Trace fossils from proglacial lake sediments

Trace fossils from proglacial lake sediments P t l l l 11' 1.. C;IDBARD A N D ANTHONY J . STUART

In the coursc of stratigraphical invcstigations i n the Pleistocene of the Vale of St. Albans, Hcrtfordshirc, England, trace fossils were discovered in laminated silty clays at Moor Mill pit (G.R. 1'Q 143025) 5 km ( 3 miles) south of St. Albans (Fig. I ).

Stratigraphy The succession at thc fossil locality is as follows: The pit surface is at 77.4 m above O.D. Orange flint gravel and sand . . . . . . 5.2 m H row n decalcified , part Iy w a t c r- Iai n , till (Chalky Boulder Clay) . . . . . . . . 0.8 m Brown-grey laminated calcareous silt y clays showing small-scalc faulting and the top 1.5 m contorted; trace fossils occur between 0.30-0.46 m abovc base . . . . . . . . . . . . . . . . . . . . 2.38 in Yellow coarse flint gravel and sand 6.5 m Chalk Similar sections in thc samc pit were descrihcd by Evans (1953). These sediments can be assigned to the Anglian glaciation ('! Elsterian) because corre-

lative glacial scdiments are overlain by pollenifcrous Hoxnian (?Holsteinian) deposits at Hatfield (Sparks et al. 1960) and Bell Lane, London Colney (Gibbard, i n prcparation) i n the immediatc vicinity. Bcd 2 shows distinct varve-like coarse-fine couplets up to 2 mm in thickness, probably of seasonal origin. They are cspecially well-developed between 0.27-0.68 m above the base of the bcd. Both their sedimentary structure and intimatc association with glacial deposits indicatc that thc silty clays were laid down in a proglacial lake. T h e laminated silty clays are known to cover an area of approximately 4 km by 1.2 km.

Trace fossils Trace fossils werc collected by splitting sediment which breaks through the coarser, probably summer horizon. T h c non-bioturbated sediments are devoid of pollen, plant detritus and diatoms and no body fossils were found. Six types of trace fossil can be distinguished. Two of these can be assigned with some confidence to particular groups of invertebrates.

S T ALBANS

\ N

HATFIELD

ST

2 miles ' 2 k m

'

I LON DON

i

50 miles

n 58 krn /i,q I.

Location map.

( a ) A series of V-shaped paired foot marks about 35 mm wide with a spacing of 3-4 mm (Fig. 2 A ) . Very similar, although slightly larger, trails were produced under laboratory S (Arthropoda, conditions by modern A S C / / L Isp. Crustacea, Isopoda) (Fig. 2 B). (See also Qni.ru s trails: Moore, 1962, fig. 127.5.) Asc4/us l'eeds on plant detritus and algae.

(b) A gently curved asymmetric trail 9 m m widc, comprising a series of V-shaped marks on the concave side paired with U-shaped marks on the convex side, s o that it resembles a figure 3 repeated at 8 mm intervals. This is probably the trail made during locomotion by a fairly large vagilc organism, possibly a n arthropod (Fig. 3).

LIORt'AS 3. (IY73)

Progltrcitrl lrike trticr fossils

71

72

Philip L. Gihhrrrd

titid

Aritlioiiy J . Stcrtrrf

BORFAS 3 (1974)

U O R t A S 3 (1974)

Proglncicil 1uke trrrc‘e fossils

73

(c) A scries of semi-circular marks 6 m m wide consisting of shallow paired elliptical depressions, with a slightly raised anterior area, rcpeated at 4.5 m m intervals. It was possibly produced by a crustacean (Fig. 4). (d) Sinuous linear trails 0.5 m m wide similar to those madc by Gastropoda (Mollusca) during locomotion (Fig. 5 ) . (e) Horizontal burrows, sometimes with a short side branch comprising a shallow central groove with a ridge on either side. Internally closely spaced ‘spreite’ are present suggesting that thcy were produced by a worm-like animal burrowing horizontally just below the sediment water interface (Fig. 6). (f) Small undulating trails 0.5 m m wide with a wave length o f about 3 m m (Fig. 7). Very similar trails were described by Andersson (1897) from proglacial lake deposits of Weichselian age in Sweden and attributed by him to the activjtitcs of chironomid midges (Arthropoda, Insecta, Diptera). These larvae,

Fig. 6 .

Worm-like animal hut-row.

74

BOREAS 3 (1974)

Philip L. Gihhtird cind Atitlioriy J. S t u w t

however, live in tubes penetrating the sediment and there is no indication of such structures at Moor Mill. Probably the larva o f some other insect genus is responsible.

Conclusions The proglacial lacustrine environment is usually rcgarded as being almost devoid of life. The occurrence of trace fossils in proglacial lake sediments in both the Anglian of England and the Weichselian of Sweden (Andersson L897) indicates that this environment is in fact capable of supporting a wide variety of benthonic animals. Presumably the primary producers in the ecosystem were aquatic plants, although none have been preserved. Any meaningful interpretation of the fossil data must await studies on the ecology of modern arctic lowland and temperate moun-

tain proglacial laltes. However, these may not be strictly comparable becausc of differences in latitude and altitude.

A~i~iiolllc.tlReii2~iIts. - We ai-c grateful t o P i ~ o l c ~ s oJ r. Haitshoi-n and DI-. M . Saarni\to [or hclpful discussion, and to DI-. R. G . Wcst, F.R.S. f o r rcading the munuscript. We acknowlcdge the suppoi~t o f an N . t . R . C . Rcscarch Studcntship (P.L.G.) and a Research Fcllowsli i p ( A .J . S . ).

REFERENCES Andci-sson, C ; . 1897: Dcii CenLraljiimtskn i\sjoii. Svcjriges g ~ o l r/m/ws. . Ajh. (ser. C ) 166. 1-38. F,vans, P. 1953: Ficld meeting i n the Vale of S t . Albana. I ' r w . G r ~ d Ass. . 6 5 , 18-22. Moc;rc, R. C. 1962: Trctrtise 0 1 1 Iiivertrbrcltc~ P d t w otrtolo,q)~. Mi: Miscdlrriieci. Geological Socicly o f America and t h e Univei-\ity of K a n u s Pt-e\a. Spat-ks, B. W., Wcst. R. C . , Williams, R . B. G . M Ran?om, M . E. 1969: I-loxnian interglacial ticposits neat. Hatficld, Hcrta. /'roc. (;eo/. Ass. 80. 243-267.