Cl in Lütker Sand. Typen A und B sind üblich und Typus C ist selten. In Typen A und B war die Strömung entweder von links nach rechts (Mikrodelta) oder senkrecht auf der Seite (seitwärtsgerichtete Aufschüttung). References BoiGK, H., DIETZ, C, GRAHLE. H.-O., HOFFMANN, K.. HOLLSTEIN, W., KÜHNE, F., RICHTER, W., SCHNEEKLOTH, H., WÄGER, R. ( I 9 6 0 ) : Zur
Geologie des Emslandes. - Beih. Geol. Jb., 37; Hannover. BOSCH, J.H.A. (1990): Toelichtingen bij de geologische kaart van Ne-
derland 1:50.000. Blad Assen West (12 W) en Blad Assen Oost ( 1 2 O). - Rijks Geologische Dienst; Haarlem. BOUMA, A.H. (1962): Sedimentology of some flysch deposits; Am sterdam. CASPERS, G., JORDAN, H., MERKT, J., MEYER, K.-D., MÜLLER, H. STREIF, H.
(1995): III. Niedersachsen. - In: BENDA, L. (Hrsg.): Das Quartär Deutschlands, Gebr. Bomtraeger, 23-58; Berlin. DAWSON, A.G. (1992): Ice Age Earth; London. DE GANS, W., DE GROOT, T., ZWAAN, H. (1987): The Amsterdam ba
sin, a case study of a glacial basin in The Netherlands. - In: VAN DER MEER, J.J.M. (Hrsg): Tills and Glaciotectonics. A.A. Balkema, 205 - 216; Rotterdam. DUPHORN, K., GRUBE, F., MEYER, K.-D., STREIF, H., VINKEN, R. ( 1 9 7 3 ) :
A. Area of the Scandinavian Glaciation, 1. Pleistocene and Ho locene. - In: SCHÖNHALS, E. & HUCKRIEDE, R. (Hrsg.): State of Re search on the Quaternary of the Federal Republic of Germany. - Eiszeitalter u. Gegenwart, 2 3 / 2 4 : 222 - 250; Öhringen/Württ. EHLERS, J. (1994): Allgemeine und historische Quartärgeologie; Stutt gart. - GRUBE, F. (1983): Meltwater deposits in north-west Germany. - In: EHLERS, J . (Hrsg.): Glacial Deposits in North-West Europe, A.A. Balkema, 249 - 256; Rotterdam. - LINKE, G. (1989): The origin of deep buried channels of Elsterian age in Northwest Gennany. - J. of Quat. Sc., 4: 2 5 5 265; Chichester. -
MEYER, K.-D., STEPHAN, H.-J. (1984): Pre-Weichselian glacia-
GIBBARD, P.L. (1988): The history of the great northwest European rivers during the past three million years. - Phil. Trans. R. Soc. Lond., B 3 1 8 : 559 - 602: London. KROOK, L., SCHWAN, J. (1994): Sediment-petrologic characteristics of
Saalian and Weichselian deposits in the Hümmling region, NW Germany. - Geologie en Mijnbouw, 72, 331 - 347; Utrecht. LILDTKE, H. (1973): Die nordischen Vereisungen in Mitteleuropa. Geologische Karte Maßstab 1:1.000.000; Berlin. MEYER, K.-D. (1982): Erläuterungen zu Blatt Nr. 2524 Buxtehude,
Geologische Karte von Niedersachsen 1:25.000. - Nieder sächsisches Landesamt f. Bodenforschung; Hannover. - ( 1 9 8 3 ) : Zur Anlage der Urstromtäler in Niedersachsen. - Z. Geomorph. N.F., 27-2: 147 - 160; Berlin. MIAU., A.D. (1977): A Review of the Braided-River Depositional En vironment. - Earth-Science Reviews, 13: 1 - 62; Amsterdam. SCHRÖDER, E. (1978):
Geomorphologische Untersuchungen
SMITH, N.D., ASHLEY, G.M. (1985): Chapter 4 Proglacial lacustrine en
vironment. - In: ASHLEY, G.M. et al. (Hrsg.): Glacial sedimentary environments, Soc. of Economic Paleontologists and Mineral ogists, 16: 135 - 216; Tulsa, OK. TER WEE, M.W. (1979): Toelichtingen bij de geologische kaart van Nederland 1:50.000, Blad Emmen West (17 W) en Blad Emmen Oost ( 1 7 O). - Rijks Geologische Dienst; Haarlem. VAN DEN BERG, M.W., BEETS, D.J. ( 1 9 8 7 ) : Saalian glacial deposits and
morphology in The Netherlands. - In: VAN DER MEER, J.J.M. (Hrsg.): Tills and Glaciotectonics, A.A. Balkema, 2 3 5 - 251; Rot terdam. ZAGWIJN, W.H., BEETS, DJ., VAN DEN BERG, M., VAN MONTFRANS, H.M.,
VAN ROOJIEN, P. (1985): Atlas van Nederland, deel 13: Geologic - Stichting Wetenschappelijke Atlas van Nederland; Den Haag.
tions of North-West Europe. - Quat. Sei. Rev., 3: 1 - 40; Oxford. EYLES, N., EYLES, C. H. (1992): 5. Glacial Depositional Systems. - In:
WALKER, R.G. &JAMES, N.P. (Hrsg.): Fades Models: response to sea level change, Geological Association of Canada, 7 3 - 100; St. John's, Newfoundland.
im
Hümmling. - Göttinger Geographische Abhandlungen, 7 0 : 5 111; Güttingen.
Manuskript e i n g e g a n g e n a m 1 3 . 0 5 . 1 9 9 6