| Matanuska Formation Stratigraphic range: Late Cretaceous |
|
|---|---|
| Type | Geological formation |
| Location | |
| Region | North America |
The Matanuska Formation consists of more than 3 km (1.9 mi) of sedimentary strata exposed in the northern Chugach Mountains, Matanuska Valley, and southern Talkeetna Mountains of South-Central Alaska.[1] The Matanuska Formation contains strata from Early Cretaceous (Albian) to Late Cretaceous (Maestrichtian).[2] Parts of the formation contain abundant marine mollusks, foraminifera, and radiolaria. Fossils of non-marine plants are found in some beds. Fossils of two dinosaurs have been recovered from marine mudstones in the formation.[3][4] The lower Matanuska Formation (MF) is several hundred meters thick and includes non-marine and marine sediments. Campanian-Maastrichtian graded sandstone, conglomerate, and mudstone comprise the upper 2000 meters of the Formation.[1]
Contents |
Calcareous worm tube fossils are known from the formation.[5]
Color key
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Notes Uncertain or tentative data are in small text; |
| Annelids reported from the Matanuska Formation | ||||||
|---|---|---|---|---|---|---|
| Genus | Species | Location | Stratigraphic position | Material | Notes | |
|
Indeterminate[6] |
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Color key
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Notes Uncertain or tentative data are in small text; |
| Bivalves reported from the Matanuska Formation | ||||||
|---|---|---|---|---|---|---|
| Genus | Species | Location | Stratigraphic position | Material | Notes | Images |
|
Acila[5] |
A. (Truncacila) sp.[5] |
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I. cuvieri[7] |
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I. hobetsensis[8] |
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I. mamatensis[9] |
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I. teshioensis[10] |
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Indeterminate[5] |
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Indeterminate[5] |
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Color key
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Notes Uncertain or tentative data are in small text; |
| Cephalopods reported from the Matanuska Formation | ||||||
|---|---|---|---|---|---|---|
| Genus | Species | Location | Stratigraphic position | Material | Notes | |
|
E. japonicum[11] |
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Gaudryceras[12] |
G. denseplicatum[12] |
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Mesopusozia[13] |
M. indopacifica[13] |
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M. yezoense[14] |
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O. teshioensis[5] |
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Sciponoceras[5] |
Indeterminate[5] |
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Tetragonites[15] |
T. glabrus[15] |
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Y. puerculus[5] |
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Color key
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Notes Uncertain or tentative data are in small text; |
| Cnidarians reported from the Matanuska Formation | ||||||
|---|---|---|---|---|---|---|
| Genus | Species | Location | Stratigraphic position | Material | Notes | |
|
Platycanthus[16] |
Indeterminate[16] |
Small solitary hexacoral.[16] |
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An unidentified naticid snail is known from the formation.[5]
Color key
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Notes Uncertain or tentative data are in small text; |
| Gastropods reported from the Matanuska Formation | ||||||
|---|---|---|---|---|---|---|
| Genus | Species | Location | Stratigraphic position | Material | Notes | |
|
Biplica[17] |
Indeterminate[17] |
Opisthobranch |
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A sponge spicule fragment is known from the formation.[5]
| Scaphopods reported from the Matanuska Formation | ||||||
|---|---|---|---|---|---|---|
| Genus | Species | Location | Stratigraphic position | Material | Notes | |
|
Indeterminate[5] |
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Mako-like shark teeth are known from the formation.[5]
Other fish fossils include teeth, jaw fragments and scales.[5]
Pasch and May reported that a hadrosaurid was found in the Matanuska Formation.[18] Its discovery heralded the knowledge of a new high-latitude source of dinosaur fossils.[18] This was the first occurrence of a hadrosaur in south-central Alaska, one out of only four vertebrate fossils from the entire Wrangellia Composite Terrane, and the first associated skeleton of an individual dinosaur in Alaska.[18] The carcass appeared to have been deposited in a bathyal or outer shelf environment.[18] Scavenged bones are preserved in soft mudstone while intact bones were presevered in calcareous concretions.[18] Some calcareous concretions may be the result of pieces of meat falling off as the carcass disintegrated.[18] The hadrosaurid was a geographic link between the hadrosaurs of north america and asia.[18]
The "Talkeetna Mountains Hadrosaur" specimen was discovered in 1994 in a quarry being excavated for road material.[19] That fall, excavation began, and was resumed in the summer of 1996.[19] The specimen is housed at the University of Alaska Museum.[19] The quarry is near the Glenn Highway, approximately 150 miles northeast of Anchorage.[20] The layer consists of dark grey marine mudstone that weathers easily.[20] The mudstone contains highly indurated calcareous concretions and finely disseminated pyrite crystals.[20] Fresh wet surface display horizontal lamina, ripples and evidence of bioturbation.[20] Evidence that these strata have experienced deformation after deposition include "joint sets, faults, secondary deposition of calcite, and degree of induration."[20]
The strata's foraminiferans and mollusks all point to a middle Turonian age for the formation.[21] Muramotoceras is an unusual heteromorph previously known only from Japan.[21] It is also middle Turonian. Eubostrychoceras is known from Japan, Germany, and Madagascar.[22] E. japonicum is Turonian, and likely confined to the middle Turonian.[23] The authors reported teleost tooth and jaw fragments as well as wood fragments from the formation.[24] Other Matanuska fossils include scaphopods, foraminifera, and palynomorphs.[24] The quarry belongs to the lower region of C-1, an informal stratigraphic unit, and Member Four.[25]
The "Talkeetna Mountains Hadrosaur" was an juvenile animal about 3 m (10 feet) long.[26] The bones of the forelimbs with partial manus and ribs have been positively identified, as have most of the tail vertebrae Pelvis and dorsal vertebrae tentatively identified.[27] The specimen could not be distinguished as a hadrosaurid or lambeosaurid.[28] Aspects of its anatomy greatly differed from Edmontosaurus.[28]
The thin shelled heteromorphic ammonites probably lived at depths 36–183 m.[29] Inoceramids tended to be at upper bathyal and neritic environments.[30] The pyrite around the animal may have been the result of sulfur produced by bacteria consuming the carcass.[31] Invertebrate shells lie parallel to bedding.[31] The high quality of the invertebrate fossils and their lack of any sign of biological action on the part of encrusting or boring invertebrates suggest that there weren't many animals in the environment and only arrive at the location after dying.[32] It is possible that mudslides could have transported the shells to the location, meaning that the preserved collection of species might not represent a specific ecosystem.[31] It may have also been possible for dead ammonites to float long distances.[31] Heteromorphic ammonites probably didn't drift as far due to their potentially benthic mode of life or the septum closing off the living chamber not being calcified and decomposing after death.[33] The most abundant mollusks in the quarry are inoceramids.[34] The ecological context indicates that the "Talkeetna Mountains Hadrosaur" was buried at a depth greater than 35 m.[34]
The skull fell off before the carcass sank.[35] The body came to rest on its left side with limbs extended to the east.[35] About 20% of the hadrosaur's bones were enveloped by calcareous concretions.[36] There were 20 concretions recovered.[35] Every element not found in a concretion bore many closely spaced ovualar conical depressions ranging in diameter from 2.12 to 5.81 mm and 1.64 to 3.62 deep.[36] The depressions are probably bite marks.[37] The depressions are not symmetrical enough for gastropod drill marks and are not shaped like sponge borings.[37] None of the preserved fish fossils of the formation fit the size or geometry of the borings.[37] The size and spacing and shape by contrast resembles closely the teeth of the mosasaur species Tylosaurus proriger.[38] If the damage to the body had been done prior to being washed out to sea, it likely would have punctured the body, preventing the build up of bloating gases that allowed the carcass to drift out to sea in the first place.[38] The distribution of bite marks corresponds inversely to the presence of flesh in the animal.[38] For instance, lower limb bones sustained the most damage because there was the least amount of flesh shielding the bones at those locations.[38] The concretions formed as the flesh chemically reacted to the seafloor on the largest parts of the animal where the scavenging mosasaur would be unable to fully wrap its jaws around the carcass.[38] Bones pulled free from the carcass were buried in the mud and preserved in mudstone.[38] The organisms occurring with the Talkeetna Mountains hadrosaur fit the description of the second of three types of benthic communities identified by Hogler as forming around large reptile carcasses that would eventually fossilize.[39]
Every element of the Talkeetna Mountains Hadrosaur specimen that was not found in a concretion bore many closely spaced ovualar conical depressions ranging in diameter from 2.12 to 5.81 mm and 1.64 to 3.62 deep.[36] These depressions are probably bite marks.[37] The depressions are not symmetrical enough for gastropod drill marks and are not shaped like sponge borings.[37] None of the preserved fish fossils of the formation fit the size or geometry of the borings.[37] The size and spacing and shape by contrast resembles closeley the teeth of the mosasaur species Tylosaurus proriger.[38]
Fragments of petrified wood are known from the formation.[5]