Mystic structural complex, undivided

Unit symbol: JDmc
Age range Jurassic to Devonian (419.2 to 152.1 Ma)
Lithology: Sedimentary
Group name: Mystic structural complex, undivided
“A depositionally and structurally complex terrane of chiefly marine flysoid sedimentary rocks which include (1) trench assemblages (and possibly intra-oceanic arc deposits) characterized by terrigenous turbidites, cherty pelagites, and basaltic pillow lavas (all of which underwent complex undersea sliding and later multiple thrusting and folding), (2) slope and shelf assemblages that include chert, shale, reefoid limestone, and, locally, terrestrial conglomerate and redbeds, and (3) a thick, locally terrestrial conglomerate and sandstone assemblage. These dissimilar middle and late Paleozoic depositional environments are now juxtaposed by large dislocated nappes and thrusts on all scales” (Reed and Nelson, 1980). Unit includes rocks in the Healy, Mount McKinley, and Talkeetna quadrangles, which are separated and offset along a strand of the Denali Fault System an apparent 110 to 160 km, depending on the piercing point chosen. Lithologies in the Healy and Mount McKinley quadrangles include medium- to dark-gray, thinly graded, bedded to laminated, medium- to fine-grained sandstone; dark-gray to black argillite; intercalated dark-gray, generally thinly bedded, limestone and dark-gray argillite; and medium- to light-gray, massive, fine- to medium-crystalline, partly dolomitic limestone, (Csejtey and others, 1992). These rocks in the Healy and Mount McKinley quadrangles, in the past, have been assigned to either the Nixon Fork or Dillinger terranes (both now abandoned and considered part of the Farewell terrane [Decker and others, 1994]; we assign them to the Mystic structural complex because of their mixed lithologic character and similar structural disruption. An alternative correlation suggested for the classic Mystic terrane rocks may be in the Yanert Fork unit (Dmvs) of the eastern Healy and western Mount Hayes quadrangles. Other units traditionally assigned to the Mystic terrane in the Lime Hills and McGrath quadrangles (Bundtzen and others, 1997a) do not include the mafic volcanic rocks here; these units have been assigned to units Dls and DCd here. Unit locally sub-divided here into map units JTrmv, JTrv, PDmc, and Dls

Source map information

Source map Csejtey, Bela, Jr., Mullen, M.W., Cox, D.P., and Stricker, G.D., 1992, Geology and geochronology of the Healy quadrangle, south-central Alaska: U.S. Geological Survey Miscellaneous Investigations Series Map I-1961, 63 p., 2 sheets, scale 1:250,000.
Symbol DOs
Unit name Sedimentary sequence
Description Medium- to dark-gray, thinly graded bedded to laminated, medium- to fine-grained sandstone; dark-gray to black argillite; intercalated layers, a few tens of meters thick, of dark-gray, generally thinly bedded, locally thick-bedded limestone and dark-gray argillite; and near the top of the section, an approximately 200-m-thick interbed of medium- to light-gray, massive, finely to medium-crystalline, partly dolomitic limestone (mapped separately as unit 'ls').
Lithology Sedimentary

Correlated geologic units

Label Pzmu
Description Mystic structural complex, undivided (Jurassic? to Devonian)
Geologic age Cambrian to Carboniferous
Geologic setting Undivided
Lithology Form Importance
Sandstone < Clastic < Sedimentary Bed Major
Basalt < Mafic-volcanic < Volcanic < Igneous Flow, pillows Indeterminate, major
Argillite < Metaclastic < Metasedimentary < Metamorphic Zeolitic (prehnite-pumpellyite) Indeterminate, major
Phyllite < Metaclastic < Metasedimentary < Metamorphic Zeolitic (prehnite-pumpellyite) Indeterminate, major
Chert < Chemical < Sedimentary Bed Indeterminate, major
Shale < Mudstone < Clastic < Sedimentary Bed Indeterminate, major
Siltstone < Clastic < Sedimentary Bed Indeterminate, major
Limestone < Carbonate < Sedimentary Bed Minor
Conglomerate < Clastic < Sedimentary Bed Minor