Oandu Stage

ID: 32

General info

Unit nameOandu lade
Unit nameOandu Stage
Belongs toVinni Subseries
Overlain byRakvere Stage
OverliesKeila Stage
Statusformal standard
EtymonOandu, river (E)
Age top (Ma)453.20
Age base (Ma)454.20
Age referenceCooper & Sadler, 2004


TypeLocalityDepth (m)ReferenceRemarks
holostratotypeMännil, Ralf, 1958eOandu River bank
hypostratotypePiilse 7909 borehole9.80..12.40Põlma et al., 1988

Chemical composition and other properties

Al2O3 [%]0.310004.8365012.240003.7216220
Ba [ppm]0.0170011.5961481.0000028.334007
Calcite [%, calculated]1.1200052.3709092.9600024.58124110
CaO [%]1.9000032.6664155.4400013.77969145
carbonates [%]2.600005.528578.800001.928517
Cl [%]0.000000.036850.230000.0548714
CO2 Amp297.00000641.14285875.00000184.632017
CO2 [%]2.6000029.5703342.7500010.52916120
Cr [ppm]0.002005.8360025.000009.315288
delta13C [‰]-0.470001.028111.690000.3959069
delta13Corg [‰]-30.30000-29.95714-29.600000.244117
delta15N [‰]-0.500001.025002.400000.816628
delta18O [‰]-7.22000-4.49753-3.020000.7937369
Dolomite [%, calculated]2.3000011.8610931.990006.90786110
Dry density [kg/m3]0.000002387.666662866.00000559.8203221
Fe2O3 [%]0.340001.806006.610001.6230015
Fe2O3T [%]2.410003.251664.380000.647396
Fe2O3total [%]0.410002.321489.380002.2304827
FeO [%]0.020000.353802.040000.4185721
Grain density [kg/m3]0.000002582.238092891.00000581.7842721
illiit [%]88.0000088.0000088.000000.000001
Insoluble residue [%, calculated]4.8600029.7274272.1100018.3094366
Insoluble residue [%, calculated]1.1500044.8393192.8900027.6721944
Insoluble residue [%]1.6200027.8716783.0000018.76809149
K2O [%]0.120002.178995.120001.5353520
kloriit [%]12.0000012.0000012.000000.000001
LOI [%]9.9000024.4575040.900009.099738
LOI_450C [%]4.000007.1666610.000001.839086
MgO [%]0.290002.8128915.710002.03703145
MnO [%]0.000000.092250.500000.1032920
N [%]0.080000.150000.250000.046908
N2 Amp285.00000676.500002007.00000511.803188
Na2O [%]0.010000.094000.210000.0516120
P-wave velocity [m/sec in dry sample]2805.000004139.538465022.00000636.1339513
P-wave velocity [m/sec in wet sample]3168.040004396.772505463.18000682.4596512
P2O5 [%]0.000000.034630.120000.0267720
Porosity [%]0.900007.5334926.000005.9841520
S [%]0.030000.226491.030000.2358814
SiO2 [%]1.3600021.5955059.1000016.6413620
Size fraction 0.01-0.002 mm [%]1.5600020.5625957.200009.40601100
Size fraction 0.05-0.01 mm [%]2.3000018.3306957.2500011.42198101
Size fraction 0.1-0.05 mm [%]0.1000014.5237070.6700017.4806289
Size fraction 0.25-0.1 mm [%]0.100000.100000.100000.000001
Size fraction 0.5-0.25 mm [%]1
Size fraction 1-0.5 mm [%]1
Size fraction <0.002 mm [%]3.1700048.1652983.0000019.26225100
Size fraction <0.01 mm [%]83.1000083.0999983.100000.000001
Size fraction >1 mm [%]1
SUM [%]94.6350799.0050199.900001.817077
TiO2 [%]0.010000.284150.660000.2010720
V [ppm]0.002001.403508.000002.703738
W [ppm]18.0000018.0000018.000000.000001
weight [mg]1.760008.3943940.753009.4571415
Wet density [kg/m3]0.000002459.285712875.00000562.2653221
Zn [ppm]16.0000016.0000016.000000.000001

Specimens in collections (approximately): 6637

Samples in collections (approximately): 1373

Thickness map (partial data only)

Localitybase, mtop, mthicknessreferencepreferred
Velise V-97 borehole 176.40 157.80 18.60 Suuroja, Kalle 1991 1
Mehikoorma 421 borehole 298.50 284.80 13.70 Põldvere, Anne 1
Otepää 2 borehole 445.10 434.30 10.80 Sarv 1
Tartu 453 borehole 296.60 288.40 8.20 Põldvere, Anne et al., 1998 1
Palamuse borehole 207.20 200.60 6.60 Kõrvel 1963 1
Ristiküla 174 borehole 407.00 400.75 6.25 0
Valga 10 borehole 389.20 383.00 6.20 Põldvere, Anne (koost), 2001 1
Alatskivi 422 borehole 195.80 190.10 5.70 Kajak 1974 1
Tareste F-352 borehole 51.20 46.40 4.80 Suuroja, Kalle 1991 1
Apraksin Bor 17 borehole, Leningrad Oblast 94.80 90.00 4.80 Põlma 1984 1
Kaagvere 1 borehole 264.50 259.80 4.70 Männil, Ralf, 1966a 1
Karula 320 borehole 400.60 396.20 4.40 0
Eikla 508 borehole 300.60 296.40 4.20 Kala et al., 1967 1
Orjaku borehole 133.50 129.30 4.20 Meidla, 1996a 1
Laeva 13 borehole 219.40 215.40 4.00 1
Soovälja K-1 borehole 137.30 133.60 3.70 Põldvere, Anne (koost), 2002 2002 1
Muhu 590 borehole 237.40 234.00 3.40 Perens, H 0
Orjaku borehole 132.50 129.30 3.20 Meidla, 1996a Meidla 1
Laeva 18 borehole 237.00 234.20 2.80 Nestor, H., päevik 23 0
Männamaa F-367 borehole 141.00 138.20 2.80 Põldvere, Anne (koost), 2008a 1
Sõru 400 borehole 171.20 168.70 2.50 Kala, E., Kajak, K. & Kajak, H. (kompleksne geoloogiline kaardistamine 1:200 000) 1971 1
Kaagvere 1 borehole 259.80 257.40 2.40 Meidla, 1996a 1
Kassari F-371 borehole 126.00 123.60 2.40 Suuroja, Kalle 1988 1
Virtsu 360 borehole 224.90 222.60 2.30 Meidla, 1996a 1
Kõrgessaare borehole 46.90 44.95 1.95 0
Abja 92 borehole 398.70 396.90 1.80 Kajak 1969 1
Rapla borehole 126.90 125.50 1.40 Põlma, 1972b Põlma 1972 1
Pärnu 6 borehole 326.20 325.00 1.20 1
Viki borehole 328.50 327.50 1.00 Põldvere, Anne (koost), 2010 1
Äiamaa borehole 175.15 174.20 0.95 Sarv 0
Kingissepa GI borehole 367.60 366.80 0.80 Põlma 0
Võhma borehole, Viljandi County 222.45 221.75 0.70 Põlma 0
Kerguta 565 borehole 122.40 121.80 0.60 Põldvere, Anne (koost), 2006 Põldvere, Anne Estonian Geological Sections Bulletin 7 2006 1
Põltsamaa H-163 borehole 209.70 209.10 0.60 Põlma 1984 1
Taga-Roostoja 25 borehole 37.80 37.20 0.60 Põldvere 1997 0


Oandu Stage

L. Hints & T. Meidla

Original text from: Raukas, A., Teedumäe, A. (eds). 1997. Geology and Mineral Resources of Estonia. Estonian Academy Publishers, Tallinn. 436 pp. ISBN 9985-50-185-3. Available online at: sarv.gi.ee/geology.

In northern Estonia, the Oandu Stage comprises rocks of two different lithofacies forming the Vasalemma and Hirmuse formations. The Vasalemma Formation, distributed in northwestern Estonia, consists of bedded fine- to coarse-grained bioclastic limestones with irregular bodies of aphanitic massive limestones (carbonate buildups). These rocks were identified first as the Hemicosmites Limestone (Eichwald 1854a) or Wasalemm’sche Schicht (Schmidt 1881). Vasalemma, as the name of a chronostratigraphic unit was applied also to the rocks of another lithofacies – the argillaceous limestones and marls, named Oandu beds by Öpik (1933) and the Hirmuse Formation by Männil and Rõõmusoks (1984), which are exposed on the banks of the Oandu River in northeastern Estonia (Rõõmusoks 1953, Aaloe et al. 1958). Later studies (Männil 1958c, 1960) showed that the lower and middle parts of the Vasalemma Formation are of Keila Age (Fig. 48) and the name Oandu was proposed for chro-nostratigraphic unit of post-Keila Age. The Oandu Age of the uppermost Vasalemma Formation is presumed by the appearance of the corals Lyopora tulaensis (Sokolov), Eofletcheria orvikui Sokolov and the brachiopods Rhynchotrema? parva Oraspõld, Rostricellula nobilis (Oraspõld), Dactylogonia luhai (Sokolskaya) (Männil 1960, Rõõmusoks 1970), or it is supposed by the disappearance of Leiosphaeridia and brachiopods of the Keila Stage (Rummu core, Põlma et al. 1988). In northern Estonia, the Oandu Stage is restricted in thickness (1-4 m, Fig. 49); only in the limits of the Vasalemma Formation it is up to 6 m thick.

In the stratotype area in northeastern Estonia, the lower boundary of the Oandu Stage and the Hirmuse Formation, is known only by the core sections where it is marked by a sharp discontinuity surface with up-to-35-cm-deep pockets, on the upper boundary of the Kahula Formation (= Kahula Group, Männil & Meidla 1994). Below this level, a great number of Middle Ordovician species and even genera common with several older stages, including the brachiopods Cyrtonotella, Estlandia, trilobites Asaphus (Neoasaphus) nieszkowskii Schmidt, Pseudobasilicus, ostracodes Tetrada (Tetrada) harpa (Krause), Polyceratella spinosa Sarv (Fig. 50) disappear. Notable is the disappearance or sharp decrease in the frequency of the acritarch Leiosphaeridia which is abundant in the Keila Stage (Fig. 51). This fossil seemingly can be used for the preliminary establishing of the above-mentioned boundary in core sections, especially when the uppermost part of the Kahula Formation is more argillaceous and possibly belongs to the Lehtmetsa Member of late Keila Age (Fig. 47, Põlma et al. 1988, fig. 32). A new complex of fossils with the ostracodes Bolbina rakverensis (Sarv), Klimphores minimus (Sarv), Disulcina perita perita (Sarv), brachiopods Howellites wesenbergensis (Alichova), Equirostrata wesenbergensis (Teichert) appears near the lower boundary of the Oandu Stage (Fig. 50, see also Põlma et al. 1988). In some cases, these species are found even below the boundary discontinuity surface, seemingly they occur in the deep pockets filled with deposits of Oandu Age. Due to the essential changes in the faunal composition (Männil et al. 1966, Hints et al. 1989), several authors have suggested to use the lower boundary of the Oandu Stage as the regional subseries or series boundary (Jaanusson 1945, Rõõmusoks 1956).

The Hirmuse Formation thins out within a rather short distance in the southern direction, and in many places in central Estonia the Oandu Stage is represented only by the Tõrremägi Member of the Rägavere Formation with a thickness less than one metre (Fig. 49). This area separates the rich and diverse fauna of bryozoans, brachiopods, echinoderms and trilobites of the Hirmuse Formation in northern Estonia (Põlma et al. 1988) from the relatively rich brachiopod and trilobite fauna in the marls and argillaceous limestone in southern Estonia, corresponding presumably to the Lukštai Formation. Beside some species common for northern and southern Estonia (Howellites wesenbergensis Alichova, Rhactorthis kaagverensis Hints a.o.), several brachiopods (Reushella magna Hints, Laticrura sp. Skenidioides sp., Leptellina? sp.) have been identified only in the latter region and are also known in the Lukštai Formation in Lithuania or in the Moldå Limestone in Sweden (Jaanusson 1982, fig. 7).

Identification of the Oandu Stage is most complicated in southeastern Estonia where the black shales and the overlying marls of the Mossen Formation are distributed (Karula core in Fig. 51). The shales, encountered in various sections have been included to the Keila (Meidla 1996), Oandu (Hints 1975) or Rakvere (Männil 1966) Stage. The overlying marls of the Priekule Member are correlated with the uppermost Oandu and/or the Rakvere Stage. In some sections, the marls below the black shale (about 4 m in the Karula core, Fig. 51) comprise brachiopods known in the other sections (Otepää, Laeva) mainly in the beds presumably of Oandu Age. In favour of this age testifies also the disappearance of Leiosphaeridia at a depth of about 3 m below the shales. At the same time, the ostracode record allows to suppose Keila Age of the lowermost part of the Mossen Formation (Meidla 1996). The contradiction in interpreting the age by macrofossils and ostracodes may be caused by insufficient data available, or it may indicate the patchy distribution of deposits in the Keila - Oandu boundary interval in southeastern Estonia.

Still, in most of southern Estonia, the Oandu Stage can be identified most realiably on the basis of ostracodes. The lower boundary of the Oandu Stage is marked by the appearance of Sigmoopsis granulata Sarv, Bolbina rakverensis Sarv, Pelecybolbina illativis Neckaja and Klimphores minimus (Sarv), and a general rapid faunal change which occurred throughout the Estonian part of the palaeobasin (Meidla 1996).