Kodiak Formation Research Articles

The role of silica redistribution in the evolution of slip instabilities along subduction interfaces: Constraints from the Kodiak accretionary complex, Alaska

Quartz veins from a subhorizontal shear zone in the Kodiak accretionary complex in southwest Alaska record cycles of cracking and sealing contemporaneous with simple shear, and analysis of the vein array in the context of silica dissolution, diffusion, and precipitation leads to the conclusion that the time needed to seal after a fracture event overlaps with the time scales associated with the recurrence of slip instabilities along active megathrusts. The central belt of the Kodiak accretionary complex is interpreted as a shear zone developed adjacent to a paleo-decollement that was active in the Late Cretaceous based on large strain magnitudes, recumbent isoclinal folds, and regional cleavage patterns that depict rotation of the steep cleavage that characterizes most of the Kodiak Formation into the horizontal fabrics of the central belt. Within this regional shear zone, crack-seal veins composed of quartz with phyllosilicate inclusion bands are spaced at less than a cm but en echelon vein arrays are spaced at 500 cm. These arrays indicate top-to-the southeast shear on southeast dipping brittle-ductile shear zones. Vein textures indicate cracking and sealing of thin veins in the matrix, but thicker veins in en echelon sets record initial cracking and sealing followed by periodic collapse of fracture porosity at a frequency that is lower than the frequency of crack-seal cycles. Silica depletion zones in the wall rock adjacent to veins indicate local transport of silica by diffusion. By combining the maximum time in which diffusion could dominate over advection with the time needed to precipitate the quartz associated with one crack-seal band, we show that a reasonable range of ΔP, or pressure difference between matrix and crack, can produce crack seal bands in less than 10 days. This time frame is less than the recurrence of earthquakes, and similar to the duration of some slow earthquakes in nature. Therefore, silica redistribution could play a role in modulating the frequency of plate boundary slip instabilities along convergent plate boundaries.

Tectonic and petrologic evolution of the Kodiak batholith and the trenchward belt, Kodiak Island, AK: Contact fault juxtaposition?

Two fore‐arc magmatic belts exist in Kodiak Island, the granitic Kodiak batholith and the basaltic trenchward belt. Both are part of the 2100 km long Sanak‐Baranof belt, interpreted to result from trench‐ridge‐trench (T‐R‐T) triple‐junction migration. In this hypothesis, trenchward belt basaltic rocks formed as the spreading ridge first entered the accretionary prism, and the larger Kodiak batholith resulted from a slab window at depth. However, radiometric dates between the two belts differ in age by up to 1–3 Ma. Kodiak batholith U/Pb zircon ages range from 59.2 to 58.4 ± 0.2 Ma (SW‐NE), whereas trenchward belt ages range from 62.6 ± 0.6 (K‐Ar) to 60.15 ± 0.86 (Ar/Ar whole rock) SW‐NE. This suggests that both fore‐arc magmatic belts did not form from a single triple‐junction passage, because at 60–62 Ma the triple junction was several hundred kilometers west of Kodiak Island. Multiple explanations exist for the age disparity between the two belts, including oblique ridge subduction, Contact fault tectonic juxtaposition, slab window thermal lag, multiple triple‐junction passages, or off‐axis ridge magmatism. Geochemical data such as rare earth metal (REE) curves and Zr/Nb of >30 indicate the trenchward belt rocks have a mid‐ocean ridge basalt (MORB)–like character. Major and trace element modeling indicate that trenchward belt rocks resulted from assimilation‐fractional‐crystallization of MORB with Ghost Rocks Formation argillite, whereas the Kodiak batholith formed via partial melting and en masse mobilization of Kodiak Formation metasedimentary rocks with a small basaltic component and subsequent equilibrium‐crystallization and restite separation. The petrologic origin of both belts is compatible with spreading ridge subduction. Fault zone structural data suggest the Kodiak batholith/trenchward belt age differential resulted from oblique ridge subduction and Contact fault displacement.

A larger agglutinated foraminifer originally described as a marine plant: the case of <i>Arthrodendron</i> Ulrich, 1904 (Foraminifera), its synonyms and homonyms

Abstract. The large, agglutinated foraminiferal genus Aschemocella Vialov, 1966 (type species Aschemonella carpathica Neagu, 1964) and the body fossil Halysium Świdziński, 1934 (type species Halysium problematicum Świdziński, 1934) are herein synonymized with the genus Arthrodendron Ulrich, 1904 (type species A. diffusum Ulrich, 1904), a form originally described as a marine alga from Upper Cretaceous (Maastrichtian) flysch sediments of the Kodiak Formation of the Yakutat Group (formerly Yakutat Formation) on Pogibshi Island, Alaska. The species Aschemonella carpathica Neagu is regarded as a subjective junior synonym of Arthrodendron diffusum Ulrich, which is herein lectotypified and transferred to the Foraminifera.

The role of ridge subduction in determining the geochemistry and Nd–Sr–Pb isotopic evolution of the Kodiak batholith in southern Alaska

The Paleocene Kodiak batholith, part of the Sanak–Baranof belt of Tertiary near-trench intrusive rocks, forms an elongate body (~ 150 km long) that transects Kodiak Island from SW to NE. The batholith consists of three zones (Southern, Central, and Northern) of kyanite-, muscovite-, and garnet-bearing biotite tonalite and granodiorite and less abundant granite that intruded an accretionary prism (Kodiak Formation, and Ghost Rocks Formation). Small and likely coeval bodies (Northern, Western, and Eastern satellite groups) of quartz gabbro, diorite, tonalite, granodiorite, and leucogranite flank the batholith. The batholith is calc-alkalic, has an aluminum saturation index of > 1.1, FeO t/(FeO t + MgO) ~ 0.65 (at SiO 2 = 65 wt.%), and increases in SiO 2 (~ 61 wt.%–73 wt.%) and decreases in TiO 2 (~ 0.9 wt.%–0.3 wt.%) from SW to NE. As a group, the granitic rocks have light REE-enriched chondrite-normalized patterns with small or no negative Eu anomalies, primitive mantle-normalized negative anomalies for Nb and Ti, and positive anomalies for Pb. Small to large negative anomalies for Th are also distinctive. The quartz gabbros and diorites are generally characterized by generally flat to light REE chondrite-normalized patterns (no Eu anomalies), and mantle-normalized negative anomalies for Nb, Ti, and P. Pb isotopic compositions ( 206Pb/ 204Pb = 18.850–18.960; 207Pb/ 204Pb = 15.575–15.694; 208Pb/ 204Pb = 38.350–39.039) are intermediate between depleted mantle and average continental crust. The Southern zone and a portion of the Central zone are characterized by negative ε Nd values of − 3.7 to − 0.3 and T DM ages ranging from ~ 838 Ma to 1011 Ma. Other granitic rocks from the Central and Northern zones have higher ε Nd values of − 0.4 to + 4.7 and younger T DM ages of ~ 450 to 797 Ma. Granitic and mafic plutons from the Eastern satellites show a wide range of ε Nd values of − 2.7 to + 6.4, and T DM ages from 204 Ma to 2124 Ma. 87Sr/ 86Sr values of the Southern and Central zones overlap and tend to be slightly more radiogenic ( 87Sr/ 86Sr > 0.70426) than the Northern zone ( 87Sr/ 86Sr < 0.70472). 206Pb/ 204Pb values increase slightly from the Southern and Central zones toward the Northern zone. There is no clear correlation of the major or trace elements with ε Nd, Pb or Sr isotopic values. Kodiak Formation and the Ghost Rocks Formation overlap the isotopic compositions (e.g., 206Pb/ 204Pb = 18.978 to 19.165, 87Sr/ 86Sr of 0.705715 to 0.707118, and ε Nd of − 6.7 to − 1.5 at 59 Ma) and T DM values (959 to 1489 Ma) of the batholith. Production of large volumes of granitic rocks in the Sanak–Baranof belt, and particularly on Kodiak Island, reflects a sequence of processes that includes underplating of mantle-derived mafic (possibly from the mantle wedge) and intermediate rocks under the accretionary flysch, interlayering of mantle-derived and flyschoid rocks, and partial melting of the mixed lithologic assemblages. Limited degrees of fractional crystallization or assimilation and fractional crystallization influenced compositions of the granitic rocks. The contribution of mantle-derived rocks that resided in the accretionary prism for only a short period of time prior to partial melting likely exceeds 40% (up to 80%). The balance (60 to 20%) is from a recently recycled crustal component represented by the Kodiak Formation. This type of progressive intracrustal melting from mixed sources controlled the geochemical character of the batholith and is most consistent with the hypothesis that the granitic rocks are associated with a slab-window produced by collision of a spreading oceanic center and a subduction zone and migration beneath the accretionary prism.

Cyclic fluid flow through a regionally extensive fracture network within the Kodiak accretionary prism

Two types of periodic textures observed in veins from the Kodiak accretionary prism attest to cyclic fluid flow through a regionally extensive fracture network buried at 8–12 km depth: (1) crack‐seal microstructures with bands of mica inclusions and (2) collapse microstructures with jagged bands of residue embedded within euhedral crystals of quartz. The difference in texture reflects the closure of cracks: crack‐seal microstructures record the complete chemical sealing of the crack after each fracture event, whereas the collapse features record longer fluid‐filled periods followed by more rapid draining of fractures. Collapse features consist of pressure solution selvages trapped within veins and in the wall rock adjacent to euhedral growth terminations; the high concentrations of immobile elements in these selvages indicate that these fractures closed by collapse and penetration of quartz crystals into wall rock. Analysis of chemical composition on either side of four large euhedral growth veins and whole rock analysis of slates across the Kodiak Formation reveal local depletion of silica adjacent to veins but no evidence for long‐distance silica transport within the system. Both crack‐seal and collapse textures are observed in a regionally extensive vein system that displays a regular geometry, with thin, closely spaced (0.5–3 cm), near‐vertical crack‐seal veins that connect vertically and laterally with thicker euhedral growth veins arranged in widely spaced (∼500 mm) southeast dipping en echelon sets. The mesoscopic distribution and textural variability of the vein network suggests that the development of the vein system involved early nucleation and growth of vertical hydrofractures. As the fracture density increased, arrays of fractures locally provided zones of weakness and southeast dipping brittle‐ductile shear zones nucleated. These en echelon cracks remained open and provided small reservoirs of fluid. Textures show that en echelon fractures remain open but periodically grow by upward and downward propagation. Crack tips are then sealed with locally derived silica, and fluid drains back into en echelon fracture arrays. This local fluid movement is punctuated by less frequent events where the system links up over a greater distance, fractured reservoirs become interconnected, and the fluid within reservoirs is drained upward or laterally. Periodic inflation and deflation of en echelon arrays may reflect periodic slip on crosscutting faults and rupture of the seals that separate reservoirs.

Models of quartz overgrowth and vein formation: Deformation and episodic fluid flow in an ancient subduction zone

Steady state models of overgrowth and vein formation are developed using kinetic data for quartz dissolution and precipitation and estimates of fluid advection, pore‐fluid and grain‐boundary diffusion. Application of these models to overgrowths and veins in the Kodiak accretionary complex suggests that the Kodiak Formation deformed continuously by a grain‐boundary diffusion‐limited mechanism, accompanied by episodic pore fluid diffusion of quartz from the matrix to vertical fluid‐filled fractures near the base of the accretionary wedge. These processes produced two types of syntectonic crystal textures within the Kodiak Formation: overgrowths containing displacement‐controlled fibers, and throughgoing veins composed of face‐controlled elongate blocky quartz crystals. Based on textural observations, displacement‐controlled quartz growth in overgrowths is rate‐limited by either diffusion along a cohesive interface or the rate of matrix strain. The magnitude of elongation recorded by displacement‐controlled crystal growth varies smoothly (elongation of 1 to 3) from the shallowest to the deepest structural levels of the Kodiak Formation, suggesting that the diffusional component of deformation in the accretionary wedge increases with depth. In contrast, face‐controlled quartz growth is largely restricted to Veins within the deepest level, where the cleavage is subhorizontal and deformation involves a component of simple shear, suggesting proximity to a decollement. The face‐controlled quartz veins represent mode I cracks which seal periodically and contain continuous planar solid inclusion bands, cracks which partially seal periodically and contain discontinuous solid inclusion bands, or cracks that remain open and contain euhedral quartz crystals with no solid inclusions. The initial crack aperture, inferred from spacing of inclusion bands, varies from 8 μm in crack seal features to much larger values in euhedral growth veins. Euhedral growth veins remain open throughout their development (105 to 106 years), and crack seal veins develop as a consequence of many crack‐seal events over a 103–106 year period. In both cases, textural evidence suggests that most transport of silica occurs by local pore‐fluid diffusion from matrix to vein. Wall rock inclusion bands suggest that “crack” events and “seal” events each occurred within periods of 102–104 years. A picture emerges of intermittent fluid flow upward from the decollement into a branching hierarchy of vertical fractures in the accretionary wedge during hydrofracturing events, followed by local transport and precipitation of silica causing sealing of the fractures at depth and propagation of pulses of fracture fluid upward.

Strain variations in an ancient accretionary complex: Implications for forearc evolution

Strain magnitudes and orientations and incremental strain histories are characterized throughout the Kodiak Formation, a portion of a Late Cretaceous‐early Tertiary accretionary prism. The results of these analyses are used to evaluate the history of underplating, penetrative strain, and unroofing in the forearc of the southwest Alaskan convergent margin. Five major deformation events are recognized in the Kodiak Formation: (1) layer‐parallel extension and layer‐parallel shear (D1) related to underthrusting, (2) seaward verging fold and thrust deformation (D2) with development of both a slaty cleavage (S2) and a downdip stretching lineation (L2) related to underplating and early penetrative strain within the wedge, (3) development of a regional scale antiform (D3), (4) development of a shear zone along the northwest limb of the D3 antiform (D4), and (5) right‐lateral strike‐slip faulting and penetrative right‐lateral simple shear in the seaward half of the Kodiak Formation (D5). Based on sharp differences in structural style and the magnitude and orientation of strain associated with D2, the Kodiak Formation has been subdivided into three structural units informally called the landward, central, and seaward belts. The structural variations observed in these three belts correspond to differences in structural relief of 1–5 km associated with the D3 antiform; the lowest D2 structural level is in the core of the antiform (central belt) and progressively higher structural levels are exposed on both limbs (landward and seaward belts). The regional distribution of incremental strain histories associated with D2, D3, D4, and D5 has been quantified by using syntectonic fibers in pressure shadows around spherical pyrite framboids. From the highest to the lowest structural level, D2 strain is characterized by (1) an increase in the magnitude of finite strain, (2) a change in the orientation of finite extension from nearly vertical to gently northwest plunging, (3) a change in the incremental extension direction from nearly vertical to moderately northwest plunging, and, finally, (4) a change in the strain history from coaxial to noncoaxial with a sense of rotation consistent with southeast directed thrusting. We interpret these D2 strain patterns to reflect a strain field that was fixed relative to the basal decollement, with coaxial subhorizontal shortening in the interior of the prism and noncoaxial strain (probably simple shear) near the base. Subsequent variations in the orientation of incremental extension associated with D3 are consistent with relatively rapid, vertical translation of the accreted sequences relative to the D2 strain field. We specifically propose two periods of wedge thickening in response to a combination of underplating and penetrative strain. These two wedge thickening events correspond with episodes of subaerial exposure and erosional unroofing in the Late Cretaceous and middle Tertiary.

Petrology and provenance of the Cretaceous Kodiak Formation, Alaska

Petrographic analysis of 30 sandstone samples from the Cretaceous Kodiak Formation indicates that they consist mainly of intermediate to basic volcanic and plutonic detritus derived from a source area east of Kodiak Island. This source area had extensive volcanic cover with rare exposure and weathering of cogenetic plutons. The petrology of the Kodiak Formation is similar to that of related rocks in the Alaskan Peninsula and is indicative of the longevity of the volcano-plutonic source area.

The character and distribution of mineralized fractures in the Kodiak Formation, Alaska: Implications for fluid flow in an underthrust sequence

The Kodiak Formation, an accreted slate belt in southwest Alaska, experienced two major episodes of fracturing and mineralization in the Late Cretaceous‐early Tertiary. The earlier of these episodes is related to underthrusting of sediments below a major décollement (D1), whereas a later episode is marked by development of fold and thrust structures and a regionally pervasive slaty cleavage within the accretionary wedge (D2). D1 veins are restricted to sand layers and are concentrated at the highest structural level of the Kodiak Formation. The most pervasive D1 fractures occur within a mélange zone, where the Kodiak Formation is thrust beneath the older Uyak Complex. Microstructures in the mélange suggest that these veins formed during the later stages of a deformation that involved layer parallel shear in conjunction with progressive lithification. D2 veins cut sand and shale layers and are concentrated at the lowest structural levels of the Kodiak Formation. Microstructures within these veins indicate that D2 veins are hydrofractures that were either (1) intermittentiy sealed along the length of the crack (e.g., in the smallest veins (&lt;0.5 cm width)), (2) intermittently sealed at asperities along the fracture surface, or (3) open along the length of the crack through most of the vein history (e.g., in the largest veins (&gt;0.5 cm width)). Fracturing coincided with development of a slaty cleavage in a regime of progressive simple shear. From highest to lowest structural levels, there is a gradual increase in measured finite strain, the abrupt appearance of pervasive D2 veins, and a greater noncoaxiality of strain related to southeast directed thrusting. These observations suggest that D2 veins formed above a décollement that was active at the base of a Late Cretaceous‐early Tertiary accreted package. In total, the D1 and D2 veins suggest substantial fluid flow within an ancient subduction complex, with development of hydrofractures first in the footwall and then in the hanging wall of a basal décollement that stepped from shallow to deeper structural levels.

The effect of carbonate cementation of underthrust sediments on deformation styles during underplating

Fluids expelled during deformation at convergent margins lead to a variety of diagenetic reactions, including carbonate cementation, which alter the rheology of the deforming sediment. The volume of carbonate cement in shallow sediments at modern convergent margins is small, but sandstones in an ancient accretionary complex in Alaska contain significant amounts of carbonate cement that formed before underplating occurred. Other than vein calcite, carbonates are early calcite replacing framework grains or filling pore spaces, followed by ankerite that occurs mainly as pore‐filling cement. Amounts range from 2–16 weight percent in typical samples, averaging about 9 weight percent. Mean porosity filled by carbonate in Kodiak sandstones corresponds to porosities expected at depths of 4–5 km in normal compacting sands underthrust at modem convergent margins. The temperatures of formation are difficult to determine precisely, but were probably between 100°C and 200°C. The bulk of cementation occurred before the formation of thrust faults, folds, and slaty cleavage related to underplating. Lithification by cementation changed the rheology of sandstones by (1) filling pore space and increasing peak strength, and (2) increasing cohesion so that paniculate flow was no longer a viable deformation mechanism. Lithification before significant deformation contributed to underplating of the Kodiak Formation as relatively coherent duplexes.

Orientation history and rheology in slates, Kodiak and Afognak Islands, Alaska

Theoretical and experimental studies have shown that slates have a roughly parabolic yield curve in φ - σ and Φ - σ space, where φ is the orientation of cleavage relative to the maximum compressive stress, Φ is the orientation of cleavage relative to the incremental shortening direction and σ is the deviatoric stress. Deformation mechanisms that operate in low-grade rocks such as diffusive mass transfer in a grain boundary fluid, dislocation creep and frictional grain boundary sliding may have stress-strain rate relationships that depend on φ. During a progressive strain history, φ and Φ may vary as a function of the strain path. Thus, the sequence of dominant deformation mechanisms as well as the stress-strain history may depend on the orientation history or variations in Φ as strain accumulates. In the Kodiak Formation, a slate belt in southwest Alaska, the orientation history and deformation mechanisms were evaluated using incremental strain indicators and microstructures. There are four successive textural events: (1) development of a slaty cleavage ( S 2) with antitaxial growth of fibrous quartz and mica in pressure shadows ( Fi 2); (2) static growth of siderite and chlorite porphyroblasts; (3) 10–70° rotation of porphyroblasts relative to S 2 with growth of curved syntaxial siderite fibers; and (4) antitaxial growth of fibrous quartz and micas with local development of a crenulation cleavage ( S 3). Cumulative incremental strain and progressive finite strain histories were determined in 23 samples from syntectonic fibers, and orientation histories were reconstructed from information provided by cumulative incremental strain histories. Textural and strain analyses indicate that: (1) the strain history of nearly all samples can be approximated by two coaxial strain accumulations separated by a rigid rotation; (2) development of crenulation cleavages was dependent on the orientation history and the magnitude of the early strain accumulation (i.e. the degree of mica alignment at the completion of D 2); (3) unstable behavior, or buckling, occurred when suggesting that the angle of internal friction (Ψ) was close to 0° (i.e. basal slip in micas may have been more important than frictional grain boundary sliding); and (4) diffusive mass transfer was accompanied by anisotropic plastic deformation, so the orientation history could be used to reconstruct the σ - ϵ history.

The development of folds and cleavages in slate belts by underplating in accretionary complexes: A comparison of the Kodiak Formation, Alaska and the Calaveras Complex, California

The development of folds and cleavages in slate and graywacke belts is commonly attributed to arc‐continent or continent‐continent collisions. However, the Kodiak Formation of southern Alaska and the Calaveras Complex of the western Sierra Nevada, California, are two slate and graywacke belts in which folds and slaty cleavages developed during simple underthrusting and underplating within accretionary wedges. The Maastrichtian Kodiak Formation is composed dominantly of coherent turbidites but includes lesser pebbly mudstone, minor conglomerate, and rare chert. The Kodiak Formation is part of a large accretionary complex that youngs in age seaward, but bedding tops generally show landward younging. A progression of structures has been determined by crosscutting relationships and includes (1) syndeformational depositional features; (2) broken formation; (3) slaty cleavage, folds, and thrust faults; (4) crenulations and crenulation cleavage; (5) late brittle thrust faults; and (6) right‐lateral strike‐slip faults. Broken formation, slaty cleavage, thrust faults, and folds developed during underthrusting and underplating within an accretionary wedge. Crenulations and brittle thrust faults are related to subsequent intrawedge shortening. Based on peak metamorphism in the uppermost zeolite to prehnite‐pumpellyite facies, underplating occurred at a minimum depth of 10 km. The Calaveras Complex is composed of argillite, chert, graywacke, pebbly mudstone, limestone, and volcanic rocks. Its age of deposition has a maximum range from Permian to Early Jurassic. Overall, the unit appears to young westward, but local facing indicators show eastward younging of individual blocks. The sequence of structures developed in the Calaveras Complex is (1) syn‐depositional olistostromes; (2) broken formation; (3) slaty cleavage, folds, and thrust faults; and (4) younger Jura‐Triassic folds and crenulation cleavages. Broken formation and slaty cleavage developed during underthrusting and underplating in an accretionary wedge, in an analogous fashion to the Kodiak Formation. Greenschist facies metemorphism during underplating suggests that the Calaveras Complex was accreted at deeper levels, or in a wedge with a higher geothermal gradient, than the Kodiak Formation. Some other probable examples of slate/graywacke belts that became folded and well‐foliated during underplating include part of the Dunnage Zone (Newfoundland), the Southern Uplands complex (Scotland), the Torlesse terrane (New Zealand), part of the Hamburg klippe (Pennsylvania), and the Taconic allochthons (New York). Peak metamorphism in these belts indicates that they were accreted in accretionary wedges with geothermal gradients higher than usually assumed for such tectonic settings; such wedges may have been fairly common features at convergent margins.

Flysch trace fossils from the Cretaceous Kodiak Formation of Alaska

The Cretaceous Kodiak Formation at the northeastern end of Kodiak Island, Alaska, is interpreted to have been deposited in a distal deep-sea fan complex. Facies analysis of the sequence suggests deposition in channels (both major and distributary), lobe fringe (proximal and distal), and interchannel and interchannel/channel mouth environments. The strata contain a diverse and relatively abundant trace fossil assemblage consisting, at the ichnogeneric level, of twenty-six forms: Acanthorhaphe, Chondrites, Circulichnis, Cochlichnus, Cosmorhaphe, Desmograpton, Glockerichnus, Gordia, Helminthoida, Helminthopsis, Muensteria, Neonereites, Nereites, Paleodictyon, Palaeophycus, ?Phycosiphon, Planolites, Protopaleodictyon, Scolicia, Spirophycus, Spirorhaphe, Taenidium, Taphrhelminthopsis, Terebellina, Thalassinoides, and Yakutatia.Thirty-eight ichnospecies are described, three of which (Acanthorhaphe ichnosp., Cosmorhaphe helicoidea, and Desmograpton fuchsi) are recorded for the first time in North America. Final channel-fill (levee) and interchannel environments contain the most abundant and diverse ichnoassemblages, a reflection of favorable environmental parameters for inhabitation by benthic organisms, but more importantly the presence of diverse lithotypes conducive to the final preservation of their activity. Channel-fill sequences are devoid of trace fossils, a reflection of substrate mobility and lack of shale interbeds precluding toponomic preservation. Lobe fringe sequences contain only isolated trace fossils, a reflection of their poor exposure and largely monolithologic character.The ichnoassemblage is compared to 20 previously described and taxonomically well-documented deep-water flysch trace fossil studies from the Phanerozoic. Comparison at the ichnogeneric level is favorable with only Acanthorhaphe, Terebellina, and Yakutatia being relatively unique to the Kodiak sequence.

Structural style and kinematics of an underplated slate belt, Kodiak and adjacent islands, Alaska

The Kodiak Formation, composed of coherent Maastrichtian turbidites, is a slate belt whose dominant structures developed during underplating to an accretionary wedge in the latest Cretaceous. It consists of about 80% coherent landward-dipping thrust packets; zones of disrupted sandstone associated with a scaly argillite matrix constitute the remainder. About half of these disrupted sandstone zones are related to pre-accretion deformation, and the rest formed along late-stage, strike-slip faults that postdate development of slaty cleavage. The formation is divided into three structural belts. The landward and seaward belts include steeply dipping structures, and the central belt contains shallowly dipping structures and rocks that have experienced the highest strain. The central belt probably acted as a low-angle, southeast-verging, floor thrust zone beneath the landward belt. The structural history of the Kodiak Formation includes (1) early soft-sediment disruption; (2) tectonic stratal disruption; (3) thrust faulting, slaty cleavage (S 1 ) development, and folding (F 1 ); (4) intrusion of granodioritic plutons, dikes, and sills and associated normal faulting; (5) development of crenulations (F 2 ) and crenulation cleavage (S 2 ); (6) thrust faulting; and (7) development of right-lateral, strike-slip faults. Event 3 produced the dominant northwest-dipping structural grain and caused the greatest amount of shortening; the timing of event 7 relative to events 5 and 6 is not certain. The dominant structures of the Kodiak Formation are interpreted as developing during underthrusting, underplating, and intra-wedge shortening during latest Cretaceous time within an accretionary wedge. The maximum time from deposition to emplacement was about 12 m.y., and the timing is constrained by the maximum depositional age (74 m.y.) and the age of plutonic rocks (62 m.y.) that crosscut the dominant fabric. Pre-cleavage zones of stratal disruption reflect deformation on the lower plate during underthrusting. Slaty cleavage, thrusts, and F 1 folds developed during underplating that also resulted in the formation of duplexes. Crenulations are probably related to post-underplating subhorizontal shortening of sediments within the accretionary wedge. Syn-deformation dynamic recrystallization of quartz, minimum syn-accretion metamorphic temperatures between 205 and 250 °C, and pressures of at least 2.65 kb recorded during the earliest stages of deformation suggest underplating at >10 km.

The trace fossil Yakutatia emersoni from the Cretaceous Kodiak Formation of Alaska

The enigmatic trace fossil Yakutatia emersoni has previously only been briefly described from the Cretaceous Kodiak Formation of Kodiak Island, Alaska. Herein we examine the trace fossil in detail, confirm that it is indeed a graphoglyptid, note the associated trace fossils, interpret the depositional environment of the strata in which it occurs as most probably a lobe fringe environment in the distal part of a large submarine fan, and designate a lectotype.

Duplex accretion and underplating in an ancient accretionary complex, Kodiak Islands, Alaska

The landward belt of the Kodiak Formation represents a sequence of lower Maastrichtian turbidites that was underplated to the Kodiak accretionary complex by duplex accretion. Evidence for duplexes exists on the mesoscopic and map scale. At the microscopic scale, incremental strain histories are consistent with early shear during underthrusting abrupt landward rotation of individual thrust slices over fault ramps, and locking of thrust packets in steeper orientations. Duplex geometries at the mesoscopic scale are represented by high-angle reverse faults bounded either above or below by low-angle, southeast-verging detachments. At the map scale, different structural belts can be related to a low-angle detachment zone (central belt) flooring a more landward duplex (landward belt). Duplex underplating can result in the uplift and thickening of accretionary prisms without necessarily deforming the overlying, previously accreted material and slope sediments.

Stratigraphic Relationships and Detrital Composition of the Medial Ordovician Flysch of Western New England: Implications for the Tectonic Evolution of the Taconic Orogeny: A Discussion

Previous articleNext article No AccessDiscussion and ReplyStratigraphic Relationships and Detrital Composition of the Medial Ordovician Flysch of Western New England: Implications for the Tectonic Evolution of the Taconic Orogeny: A DiscussionJohn RodgersJohn Rodgers Search for more articles by this author PDFPDF PLUS Add to favoritesDownload CitationTrack CitationsPermissionsReprints Share onFacebookTwitterLinkedInRedditEmail SectionsMoreDetailsFiguresReferencesCited by The Journal of Geology Volume 90, Number 2Mar., 1982 Article DOIhttps://doi.org/10.1086/628670 Citations: 14Citations are reported from Crossref Copyright 1982 The University of ChicagoPDF download Crossref reports the following articles citing this article:Mark H. Anders, John R. Schneider, Christopher H. Scholz, Steven Losh Mode I microfracturing and fluid flow in damage zones: The key to distinguishing faults from slides, Journal of Structural Geology 48 (Mar 2013): 113–125.https://doi.org/10.1016/j.jsg.2012.11.010John Rodgers, Thomas X. Grasso, William M. Jordan, Christopher Hepburn, Edward S. Belt, Markes E. Johnson, Gerald M. Friedman Boston to Buffalo, in the Footsteps of Amos Eaton and Edward Hitchcock: Boston, Massachusetts to Washington, D.C. June 28–July 8, 1989, (Jan 1989).https://doi.org/10.1029/FT169John Rodgers General introduction, (Jan 1989): 1–3.https://doi.org/10.1029/FT169p0001Richard S. Naylor New England mapping Boston: An informal history, (Jan 1989): 4–13.https://doi.org/10.1029/FT169p0004Edward Belt A brief sketch of Edward Hitchcock (1793–1864), (Jan 1989): 14–20.https://doi.org/10.1029/FT169p0014Markes E. Johnson Williams College and the foundation of geology in western Massachusetts and New York, (Jan 1989): 20–31.https://doi.org/10.1029/FT169p0020John Rodgers The Taconic controversies, (Jan 1989): 32–38.https://doi.org/10.1029/FT169p0032John Rodgers Geology and the Saratoga Campaign of 1777, (Jan 1989): 38–39.https://doi.org/10.1029/FT169p0038Gerald M. Friedman New York State Troy, New York and the Van Rensselaers, (Jan 1989): 39–43.https://doi.org/10.1029/FT169p0039Thomas X. Grasso Amos Eaton and early New York State geology, (Jan 1989): 43–48.https://doi.org/10.1029/FT169p0043Thomas X. Grasso Erie Canal history and geology, (Jan 1989): 48–83.https://doi.org/10.1029/FT169p0048William M. Jordan Pennsylvania, (Jan 1989): 84–93.https://doi.org/10.1029/FT169p0084Scott R. Paterson, James C. Sample The development of folds and cleavages in slate belts by underplating in accretionary complexes: A comparison of the Kodiak Formation, Alaska and the Calaveras Complex, California, Tectonics 7, no.44 (Jul 2010): 859–874.https://doi.org/10.1029/TC007i004p00859 William Bosworth , D. B. Rowley , W. S. F. Kidd , and C. Steinhardt Geometry and Style of Post-Obduction Thrusting in a Paleozoic Orogen: The Taconic Frontal Thrust System, The Journal of Geology 96, no.22 (Sep 2015): 163–180.https://doi.org/10.1086/629207

Uyak Complex, Kodiak Islands, Alaska: A Cretaceous subduction complex

The Uyak Complex is a chaotic assemblage of gray chert and argillite, wacke, greenstone, radiolarian chert, and gabbroic and ultramafic rocks. The simplest interpretation of these rocks is that the gabbroic and ultramafic rocks and greenstone represent basal oceanic crust upon which the radiolarian chert, gray chert and argillite, and wacke were deposited, respectively, at a mid-ocean rise, on the abyssal ocean floor, and in an oceanic trench. Fossils are scarce in the complex and range in age from mid-Permian to mid–Early Cretaceous. The Uyak Complex was emplaced by underthrusting to the northwest beneath lower Mesozoic metamorphic, igneous, and sedimentary rocks. During underthrusting, brittle rock types were broken into phacoids of all sizes and suspended in the less competent matrix of gray chert and argillite with their longest dimensions aligned subparallel to cataclastic foliation. Prehnite and pumpellyite developed extensively in lithologies of suitable composition. The Uyak Complex correlates to the northeast with a similar assemblage of deep-sea rocks on the Barren Islands and with the McHugh Complex on the Kenai Peninsula and near Anchorage. The Uyak-McHugh belt defines a probable subduction complex trending northeast for at least 600 km along the margin of southwestern Alaska. The time of emplacement of this melange is uncertain, but fossils present indicate that it occurred after mid–Early Cretaceous time. To the southeast, the Uyak is underthrust by deformed turbidites of the Kodiak Formation, which are interpreted to have been deposited in an oceanic trench and accreted to the Alaskan margin in Late Cretaceous time. The relationship between the Uyak Complex and Kodiak Formation is uncertain, but they may represent two phases, or two facies, of Late Cretaceous accretion. Two large bodies of schist, referred to as the Kodiak Islands schist terrane, occur along the northwest border of the Uyak. The Kodiak Islands schist terrane is a blueschist-bearing metamorphic belt that has yielded mainly Early Jurassic K-Ar mineral ages. Similarities in K-Ar ages, metamorphism, and tectonic setting support a correlation between the Kodiak Islands schists and the Seldovia schist terrane on southern Kenai Peninsula. The Upper Triassic Shuyak Formation structurally overlies the Kodiak Islands schist terrane and Uyak Complex but is separated from them by a long, narrow pluton emplaced in Early Jurassic time. The Shuyak is a little-deformed formation of volcanic and volcaniclastic rocks. It correlates with similar rocks on the Alaska and Kenai Peninsulas, which together outline a lower Mesozoic forearc basin; K-Ar ages show that much of the Alaska-Aleutian Range batholith was intruded coeval with deposition in this forearc basin. A likely interpretation of these rocks is that the Kodiak-Seldovia schists are the only vestige of a subduction complex emplaced along the margin of southwestern Alaska during the prominent early Mesozoic volcanoplutonic activity recorded on the Alaska and Kenai Peninsulas and the Kodiak Islands.