The stratigraphic and fossil record of Böttstein was previously presented by Wohlwend and colleagues in 2019 as part of a broader study of the Passwang Formation in the eastern Folded and Tabular Jura mountains. Earlier descriptions of the strata and ammonites of the area, mostly under the locality name Beznau, were published by Moesch in 1867 and 1874, Mühlberg in 1900, Lieb in 1960, Schneider and Matousek in 1990 and Christ in 1995. The new account builds on this long history but adds a level of detail that earlier workers could not achieve, thanks to favourable conditions for excavation that have enabled further sections to be recorded since 2017.

The majority of the ammonites used in the study were excavated by a group of collectors from Zurich, R. Chiarini, T. Chiarini and B. Huber, in 1981. The section was excavated by hand with the aid of a diesel-powered jackhammer, with the layers outcropping on the steep quarry wall removed in terraces and the fossils collected bed-by-bed. H. Rieber of Zurich University was responsible for the scientific management and record of that section, which is now designated section I and was recorded approximately in the centre of the clay pit. Section II lies about 60 metres further east and section III a further 40 metres east again, while the section published by Wohlwend and colleagues sits halfway between sections I and II. A fourth profile, referred to as section West, was recorded at the western end of the pit for comparison, but its steepness made bed-by-bed collection impossible.

The ammonite specimens, several hundred in total, were prepared mechanically using pneumatic chisels, abrasive iron powder and potassium hydroxide, and the entire collection is now housed in the Natural History Museum in Bern. Preparation of this kind is essential for iron-oolitic limestones and marls, where delicate ammonite shells are often encrusted with limonite and embedded in dense ooid-rich matrices. The resulting assemblages allow the researchers to identify morphospecies of the family Graphoceratidae and other ammonite groups with a precision that transforms the correlation potential of the Middle Jurassic strata of northern Switzerland.

Lithostratigraphically, the base of the investigated outcrop belongs to the upper part of the Opalinus Clay, which in Switzerland is termed without the suffix Formation in accordance with the guidelines of the Swiss Stratigraphic Commission, in contrast to Germany where the equivalent sequence is referred to as the Opalinuston Formation. Above it, the parts of the Passwang Formation investigated correspond to the formal Sissach Member and incorporate the combined Waldenburg to Brüggli members. Because the discontinuities observed in the central Swiss Jura can hardly be traced in the eastern Aargau Jura due to significant condensation, the authors adopt the preliminary lithostratigraphic classification of Wohlwend and colleagues and informally subdivide the interval into two new units, the Schmidberg and Frickberg members, whose formal definition will follow in further work that will redefine the entire Middle Jurassic Dogger Group of northern Switzerland.

The new Schmidberg Member can be well defined at Böttstein as an approximately 5-metre-thick, predominantly uniform iron oolitic limestone and marl sequence overlying the Opalinus Clay. It comprises the two previously distinguished sequences of Unit 1 and the condensed Unit 1-3k, and its iron-oolitic layers can be traced west-southwest beyond the Fricktal region. It is overlain by a clearly distinguishable clayey and calcareous sequence, the newly proposed Frickberg Member, which represents the combined section of the condensed time equivalents of the Waldenburg Member and the lower part of the Brüggli Member in the eastern Aargau Jura. Following the Frickberg Member from Böttstein via a possible type locality near Ittenthal at the Frickberg as far as the Fricktal, or along the southern part of the Jura via Riniken, Bözberg and Staffelegg, the basal clayey sequence increases in thickness, from more than 1.5 metres to about 16 metres at Frickberg, a distance of roughly 5.4 kilometres between the Cheisacher and Frickberg sections.

The bed-by-bed record in section I, with a total thickness of approximately 5 metres, reveals a strikingly condensed succession. Bed 2 is an echinodermal biosparite dominated by crinoid skeletal elements, with bivalves including Oxytoma inaequivalvis, while beds 3a and 3b are rust-brown marls with iron ooids alongside chamosite ooids. Bed 5 is a monotonous clay marl with limestone concretions at its base, yielding only two belemnite rostra of the genus Brevibelus. Bed 6b, a weathered light brown biodetritus and iron ooid-bearing limestone, is exceptionally rich, containing numerous ammonites together with bivalves such as Idonearca, Plagiostoma, Chlamys textoria, Coelastarte and Gryphaea sublobata, with fist-sized intraclasts encrusted with limonite. Of approximately 80 Ludwigia, Brasilia and Ludwigella specimens recovered from that bed, the more common and typical morphospecies include Staufenia staufensis, Brasilia platychora, B. crinialis, B. effricata, B. bradfordensis, B. aff. baylii, B. floccosa, Ludwigia gradata, L. reflua, Ludwigella rudis, Accardia noszkyi and Abbasites aff. abbas.

The ammonite faunas are dominated by the family Graphoceratidae, which the authors, concurring with Chandler and colleagues and with Dietze and Schweigert, interpret as two main phyletic lineages. The first runs from Leioceras through Ludwigia, Brasilia and Graphoceras to Hyperlioceras, while the second is a side branch dating back to around the Lower and Upper Aalenian boundary that ends with the oxyconic Staufenia staufensis. The artificial distinction between these morphological genera should not obscure the fact that they are linked by transitional forms and even occur in parallel for some time. At Böttstein, Leioceras goetzendorfense and the extremely coarse-ribbed, unusually large Leioceras uncinatum occur in bed 3b of the Bifidatum Subzone, while bed 4 yields Ludwigia obtusiformis and Staufenia sehndensis, allowing the Obtusiformis Subzone of the Murchisonae Zone to be proposed for the first time for the Swiss Jura, based on highly comparable Staufenia ammonites from southern England and the W Swabian Alb in southwestern Germany.

Biostratigraphically, the succession is finely resolved. Beds 2 to 3b are placed in the Bifidatum Subzone of the Opalinum Zone, bed 5 with Brasilia baylii and Ludwigia aff. murchisonae in the Bradfordensis Subzone, and bed 6b is interpreted as containing a condensed ammonite fauna from both the Bradfordensis and Gigantea subzones of the Bradfordensis Zone, an observation consistent with the palynological data of Wohlwend and colleagues. Beds 6cu, 6cm and 6co belong to the Gigantea Subzone, where the genus Staufenia has disappeared and the assemblage is dominated by ammonites of the species grouped around Brasilia platychora and B. effricata, alongside small, slender Brasilia such as B. tutcheri and B. crinialis. Beds 7 to 9, characterised by the abundance of Euaptetoceras amplectens, Eudmetoceras ex gr. eudmetum-masticonnensis and graphoceratids, are assigned to the amplectens Biohorizon of the Formosum Subzone of the Concavum Zone, still within the latest Aalenian. Bed 10 belongs to the older Discites Zone, where Graphoceras co-occurs with Hyperlioceras, while beds 11 and 12 belong to the younger Discites Zone due to the abundance of Euhoploceras, the virtual absence of Graphoceras and the occurrence of characteristic Hyperlioceras species such as H. cf. subsectum. Ex-situ finds of Sonninia cf. nenningensis, Witchellia jugifera and Emileites from beds 17 to 20 probably originate from the Trigonalis Subzone of the Laeviuscula Zone in the Lower Bajocian.

The correlation of the sections shows that the strongly iron-rich basal section of the Schmidberg clay pit is almost identical to the corresponding section in the Riniken borehole, confirming the regional occurrence of this specific facies in the eastern Aargau Jura. At the same time, section West demonstrates how quickly the iron-oolitic facies can change into a sandy and calcareous-dominated facies, which corresponds more to the typical lithology found at the type locality of the Sissach Member in the Grütsch section near Thürnen southeast of Sissach. A sequence dominated by sandy limestones can also be observed on the eastern side of the Aare River at Acheberg and then along the Surb River from Döttingen to Tegerfelden. The amplectens Biohorizon, first used for strata in southern Alaska, offers an almost worldwide correlation potential for the Aalenian and Bajocian boundary, and the authors note that further investigations are needed to clarify whether beds containing Euaptetoceras at Ringsheim, Lörrach-Stetten, Geisingen and Metzingen should be assigned to the Aalenian or already to the Bajocian. The planned description of further outcrops in the Middle Jurassic of northern Switzerland will further improve understanding of the development of the complex stratigraphic sequences there, in which the ammonite fauna of the Aalenian and lowermost Bajocian is dominated by poorly ornamented platycones, frequently of large size, with ribbing usually restricted to early whorls or shown by microconchs.

Subject of Research: Ammonite biostratigraphy of the Aalenian and Lower Bajocian (Middle Jurassic) near Böttstein (Canton Aargau, Northern Switzerland)

Article Title: Ammonite biostratigraphy of the Aalenian and Lower Bajocian (Middle Jurassic) near Böttstein (Canton Aargau, Northern Switzerland)

Article References: Dietze, V., Hostettler, B., Wohlwend, S., & Menkveld-Gfeller, U. (2026). Ammonite biostratigraphy of the Aalenian and Lower Bajocian (Middle Jurassic) near Böttstein (Canton Aargau, Northern Switzerland). Zitteliana, 100, 129-162. https://doi.org/10.3897/zitteliana.100.199140

Image Credits: AI Generated

DOI: 10.3897/zitteliana.100.199140

Keywords: Ammonite, biostratigraphy, Aalenian, Lower, Bajocian, Middle, Jurassic, near, ttstein, Canton, Aargau, Northern