LATE WEICHSELIAN MARINE SEDIMENTS CONTAINING SHELLS,FORAMINIFERA, and Pollen, at ACOTNES, WESTERN NORWAY

LATE WEICHSELIAN MARINE SEDIMENTS CONTAINING SHELLS,FORAMINIFERA, AND PoLLEN, AT ACOTNES, WESTERN NORWAY JAN MANGERUD Mangerud, J.: Late Weichselian marine sediments containing shells, foramitli- fera, and pollen, at Agotnes, western Norway. Norsk Geologisk Tidsskrift, Vol. 57, pp.23-54. Oslo 1977. Marine sediments, mainly clay' and silt, deposited at a water depth of 20-30 + m are describeil. The base of the succession is radiocarbon dated to 12.220 150 B.P., the upper part to 10,230 + 180. The shells indicate water tempera- tures similar to those of northern Norrvay today. These shells and shells from other localities suggest that warm Atlantic water entered the Norrvegian Sea prior to 12,600 B.P. The pollen diagram from the sediments has the same main trends as in cliagrams from limnic sediments covering the same period, except for high percentagesof Alnus. The age and correlation of several Late Weichselian events are discttsscd. J. Mangerud, Geologisk instittttt, Avd.B, Olal Ryesvei 19' N-5014 Bergett- U niv ersitetet, N orw aY. Large areasof Hordaland were deglaciatedduring the A11er0dChronozone (Figs. I and l4) (Mangerud 1910,1912b, Aarseth & Mangerud 1974).LaIer' during the Younger Dryas Chronozone, the ice front re-advancedby at least 40 km. The maximum extent of the inland ice during this re-advance is marked by the Herdla moraines(Aarseth & Mangerud 1974). The Agotnes locality discussedin the present paper lies 2 3 km outside the Herdla moraines (Fig. 1), and includes sedimentsof both Allerpd and Younger Dryas age. The stratigraphicalterminology used here is in accordancewith the pro- posals of Mangerud et a]l. (1974). The boundaries of the chronozones (Younger Dryas, Allerod, etc.) are defined in conventional radiocarbon years. The Agotneslocality The coast of Hordaland, including Sotra, is characterizedby bare bedrock. Loose, Quaternary sedimentsare mainly found in topographicaldepressions. Investigation of these sedimentsis usually possibleonly by coring. In the investigatedlocality, an excavatedsection was accessiblefor just one day in the summer of 1970.Field work was, however, limited by dangerousslides in the clayey sediments. The locality is situated on the landward side of the island Sotra (Figs. I and 2). close to the shore of Hjeltefjorden, however, only 6 km from the open North Sea. J. MANGERUD --:NbBTH= :SEA LOnd Herdlo endmoroines {Younger Dryos) lTTl ,ornn.,Dryos isoboses non ot tce_itow , 10km Fig. 1. Location of the Agotnes locality in Hordaland. younger Dryas isobasesare extrapolated from Aarseth & Mangerud (1974, fig. ll). It lies on a small peninsula (Fig. 2), with a maximum height of 29 m a.s.l.From the highestpoint the bedrock,with small patchesof till and littoral sand, slopesgently eastwardtowards the investigatedlocality (Fig. 3) at ca.12 m a.s.l. The bedrock is gneiss,forming smal escarpmentswith crestsparallel to the NNw strike, and dip slopeinclined eastwards.The locality lies at the foot of one of theseescarpments (Figs. 3 and 4). Details of the locality are shown at Figs. 4 and 5. The sedimentswere best exposedin the southernwall of the excavation,and the sequencein this wall is shown at Fie. 6. Grain-size analysis Particlesabove 16 mm were excludedfrom the analysis,those coarser than 0.063 mm were sieved,with a sieveinterval of one phi unit. The finest (< 0.063mm) were analysedby the pipettemethod, with readingsfor each phi unit. In all samplesorganic material was removedwith HoO" before analvsis. NORWAY L) LATE WEICHSELIAN SEDIMENTS IN WESTERN ! locaritv [.--q,r.;....',o.T.runouoou" .o.u ",. [I.Tl Lando-3o ma s.l. = "uu rhe Asotnes Fig.2.MapoftheareaaroundAgotnes.DuringtheLateWeichselianthesealevelwas and the map therefore gives an im- "p-pt"*mrLfv 30 m above that oithe present day' pressionof the contemporarypaleogeography' Fig.3.PhotographyoftheAgotneslocalityafterconstructionworkhadremovedmost The photograph is of the sediments described. ihe bedrock wall is thus man-made. northern edge of the sediments iut"r, to*u.d, the summit of the peninsula (NW). The is marked with stippled line along the base' J. MANGERUD Bedrock l-l un"on.ol,,lut"o L*-J sedrments 7 r^"uuuuon PI]OFiLE ] the geologicalpro- Fig. 4. To the left sketchmap of the Agotneslocality. To the right 6' fills. The'describedsection' indicated on themap is shownin Fig' (1968' fig' 1) In Norway a modified Wentworth scaleproposed by Doeglas is now used,and this scaleis applied here (Fig' 7)' McCammon(|962)andFolk(|966)discussedtheefficiencyofthedif- ferentparametersusedforgrain-sizedistribution.However,inroutine obtain the neces- analysisof fine-grained sediments,one does not always the present study, I sary percentilesfor the most efficient parameters.In (1957)for the mean grain have ttreretoreused the measureof Folk & Ward sizs: M2:(iDl6+o50+084)/3, and that of Inman (1952) for sorting (standard deviation) 5: (A84-Alol2. to be extrapolated' Even so, to obtain the value for iD16,some curves had Selmer-olsen(1954) analyseda large number of Norwegian Quaternary in a useful Md - sedimentsof differeni geneses,and presentedthe results Sodiagram.InordertocomparethesedimentsfromAgotneswithhisre- (Fig. assuming that sults, I have constructed a similar diagram 9), Mz: Md and recalculating the sorting coefficient Q75 so : log," Q25 LATE WEICHSELIAN SEDIMENTS IN WESTERN NORWAY 27 Fig. 5. Photograph of the excavation taken towards the sE. The main section shown in Fig. 6 is in the shadow, with the bedrock wall to the left. of Selmer-olsen(1954) to be relatedto the standarddeviation expressed in phi units (S): S : 2.465o Co.12mo.s I . co.5m LITHOST RAT I GRA PHY XX Chrono- Beds Inlormal stratrgraphy 0T Bed /\ names rock X Boreal XX Peat x/ Sand upper Agotnes )- XL Preboreal Gravel ( 10cm) sano 6 '4,i* Agotnes Greenish YoungerDrVas Xkq.,."t grey srrr srlt \ a-rzo 6 ..::. ::x25. :1.21. ;.:':::: i)3 ["*"rr,,," Agotnes Allet@d ,22 cray clay 5 I L sano I ano Agolnes with Older Drvas lgravel ano boulderl sano X g ravel lsome i. /2) . : lv_l Marlne sheils/ . .rn tenses Radiocarbon datesr sample 21t 10 230 ! 1go (T_1s74) X Sample 20: 12 22O ! 1SOfi_1oal Fig. 6. The described section. The position for all samplesdiscussed is given in numbers. J. MANGERUD CLAY SILT SAND GRAVEL - I ./ 95 l-_1-..- 1: /"1 I ,/i -t- I 2a 2a iI 20 14 IJ if ll ilT I I 109 876543210-t-2 -3 phi tAtz. t/ta .t/4 vtz l/e vz 1248mm t248163163 rooo x, micron Fig. 7. Grain-size distribution. Cumulative curves on probability-paper. For locations of samples, see Fig. 6. Sample 27: Sand and gravel in the bottom of the excavation. Sample 16: The lense of sand and gravel between Agotnes clay and Agotnes silt. Sample 10: The sedimentsequence The sequenceconsist of 5 main units (Fig. 6). At the baseis a bed of sand and gravel at least 2 m thick, followed by a ca. I m thick bed of brown gyttja clay, with a gradual upward transition to a bed of greenishgrey silt. Marine shellsare frequent in both the clay and silt beds. Above the silt is a I m thick bed of well-sortedsand (sample 10, Fig. 7) with a thin underlying bed of gravel. This sand is a littoral sediment,deposited during the Holocene uplift. The sequenceis cappedby a I m thick bed of peat of Holocene age. NORWAY LATE WEICHSELIAN SEDIMENTS IN WESTERN I 1_-- I -3 phi 7654321 Vzz t/to \/a 1/a Vz 4 8mm micron 8 16 31 63 * gyttja clay (Agotnes cla,v'').Sample 13 Upper Agotnes sand. Sample 23 & 25: Brownish & 14: Greenish grey silt (Agotnes silt)' Allthebedsarephysicallyrestrictedtothesmalldepression'andlwill thereforenotdefineanyformallithostratigraphicunits.Forthecorrelation to each bed with other sequences,the name Agotnes is, however, attached (Fig. 6) in an informal nomenclature(Hedberg 1970:16)' gravel) The sand and Sravel (lower Agotnes sand ond sedimentscon- This bed was poorly exposed,partly becausethe overlying of the excavationwas stantly slumpeddown, and partly becausethe bottom gravelwere seenin direct filled with water. In the northern part the sandand 30 J. MANGERUD contact with the beclrock, and I assume this to be the case also in the the deeper parts of the pit, though possiblywith a veneer of till between bedrock and this bed. It consistsmainly of very poorly sorted sand and gravel (sample 27, Ftg. 7). However, some boulders were also found throughout the bed, and near the beclrockwall (to the east in Fig' 6) the a bouldersdominate. All the pebblesare angular or sub-angular,suggesting short clistanceof water transport. An encrustationof calcareousalgae was observedon some Pebbles. I assumethat this beclwas depositedmore or lessdirectly from the glacier front, followecl by some rolling and sliding on the sea floor' From other field observationsin Hordaland the water depth during the deposition can be estimatedto 20 -30m. The brown gytt ja clot- (Agotnes clay) This was the most striking sediment of this sequenceas marine sediments from the cleglaciationperiod very seldomly have brownish colours. Shells occur frequently throughout the bed, giving clear evidence of its marine origin. The shellsoccurred partly in small lenses,and partly as isolatedin- dividuals,many of them in growth positions.The variable thicknessof the bed, from 30 cm up to 100 cm, and the variable strike and dip of some internal bedding planesare to be expectedin such a small sedimentaryba- sin. However, due to the slumping, it was not possibleto map the internal structures. I rds I .' '1 ro ?o lo 1o 5c ba i.) "u to 8 6 4 2 T t0 I l2 ''l ll 1t 2^ 23 22 27 Fig. 8. Parametersof grain-sizedistributions and losses on ignition for some of samplesarranged in stratigraphicalorder (Fig. 6). The same samPles rvere used in polien analysis(Fig. 12). LATE WEICHSELIAN SEDIMENTS IN WESTERN NORWAY 31 The brownish colour is most probably causedby the high content of or- ganic material. The loss on ignition (Fig. 8) is generally higher in the brown Agotnes clay than in the greenish grey silt above.

Load more