Tambach Formation

From 1887 to 2010, the Ducal (Herzogliches) Museum in Gotha housed most of the Tambach Formation's most famous fossils.

Starting in the 1840s, five-toed footprints had been described from various sandstone quarries and roadcuts in the Gotha district of Germany. The stratigraphy of rocks near Tambach-Dietharz was mapped out in 1876,[5] and named as the "Tambacher Schichten" (Tambach strata) in 1895, although at the time it was believed to include several additional rock layers (now termed the Elgersberg and Eisenach Formations) which have since then been separated from it.[2] in 1887 the first fossilized footprints from Bromacker were discovered by a local fossil collector named Heinrich Friedrich Schäfer. The find was donated to the Ducal Museum in the city of Gotha, after which it was independently reported on by various German paleontologists who had received photographic evidence.[6] Wilhelm Pabst, the curator of the Ducal Museum's natural history department, collected and described 140 sandstone slabs from the Tambach Formation from 1890 until his death in 1908. The collection was rediscovered in the 1950s, and subsequently restudied by GDR paleontologists such as Hermann Schmidt, Arno Hermann Müller,[7] and Hartmut Haubold.[8]

Fossilized tetrapod bones were discovered in the upper beds of Bromacker by Thomas Martens in 1974, prompting further attention from Gotha paleontologists. These include Harald Lutzner, who formally delineated the Tambach Formation as a sequence including two conglomerate layers separated by a sandstone layer.[2] In the coming years, research contacts were made with Western paleontologists such as Jürgen Boy (University of Mainz) and David Berman (Carnegie Museum of Natural History). This allowed excavation to ramp up and the Tambach Formation to achieve worldwide fame. A 1993 German-American joint expedition recovered articulated tetrapod fossils, and trace fossil collecting was resumed after more than 80 years thanks to the excavation of a new sandstone quarry at Bromacker in 1995. The first Tambach body fossils outside of Bromacker were discovered in 2008, in a construction site in downtown Tambach-Dietharz. In 2010, the exhibits comprising Gotha's Museum of Nature began the process of being moved from the Ducal Museum (which was being converted to an art museum) to the Friedenstein Castle. Due to funding issues, collecting from Bromacker has been limited and the Tambach collection is being archived at the historic Perthesforum complex prior to the construction of a new Permian exhibit.[8]

Map of the modern Thuringian Forest Basin (in German). TB = "Tambacher Becken" (Tambach Basin)

The Tambach Formation mainly lies within a basin currently occupied by the Thuringian Forest, and it is stratigraphically younger than the Rotterode Formation and older than the Eisenach Formation. It is the oldest part of the Thuringian Forest Basin's Upper Rotliegend succession, a name referring to a sequence of purely sedimentary rocks in the Lower Permian of Germany. The sediments of the Tambach Formation were deposited in a small Permian graben (termed the Tambach Basin), which was oriented in a northeast to southwest direction and incised into the igneous and sedimentary rocks of the Rotterode Formation. The Tambach Basin would have about 250 square km during the Permian, though modern outcrops only occupy about 50 square km, not counting the northeastern portion of the basin which has had its deposits erased by later geological processes.[1][2]

Traditionally, the Tambach Formation is considered to be divided into three discrete layers: a lower and an upper conglomerate layer (Bielstein and Finsterbergen Konglomerate, respectively), separated by a somewhat narrower layer of finer sediments such as sandstone, the Tambach-Sandstein (Tambach sandstone) member.[9][3][2] However, the borders between these layers are often imprecise, and some geologists have offered a more complex depositional picture with three stratigraphic stages defined by changes in basin-wide sedimentology, rather than specific rock types.[1]

The lowest stage (stage I) experienced a period of high tectonic activity (part of the Variscan orogeny) to the southeast, the Oberhof uplift. The formation of the basin among this tectonic backdrop initially led to powerful debris flows and sheet floods, and then active braided rivers flowing along its edge with lower-energy rivers, floodplains, and lakes at its center. The coarse (cobble/boulder-scale) and rhyolitic Bielstein conglomerate was deposited among the high-energy marginal environments while early portions of the Tambach sandstone were deposited further away from the basin's edge. The most prominent flow direction in the well-preserved eastern part of the basin is northwest, towards the center of the basin.[1][2][10]

Decreasing tectonic activity in the middle stage (stage II) leads to increased erosion, lowering the relief along the edge of the basin. As a result, the braided rivers at the edge slowed down, lowering the size of the clasts to cobble/pebble conglomerate. The center of the basin graded into small streams and marshes, depositing sandstone, siltstone, shale and mudstone in fossiliferous red beds. Although the Tambach Basin may have been hydrologically isolated during this period, with its waterways draining internally,[1] some paleontologists instead consider its waters to flow into another basin in the northeast, which was not preserved.[2] The last stage (stage III) experienced a return of tectonic activity (the Ruhla crystalline uplift) to the northwest, although the relief was still fair flat. Ruhla-sourced alluvial fans and braidplains became more common, gradually allowing mineral-rich and polymictic pebble-sized conglomerate to build up and expand into the center of the basin, forming the Finsterbergen conglomerate.[1][2][10]

Bromacker

A sandstone slab from the lower beds of Bromacker, showing cracked mudstone drapings.

The most famous and fossiliferous locality within the Tambach Formation is the Bromacker locality, a cluster of small abandoned quarries near the town of Tambach-Dietharz. Strata exposed to the surface at Bromacker corresponds to the center of the Tambach Basin, during the time of the upper portion of stage I and the lower portion of stage II. Stage I sediments at Bromacker are termed the "lower beds"[1] or "Bromacker sandstone"[3] and stage II sediments are the "upper beds"[1] or "Bromacker horizon".[3]

The lower beds are dominated by thick sheets of fine-grained sandstone, often with cross-bedding indicating that the paleocurrent was oriented towards the northeast. These sandstone layers are typically blanketed by homogeneous mudstone, which sometimes preserved mudcracks, plant fossils, invertebrate burrows, and tetrapod footprints. The thick sandstone layers (and their mudstone drapings) are not continuous, interrupted by a succession of finer and darker micaceous sediments such as shale, siltstone, and (rarely) very fine sandstone. This reconstructs a sequence of repeated flooding events, involving strong, straight rivers eroding channels through the silty floodplain at the center of the basin, leaving behind sandy channel fills (sandstone) and fine-grained overbank deposits (shale and other sediments). The mudstone drapings can be explained as precipitate from extensive ephemeral lakes that evaporated in the weeks following the floods, after which they became mudflats. Many of the mudcrack fragments were ripped up by the next flood, being incorporated into the subsequent sandstone sheets as intraclasts.[1]

A complete and fully articulated skeleton of Orobates pabsti from the upper beds of Bromacker.

The narrower upper beds also represent alternating flooding and still water conditions, albeit with finer clasts and lower topography. The sandstone channel fills are mostly replaced by homogeneous layers of red siltstone, which were prone to breaking along sharp edges. This siltstone contained mudcrack fragments, calcite-encased roots, and well-preserved partial or articulated skeletons of terrestrial tetrapods. These sediments indicate that reduced relief in stage II of the Tambach Formation had made sheetfloods the dominant erosional force, rather than individual channels. The lowest sheetflood deposit was particularly rich in diadectid fossils. The fine sediment successions were also altered, becoming dominated by finely laminated (and only occasionally micaceous) shale in which conchostracans and arthropod remains were fossilized. These indicate a transition to more permanent lakes and broad floodplain conditions at the center of the Tambach Basin, rather than the ephemeral fluvial environment of the lower beds.[1]

An extensive older sequence, the "Tambach-Wechsellagerung" (Tambach interbedding) was discovered in 2004 through borehole data. This sequence somewhat resembled the lower beds, with alternating fine micaceous deposits and thick sandstone sheets filled with a breccia of mudstone intraclasts. However, the sandstone layers had no evidence of cross-bedding, and the mud drapings responsible for most of the Tambach trace fossils were absent as well. Rare fragments of vertebrate fossils were present, along with calcite structures.[3]

Los Llanos in present-day South America, which may have had a similar climate to the Tambach Formation.

The sand and silt-rich portions of the Tambach Formation were likely deposited in a warm climate with both hot, dry parts of the year and periodic heavy rainfall events. The dry times were severe enough to evaporate the Tambach basin's flood-induced ephemeral lakes within a matter of days, restricting the ability of a permanent aquatic fauna to colonize the basin. However, most plant root fossils are horizontally (rather than vertically) oriented, indicating that the climate was generally humid enough that native plants would not need to evolve deep roots or other xerophytic adaptations. Because of this, the Tambach Formation would probably fall under the modern tropical savanna climate, despite its lack of grass. Modern climatic equivalents include the northern African savanna and the Llanos of Venezuela and Colombia.[1] There is evidence that sub-zero temperatures may have occurred on some nights during the dry season, likely as a result of the basin's high elevation.[16][2] The climate may have been drier during the conglomerate-rich periods of the Tambach Formation.[1][2]

Artist's depiction of the Tambach paleoenvironment

The ecosystem of the Tambach Formation is unusual for its lack of aquatic animals such as xenacanthid sharks, Eryops, or Diplocaulus, which are otherwise common in Early Permian red beds. This is best explained by its mountainous environment, isolated from the monsoonal lowland floodplains which deposited most of the red beds. In addition, the ephemeral nature of the Tambach Basin's lakes and rivers means that only aquatic animals adapted to such conditions, such as conchostracans, were able to flourish. The Tambach Basin did support a diverse amphibian fauna, but only terrestrially-adapted types including dissorophoids and seymouriamorphs. Large herbivorous tetrapods such as caseids and especially diadectids are the most common body fossils recovered from the formation, while carnivorous synapsids are relatively rare. This is in contrast to North American environments, where fossils of carnivores such as Dimetrodon outnumber herbivore fossils. The environmental conditions of Tambach likely created a food web which was very different from that of the lowlands. The most common plants were tough, drought-adapted types such as conifers, while seed ferns and other lowland plants were much rarer. Fibrous terrestrial plants encouraged colonization of the basin by herbivorous land animals, but the dry climate prevents the development of an aquatic food chain, inhibiting animals such as large species of Dimetrodon, which get a large portion of their food from waterways.[1][4]