Stratigraphic range: Late Tremadocian-Floian
|Thickness||>1,000 m (3,300 ft), combined for upper and lower formations, the two exceptionally preserved intervals ~25 m (82 ft) and 15 m (49 ft) respectively|
The Upper and Lower Fezouata Formations of Morocco are Burgess shale-type deposits dating to the Early Ordovician, filling an important preservational window between the common Cambrian Lagerstätten and the Late Ordovician Soom Shale. Found fossilized fauna were numerous organisms previously thought to have died out after the mid-Cambrian.
Over 1500 non-mineralized specimens, representing 50 distinct taxa that have a composition similar to earlier Burgess Shale type biotas, have been recovered from the formations in addition to a less abundant shelly fauna. The make-up of the community varies significantly through the stratigraphic sequence, with both abundances and faunal composition changing as time progresses. Small (1–3 mm wide) burrows are present in the sediment, but major burrowing is absent; this may suggest a paucity of oxygen in the water or sediment. Particularly notable is the presence of bryozoa and graptolites, forms that are absent in the Cambrian period. Diverse echinoderms indicate a normal range of salinity, and the overall shelly assemblage is not significantly different from the normal shelly fauna expected in open Ordovician waters. The non-mineralized cohort contains a range of forms familiar from the Burgess Shale: Demosponges, lobopods, barnacles, annelids, anomalocaridids, possible halkieriids, marrellomorphs, paleoscolecid worms, naraoiids, skaniids as well as the expected problematica. Other Ordovician oddballs are also present, including mitrates, machaeridia, cheloniellids and horseshoe crabs in abundance.
The fossiliferous strata were deposited just above storm wave base (offshore to lower shoreface transition), at between 50 and 150 metres (160 and 490 ft) water depth. Organisms were likely buried in situ. Because of its placement above storm wave base, storms would have mobilized sediment that could be quickly deposited, trapping animals and leading to their preservation. Consequently, the assemblage is dominated by benthic organisms.
Fossils of the Fezouata Formation, which are usually squashed flat (although some do retain some degree of their original three-dimensionality) are often coated with a dusting of pyrite, and tin; this aspect of the fossil preservation is very similar to that at Chengjiang. Non-mineralized appendages are often preserved.. While the formation as a whole is over 1,000 metres (3,300 ft) thick, only two intervals, 25 metres (82 ft) and 15 metres (49 ft) thick, provide exceptional preservation. Both of these intervals are located near the top of the lower formation, corresponding to the Araneograptus murrayi and Hunnegraptus copiosus graptolite zones respectively.
The fossils occur within an area of 500 km2, in southeast Morocco's Draa Valley, north of Zagora. Stratigraphically productive layers are found through a 1.1 kilometres (0.68 mi)-thick column of rock that spans the Tremadocian and Floian epochs.
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