Talkeetna Formation Research Articles

Subduction Polarity in Ancient Arcs: A Call to Integrate Geology and Geophysics to Decipher the Mesozoic Tectonic History of the Northern Cordillera of North America: REPLY

Subduction Polarity in Ancient Arcs: A Call to Integrate Geology and Geophysics to Decipher the Mesozoic Tectonic History of the Northern Cordillera of North America: REPLY

The Ketavik Formation: A new stratigraphic unit and its implications for the Paleogene paleogeography and paleoclimate of southwestern Alaska

A new formation, the Ketavik Formation, is proposed for Paleogene rocks of Katmai National Park near Brooks Camp. The type section is in an area previously mapped as the Jurassic Talkeetna Formation. The proposed formation was deposited in a fluvial environment. It is distinct from the coeval Copper Lake Formation on the southeast side of the Alaska Peninsula volcanic arc and was deposited in a different river system. The Ketavik Formation may include previously mapped, undifferentiated Tertiary rocks, at least some of which are similar in age, that are scattered along a belt parallel to and northwest of the present magmatic arc. The Ketavik Formation contains dicot and coniferous leaf and wood fossils that indicate a warm temperate climate.

Sedimentary Processes in Modern and Ancient Oceanic Arc Settings: Evidence from the Jurassic Talkeetna Formation of Alaska and the Mariana and Tonga Arcs, Western Pacific

Abstract Sediment deposited around oceanic volcanic ares potentially provides the most complete record of the tectonic and geochemical evolution of active margins. The use of such tectonic and geochemical records requires an accurate understanding of sedimentary dynamics in an arc setting: processes of deposition and reworking that affect the degree to which sediments represent the contemporaneous volcanism at the time of their deposition. We review evidence from the modern Mariana and Tonga arcs and the ancient arc crustal section in the Lower Jurassic Talkeetna Formation of south-central Alaska, and introduce new data from the Mariana Arc, to produce a conceptual model of volcaniclastic sedimentation processes in oceanic arc settings. All three arcs are interpreted to have formed in tectonically erosive margin settings, resulting in long-term extension and subsidence. Debris aprons composed of turbidites and debris flow deposits occur in the immediate vicinity of arc volcanoes, forming relatively continuous mass-wasted volcaniclastic records in abundant accommodation space. There is little erosion or reworking of old volcanic materials near the arc volcanic front. Tectonically generated topography in the forearc effectively blocks sediment flow from the volcanic front to the trench; although some canyons deliver sediment to the trench slope, most volcaniclastic sedimentation is limited to the area immediately around volcanic centers. Arc sedimentary sections in erosive plate margins can provide comprehensive records of volcanism and tectonism spanning < 10 My. The chemical evolution of a limited section of an oceanic arc may be best reconstructed from sediments of the debris aprons for intervals up to ~ 20 My but no longer, because subduction erosion causes migration of the forearc basin crust and its sedimentary cover toward the trench, where there is little volcaniclastic sedimentation and where older sediments are dissected and reworked along the trench slope.

Integrated ammonite biochronology and U-Pb geochronometry from a basal Jurassic section in Alaska

New results from integrated biochronologic and geochronometric studies on the basal Jurassic section at Puale Bay (Alaska Peninsula) improve the calibration of the Early Jurassic time scale. Previously, the interval around the Triassic-Jurassic system boundary was poorly dated, which hampered our understanding of geologic and biotic events, e.g., the end-Triassic mass extinction and subsequent recovery. Published suggestions for the presence of the earliest Hettangian (Planorbis Zone) and a continuous boundary section at Puale Bay are not substantiated. Although the Kamishak Formation is likely to contain an uninterrupted sedimentary record, pre‐middle Hettangian strata are locally faulted, resulting in an apparent Rhaetian to early Hettangian gap in the fossil record. The Hettangian ammonite zonal schemes developed locally for Nevada and the Queen Charlotte Islands permit reliable correlation with Alaska, but have limited applicability. The faunal succession recorded at Puale Bay is useful in the development of a regional zonation for North America. We obtained three U-Pb zircon dates that are tied into an ammonite biochronology at the zonal level. A middle Hettangian tuff layer from near the top of the Kamishak Formation is dated at 200.8 +2.7 / ‐2.8 Ma. Tuffs from the overlying Talkeetna Formation are bracketed by middle and late Hettangian ammonites and yield crystallization ages of 197.8 +1.2 / ‐0.4 and 197.8 ± 1.0 Ma. These new calibration points require that the Hettangian-Sinemurian boundary be younger than 199 Ma. The Triassic-Jurassic boundary is likely to fall between 200 and 205 Ma. Similar studies are needed for the uppermost Triassic to obtain tighter constraints. Zircon U-Pb systematics of two samples revealed strong Proterozoic and late Archean inheritance patterns that require revised tectonic models to account for the proximity of the Talkeetna arc to evolved crustal blocks.

Seismic refraction measurements within the Peninsular terrane, south central Alaska

We present an interpretation of crustal seismic refraction data from the Peninsular terrane, one of the many exotic terranes that have been accreted to the continental margin of southern Alaska in the past 200 m.y. A seismic refraction line was collected along the Glenn Highway in the Copper River Basin of south central Alaska in 1984 and 1985, as part of the U.S. Geological Survey Trans‐Alaska Crustal Transect (TACT) program. P wave velocities of 2.7–3.5 km/s and thicknesses of 1–2 km characterize post‐Lower Jurassic sedimentary rocks that underlie most of the seismic refraction line. An average crustal velocity structure includes the following five velocity divisions. Beneath the sedimentary rocks lie 1–2 km of 4.0–4.6 km/s materials, correlating with andesitic volcaniclastic sedimentary rocks and lava flows of the Lower Jurassic Talkeetna Formation. Below these rocks, seismic velocity increases rapidly, from 5.0 to 6.1 km/s, in 2–3 km. At 7–8 km depth, velocity jumps to 6.3 km/s and increments to 6.6 km/s by 10–12 km depth. Velocities increase from 6.8 to 7.0 km/s between 12 to 20 km depth. At about 22 km depth, a jump in velocity from 7.0 to 7.4 km/s is inferred but is poorly resolved. Depth to the Moho discontinuity could not be determined from our data. The absence of clear PmP reflections may indicate that Moho is deeper than 40 km. Data from two offset shotpoints northeast of the line and within the Wrangellia terrane constrain the deep structure transition between Peninsular and Wrangellia terranes. The 6.3–6.6 km/s material thickens to the northeast, toward the suture between Peninsular and Wrangellia terranes, but southwest of its mapped trace at the West Fork fault. Peninsular terrane crustal structure appears dissimilar to that of continental interiors. It is similar to velocity structures determined for accreted island arc fragments in California, such as the basement of the Great Valley and the Klamath Mountains.

High-Mg, low-Al bulk composition of the Talkeetna island arc, Alaska: Implications for primary magmas and the nature of arc crust

Major- and trace-element mass-balance calculations were used to determine the bulk composition of the Jurassic Talkeetna island arc (Talkeetna Formation and Border Ranges mafic and ultramafic complex) of south-central Alaska. A representative section of the arc was chosen near Tonsina, Alaska, where a cross section of the arc exposes 18% ultramafic cumulate rocks (dunites, wehrlites, and pyroxenites), 16% lower gabbroic (garnet- and spinel-bearing) rocks, 34% upper gabbroic to tonalitic rocks, and 32% volcanic rocks (tuffs and flows). Mass-balance calculations for the Talkeetna arc yield a high-Mg basaltic bulk composition with REE abundances approximately 8x chondrites and La/Yb ∼1.8. The bulk composition is high in mafic components (MgO = 11.3 wt. %, Ni = 247 ppm, Cr = 723 ppm) and is compatible with derivation by partial melting of a mantle-wedge source. It is too mafic and too poor in Al 2 O 3 (15 wt. %) to have been derived by partial melting of subducted oceanic crust. High-alumina basaltic compositions are present within the arc as fractionates after crystallization of dunite and pyroxenite; however, they do not represent primary magmas parental to the arc. This finding supports the idea that the mass flux of magma from mantle to crust in island arc regions is high Mg, not high Al, in composition. The arc is stratified with respect to composition and density, with a dense ultramafic to mafic cumulate lower crust, a mafic to intermediate plutonic middle crust, and a mafic to silicic plutonic and extrusive upper crust. The presence of a highly dense, deformed, ultramafic, and garnet-bearing lower crust in this arc suggests that lower crustal delamination may be a viable mechanism for removing a highly mafic component from island arc crust.

Bedrock geology and tectonic evolution of the Wrangellia, Peninsular, and Chugach Terranes along the Trans‐Alaska Crustal Transect in the Chugach Mountains and Southern Copper River Basin, Alaska

The Trans‐Alaskan Crustal Transect in the southern Copper River Basin and Chugach Mountains traverses the margins of the Peninsular and Wrangellia terranes, and the adjacent accretionary oceanic units of the Chugach terrane to the south. The southern Wrangellia terrane margin consists of a polymetamorphosed magmatic arc complex at least in part of Pennsylvanian age (Strelna Metamorphics and metagranodiorite) and tonalitic metaplutonic rocks of the Late Jurassic Chitina magmatic arc. The southern Peninsular terrane margin is underlain by rocks of the Late Triassic (?) and Early Jurassic Talkeetna magmatic arc (Talkeetna Formation and Border Ranges ultra‐mafic‐mafic assemblage) on Permian or older basement rocks. The Peninsular and Wrangellia terranes are parts of a dominantly oceanic superterrane (composite Terrane II) that was amalgamated by Late Triassic time and was accreted to terranes of continental affinity north of the Denali fault system in the mid‐ to Late Cretaceous. The Chugach terrane in the transect area consists of three successively accreted units: (1) minor greenschist and intercalated blueschist, the schist of Liberty Creek, of unknown protolith age that was metamorphosed and probably accreted during the Early Jurassic, (2) the McHugh Complex (Late Triassic to mid‐Cretaceous protolith age), a melange of mixed oceanic, volcaniclastic, and olistostromal rocks that is metamorphosed to prehnite‐pumpellyite and lower greenschist facies that was accreted by middle Cretaceous time, and (3) the Upper Cretaceous Valdez Group, mainly magmatic arc‐derived flysch and lesser oceanic volcanic rocks of greenschist facies that was accreted by early Paleocene time. A regional thermal event that culminated in early middle Eocene time (48–52 Ma) resulted in widespread greenschist facies metamorphism and plutonism.

The Johnson River Prospect, Alaska; gold-rich sea-floor mineralization from the Jurassic

The Johnson River prospect on the Alaskan peninsula is an unusually well preserved Jurassic example of gold-rich sea-floor mineralization accompanied by extensive anhydrite. The prospect is a gold-zinc-copper-lead deposit located within the dominantly dacitic middle portion of the lower Jurassic Talkeetna Formation, a complex succession of unmetamorphosed volcanic and volcaniclastic rocks. The mineralization at the Johnson prospect forms a discordant quartz-sulfide stockwork body in a series of subaqueous tuffs and debris flows.The mineralization and alteration can be divided into an early, pervasive nodular anhydrite-Mg chlorite-clay-sericite stage essentially barren of base metals and gold and a later quartz-sulfide gold stage associated with barite, Fe chlorite, sericite, and vein-type anhydrite. The abundance of sulfate far exceeds that of total sulfide at the Johnson River prospect. The anhydrite-bearing assemblages form a stratigraphically restricted zone in which anhydrite may constitute up to 80 percent of the rock. Textures indicating both replacement of fragments by anhydrite, and more commonly, open-space growth of anhydrite can probably be explained by anhydrite precipitation in highly porous unconsolidated tuffs. Early sulfide and oxide mineralization consists of jasper veins, sinuous pale sphalerite veins and chambers, and quartz-barite-sphalerite nodules accompanied by large volumes of chalcedonic quartz. Late sulfide mineralization includes chalcopyrite-rich veins and hydrothermal breccias accompanied by dark sphalerite, galena, pyrite, and native gold. High-grade gold is concentrated at the highest stratigraphic occurrences of significant copper mineralization.Whole-rock chemical analyses of anhydrite-chlorite rock shows that it is depleted in SiO 2 and enriched in MgO relative to stratigraphically equivalent unaltered tuff. The retrograde solubility of anhydrite, the prograde solubility of quartz, and the large Mg content of the chlorites suggest that this assemblage formed by heating of large volumes of cold seawater in a vigorous convection cell within the unconsolidated tuffs of the Talkeetna Formation. It is suggested that the convection cell was generated by the incursion of heated, metal-bearing fluids from below which eventually came to dominate the hydrothermal system with time as mineral precipitation gradually insulated the stockwork body from cold seawater.

The Border Ranges ultramafic and mafic complex, south-central Alaska: cumulate fractionates of island-arc volcanics

A discontinuous, elongate zone of mafic and ultramafic plutonic rock crops out in south-central Alaska for a distance of more than 1000 km. Intermediate- and detailed-scale geologic mapping, petrographic study, and compositional data suggest that the plutonic rocks are compositionally, petrologically, and mineralogically distinct from rocks in mid-ocean ridge and back-arc basin ophiolites. The mafic and ultramafic rocks instead represent part of the plutonic core of an intraoceanic island arc.The mafic–ultramafic zone, referred to as the Border Ranges ultramafic and mafic complex (BRUMC), is composed of ultramafic cumulates, gabbronorite cumulates, and massive gabbronorites. A very minor amount of tectonized ultramafic rock of mantle origin is present in the southern part of the BRUMC. A thick sequence of andesitic volcanic rocks, the Talkeetna Formation of Early Jurassic age, lies to the north of and structurally above the mafic–ultramafic zone. Voluminous calcalkaline plutons composed of quartz diorite, tonalite, and minor granodiorite intrude both the mafic plutonic complexes and the andesitic volcanic rocks.The cumulate ultramafic sections are largely composed of dunite ± chromite, wehrlite, clinopyroxenite, and websterite and are characterized by a wide range of Mg–Fe silicate compositions (Fo90–81; En45–50, Fs1–7, Wo45–49; En88–82, Fs11–17), chrome-rich spinels, and a lack of plagioclase. The gabbroic sections are composed of gabbronorites with up to 10–15% magnetite ± ilmenite. Hornblende, if present, is a very minor phase in most gabbroic rocks. The coexisting mineral compositions seen in the gabbroic rocks of the BRUMC (relatively iron-rich pyroxene—Fs6–13, En45–40; En81–63 —and calcic plagioclase An75–100) and their association with magnetite are common in plutonic xenoliths in island-arc rocks.The mineralogy and composition of the gabbroic rocks in the BRUMC are consistent with the fractional crystallization products predicted to be associated with the formation of andesite from a basaltic magma. Consideration of additional data, including detailed and regional field mapping of the plutonic and volcanic rocks and geochronology of the BRUMC and the nearby Talkeetna arc volcanic rocks, strongly suggests that the BRUMC represents relatively deep fractional crystallization products of magmas that produced the Talkeetna Formation volcanic rocks. Field relationships also indicate that intrusion of quartz diorites, tonalites, and granodiorites of batholithic proportions occurred slightly later than formation of the BRUMC.

Tectonic Implications of Some New Mesozoic Stratigraphic Data on Alaska: ABSTRACT

Increased geological exploration of Alaska during the past decade has produced a wealth of new stratigraphic data. Some of these data, particularly from Mesozoic rocks, are of fundamental importance in the interpretation of the tectonic history of Alaska. The U. S.:Geological Survey is compiling a comprehensive correlation chart of all known Mesozoic sedimentary, volcanic, and intrusive rocks. Preliminary work on the chart indicates the need for revising or refining present concepts of the stratigraphy and tectonics in several areas. For example, recent studies in northwestern Alaska suggest that the mid-Cretaceous Koyukuk geosyncline End_Page 274------------------------------ was not, as previously supposed, a single depositional trough stretching from the Brooks Range to the Yukon delta but consisted of a narrow east-trending trough along the Kobuk and upper Koyukuk Rivers and a larger southwest-trending trough that extended from the lower Koyukuk River to the Yukon delta. The two troughs were separated by an east-trending geanticline along Lat 66° N. The volcanics and granitic intrusives of Late Jurassic and Early Cretaceous age that are now exposed along this geanticlinal trend were an important source of the sediments in both troughs. Important data are also being developed by restudy of long-known Mesozoic terranes in southern Alaska. For example, the Talkeetna Formation of southeastern Talkeetna Mountains was thought to consist mainly of marine volcanics of Pliensbachian (Early Jurassic) age. However, it is now known to range in age at least from early Sinemurian to late Toarcian. Radioactivity dates indicate that, in the Talkeetna Mountains, plutons began to be intruded into the Talkeetna Formation shortly after it was deposited. Intrusion began at about the time the Matanuska epi-eugeosyncline was established in earliest Middle Jurassic time within the area of the more extensive eugeosyncline of the Talkeetna Formation. Intrusion probably continued into Late Jurassic and possibly Early Cretaceous time, although the adjacent Matanuska geosyncline was concurrently receiving a relatively complete sedimentary section. End_of_Article - Last_Page 275------------