Pliocene Deformation Research Articles

Crustal block-controlled contrasts in deformation, uplift, and exhumation in the Santa Cruz Mountains, California, USA, imaged through apatite (U-Th)/He thermochronology and 3-D geological modeling

Deformation along strike-slip plate margins often accumulates within structurally partitioned and rheologically heterogeneous crustal blocks within the plate boundary. In these cases, contrasts in the physical properties and state of juxtaposed crustal blocks may play an important role in accommodation of deformation. Near the San Francisco Bay Area, California, USA, the Pacific−North American plate-bounding San Andreas fault bisects the Santa Cruz Mountains (SCM), which host numerous distinct, fault-bounded lithotectonic blocks that surround the San Andreas fault zone. In the SCM, a restraining bend in the San Andreas fault (the SCM bend) caused recent uplift of the mountain range since ca. 4 Ma. To understand how rheologic heterogeneity within a complex fault zone might influence deformation, we quantified plausible bounds on deformation and uplift across two adjacent SCM lithotectonic blocks on the Pacific Plate whose stratigraphic and tectonic histories differ. This was accomplished by combining 31 new apatite (U-Th)/He ages with existing thermochronological datasets to constrain exhumation of these two blocks. Additionally, surface exposures of the latest Miocene to late Pliocene Purisima Formation interpreted in 18 structural cross sections spanning the SCM allowed construction and restoration of Pliocene deformation in a three-dimensional geologic model. We found that rock uplift and deformation concentrated within individual Pacific Plate lithotectonic blocks in the SCM. Since 4 Ma, maximum principal strain computed for the more deformed block adjacent to the fault exceeded that computed for the less deformed block by at least 375%, and cumulative uplift has been more spatially extensive and higher in magnitude. We attribute the difference in uplift and deformation between the two blocks primarily to contrasts in lithotectonic structure, which resulted from diverging geologic histories along the evolving plate boundary.

Miocene Paleogeography of NW Colombia: A review of the sedimentary and magmatic evolution of the Amagá Basin a century after Grosse’s work

In 1918, the geologist Emile Grosse was commissioned to conduct geological studies in the Amagá Basin, Antioquia, Colombia. In 1923, Grosse finished a comprehensive cartographic work that became the cornerstone for the geology of the northwest (NW) Colombian Andes. Today, 100 years later, the volcanoclastic strata preserved in the Amagá Basin are crucial for understanding major Oligocene to Pliocene tectonic events that occurred in the NW South-American margin, including the fragmentation of the Nazca Plate, the collision of the Panamá-Chocó Block, and the shallowing of the subducted slab. Our contribution includes new mineral chemistry and zircon petrochronological data from the Combia Volcanic Complex and published data to provide a review of the Oligocene to Pliocene deformation, sedimentation, and magmatic patterns in the Amagá Basin and their implications for the tectonic evolution of NW South America. The Amagá Basin was the result of the Eocene to Oligocene uplift of the Western Cordillera followed by the Middle Miocene to Pliocene uplift of both the Central and Western cordilleras, events that modified the Miocene drainage network in the Northern Andes. Coeval with the final Miocene deformation phases in the Amagá basin, the magmatism of the Combia Complex was the result of subduction magmas emplaced in a continental crust affected by strike-slip tectonics.

Cenozoic Fault Growth Mechanisms in the Outer Apulian Platform

This work focuses on a ca. 55 km-long extensional fault zone buried underneath the foredeep deposits of the southern Apennines, Italy, with the goal of deciphering the Cenozoic fault growth mechanisms in the Outer Apulian Platform. By considering public 2D seismic reflection profiles, well logs, and isochron maps data, the study normal fault zone is interpreted as made up of four individual fault segments crosscutting Top Cretaceous, Top Eocene, Top Miocene, and Top Pliocene chrono-stratigraphic surfaces. The computed cumulative throw profiles form either bell-shaped or flat-shaped geometries along portions of the single fault segments. The computed incremental throw profiles also show an initial fault segmentation not corresponding with the present-day structural configuration. Data are consistent with the initial, post-Cretaceous fault segments coalescing together during Miocene–Pliocene deformation and with fault linkage processes localizing at the stepover/relay zones. Pleistocene faulting determined the evolution of a coherent fault system. The computed n-values obtained for the single time intervals by considering the maximum fault throw–fault length relations indicate that the fault segments formed scale-dependent geometries. Variations of these computed values are interpreted as due to the higher degree of maturity reached by the entire fault system during Miocene to Pleistocene deformation.

Insights into the late Cenozoic structural deformation and tectonic stress field of the Qiabuqia region, Gonghe Basin, northeastern Qinghai–Tibetan Plateau

In addition to overall uplift of the Qiabuqia region during the Late Cenozoic, three deformation stages can be identified in the northeastern Qinghai–Tibet Plateau. Specifically, these deformation stages were recognized in areas east of the Gonghe Basin that surrounds Waliguan Mountain and include: 1) Late Miocene to Late Pliocene deformation—dominated by thrust napping with dextral strike-slipping; 2) Early Pleistocene to late middle period of Pleistocene deformation—fault structures were dominated by the dextral strike-slipping with thrust napping; and 3) end of Late Pleistocene to Middle Holocene deformation—thrust napping slowed down and finally braked, while the shallow surface began to loosen and extend at Qiabuqia region’s rear-edge. DR4 borehole drilling data indicate that the Xiangshuihe Formation’s buried depth is ∼1,000 m. The Xianshuihe Formation’s top section is characterized by normal fault–type (NF) tectonic stress, while the maximum principal tectonic stress (σ1) is perpendicular and the minimum principal tectonic stress (σ3) is horizontal. In contrast, the deep basement is characterized by thrust fault (TF) and strike-slip (SS) tectonic stress, while the σ1 is in the NE-SW direction, ranging from 20.1° to 75.3° with an average of 40.0°. Based on the aforementioned observations, it can be inferred that there are three sets of hidden faults in the basement of the Qiabuqia region: 1) the NW–NNW trending compressional–torsional faults; 2) the NE trending tensional–torsional faults; and 3) the NWW trending compressional–torsional faults. The findings in this study can potentially offer a robust geological basis for exploring and utilizing hot, dry rock resources within the Gonghe Basin.

Response of the Pliocene-Pleistocene carbonates to relative sea-level changes in Kish Island, Persian Gulf

Plio-Pleistocene period in the Persian Gulf, Iran was marked by the development of carbonate rocks of the Khark Formation. The shallow-water carbonates of the Khark Formation record fluctuations in sea-level. Khark limestones unconformably cover the calcareous marlstones of the Late Miocene Mishan Formation. This study used petrographic analysis and SEM-EDX analysis to determine microfacies, diagenetic products, sedimentary environments, sedimentary cycles, and relative sea-level changes during the Plio-Pleistocene. Based on the textural features and fossil content, six microfacies were recognized in the lagoon, shoal, back reef, and reef facies belts. The dominance of reef and grainstone facies shows a high-energy shallow-marine environment and the existence of a paleo-high. This paleo-high was formed due to the Late Pliocene deformation and folding in the Persian Gulf. The Khark carbonates were deposited as a reef complex on an anticline parallel to the Zagros Orogeny trend due to dominance of reef and grainstone facies, variable thickness, and irregular pre-existing topography during the Zagros folding phase. Morphology of cement crystals, the spatial distribution of cement, and their compositional variations display that the Low-Mg dogtooth calcite, gypsum cement, and moldic porosity were formed in the sandy bioclastic peloidal grainstone facies with meteoric diagenesis during the relative sea-level fall, whereas aragonite isopachous fringe of cement and High-Mg calcite (beachrocks) were developed in the reefal facies with marine phreatic diagenesis during relative sea-level rise. Khark Formation consists of two shallowing upward sequences that were marked by coral boundstone and oyster-coral floatstone facies during sea-level highstand at the top. These facies changes are in close agreement with the local sea-level changes curve during Plio-Pleistocene in the Persian Gulf. This kind of study can improve our understanding of reservoir characterization of the reefal carbonates in the subsurface sections.

Low-temperature thermochronology of the Black and Panamint mountains, Death Valley, California: Implications for geodynamic controls on Cenozoic intraplate strain

We use apatite and zircon (U-Th)/He thermochronometry to evaluate space-time patterns and tectonic drivers of Miocene to Pliocene deformation within the Death Valley area, eastern California. Zircon He ages from the footwall of the Amargosa–Black Mountains detachment in the Black Mountains record continuous cooling and exhumation from 9 to 3 Ma. Thermal modeling of data from the central Black Mountains suggests that this cooling took place during two intervals: a period of rapid footwall exhumation from 10 to 6 Ma, followed by slower (<5 mm/yr) exhumation since 6 Ma. Cumulative exhumation is estimated to be 10–16 km. Paleodepth reconstruction of cooling ages from the footwall of the Panamint-Emigrant detachment, in the central Panamint Range, also show two periods of cooling. Zircons record late Miocene cooling, whereas apatite He ages show punctuated exhumation at ca. 4 Ma. The results suggest the Panamint Range experienced a minimum of 7.2 km of exhumation since ca. 12 Ma. The new data, when evaluated within the context of published fault timing data, suggest that the transition from Basin and Range extension to dextral transtension is spatially and temporally distinct, beginning at ca. 11–8 Ma in ranges to the east and north of the Black Mountains and migrating westward into eastern Death Valley at 6 Ma. Initiation of dextral transtension was coincident with a major change in plate-boundary relative motion vectors. Data from Panamint Range and several ranges to the west of Death Valley indicate transtension initiated over a large area at ca. 3–4 Ma, coeval with proposed lithospheric delamination in the central and southern Sierra Nevada Range. Our results suggest that the transition from extension to dextral transtension may reflect an evolution in tectonic drivers, from plate-boundary kinematics to intraplate lithospheric delamination.

Pliocene deformation of the north-Ligurian margin (France) : consequences of a south-Alpine crustal thrust

The Oligo-Miocene extension phase of the Mediterranean basins rifting (30-25 Ma) [Jolivet and Faccenna, 2000] followed by the Ligurian basin oceanic crust formation (21-18 Ma) [Le Pichon et al., 1971 ; Rehault et al., 1984 ; Carminati et al., 1998 ; Gueguen et al., 1998] occurred during the western Alps compression phase. The deformations were characterised during the Miocene by the southwestward structuration of the Castellane Arc [Fallot and Faure-Muret, 1949 ; Laurent et al., 2000] and during the Mio-Pliocene by the southward structuration of the Nice Arc. This latter arc is bounded on its western side by a dextral strike-slip fault and on its southern side by a thrust inducing an uplift of this arc [Ritz, 1991 ; Guglielmi and Dubar, 1993 ; Clauzon et al., 1996 ; Guardia et al., 1996 ; Schroetter, 1998]. Fission tracks thermochronology data [Bigot-Cormier et al., 2000] suggest a general uplift at ~3.5 Ma of the Argentera massif. Stratigraphical [Irr, 1984 ; Hilgen, 1991 ; Hilgen and Langereis, 1988, 1993] and geomorphological studies [Clauzon et al., 1996 b ; Dubar and Guglielmi, 1997] show evidences for an uplift of the Ligurian coast increasing east of the Var river. The analysis of 70 seismic-reflection profiles allows us to better characterise and quantify the deformation from Antibes to Imperia (fig. 1). We then reconstruct vertical motions in space and time since the Messinian crisis in order to propose a deformation model of the margin related to crustal thickening.

Mio–Pliocene to Pleistocene paleotopographic evolution of Brittany (France) from a sequence stratigraphic analysis: relative influence of tectonics and climate

The Mio–Pliocene in Western Europe is a period of major climatic and tectonic change with important topographic consequences. The aim of this paper is to reconstruct these topographic changes (based on sedimentological analysis and sequence stratigraphy) for the Armorican Massif (western France) and to discuss their significance. The Mio–Pliocene sands of the Armorican Massif (Red Sands) are mainly preserved in paleovalleys and are characterized by extensive fluvial sheetflood deposits with low-preservation and by-pass facies. This sedimentological study shows that the Red Sands correspond to three main sedimentary environments: fluvial (alluvial fan, low-sinuosity rivers and braided rivers), estuarine and some rare open marine deposits (marine bioclastic sands: “faluns” of French authors). Two orders of sequences have been correlated across Brittany with one or two minor A/ S cycles comprised within the retrogradational trend of a major cycle. The unconformity at the base of the lower cycle is more marked than the unconformity observed at the top, which corresponds to a re-incision of the paleovalley network. A comparison of the results of the sequence stratigraphy analysis with eustatic variations and tectonic events during the Mio–Pliocene allows (1) to discuss their influence on the evolution of the Armorican Massif and (2) to compare the stratigraphic record with other west-European basins. The unconformity observed at the base of the first minor cycle may be attributed to Serravallian–Tortonian tectonic activity and/or eustatic fall, and the unconformity of the second minor cycle may be attributed to Late Tortonian–Early Messinian tectonic activity. The earlier unconformity is coeval with the development of a “smooth” paleovalley network compared to the jagged present-day relief. A single episode of Mio–Pliocene deformation recorded in Brittany may be dated as Zanclean, thus explaining the lack of the maximum flooding surface except in isolated areas. From this study, five paleogeographic maps were drawn up also indicating paleocurrent directions: three maps for the lower cycle (Tortonian retrogradational trend, Late Tortonian to Early Messinian maximum flooding surface and Messinian progradational trend) and two for the upper cycle (Pliocene retrogradational trend and Piacenzian maximum flooding surface). These maps show (1) the variations of paleocurrent directions during the Mio–Pliocene, (2) the extent of estuarine environments during the maximum flooding intervals and (3) a paleodrainage watershed oriented NNW–SSE following the regional Quessoy/Nort-sur-Erdre Fault during the retrogradational trend of the upper cycle and possibly during the progradational trend of the lower cycle. The present-day morphology of the Armorican Massif is characterized by (1) incised valleys and jagged topography, in contrast with the “smooth” morphology described for Mio–Pliocene times and (2) a main East–West drainage watershed, located to the north, separating rivers flowing towards the English Channel from rivers flowing towards the Atlantic Ocean. The Mio–Pliocene/Pleistocene paleotopographic changes seem to be controlled by climatic effects. These can be related to the change in runoff associated with warmer and wetter conditions during the Mio–Pliocene, which control the river discharge and lead to the development of extensive fluvial sheetflood deposits. Tectonic or eustatic factors exert a second-order control.

Structural and stratigraphic perspectives on the uplift and erosional history of Djebel Cherichira and Oued Grigema, a segment of the Tunisian Atlas thrust front

Abstract Djebel Cherichira, a segment of the Tunisian Atlas thrust front, was structurally uplifted by at least 1650m and possibly more than 2000m by late Tortonian and post-Messinian thrusting. After late Tortonian structural uplift and erosion, Djebel Cherichira formed a topographic high with Messinian, arid to semi-arid, debris-flow fan deposits mantling the relict topography. Oued Cherichira formed in Serravallian to early Tortonian times, prior to the growth of the thrust front, as a foreland basin fluvial system ahead of the thrust structures within the thrust belt. It maintained its antecedent drainage pattern during rapid tectonic uplift of the thrust front at Djebel Cherichira. Movement on the Kredija Strike-slip Fault records the latest tectonic event in the thrust belt. This probable Early Pliocene deformation did not displace the drainage pattern, implying that much syntectonic erosion and desert pediment formation had already occurred. Oued Grigema, a tributary of Oued Cherichira, whilst appearing to be antecedent with respect to Djebel Cherichira, is a post-tectonic, consequent stream, where at least some of the fluvial erosion and deposition is of Holocene age. Many of the present-day processes of sedimentation and erosion in Oued Grigema are analogous to those interpreted from the Messinian strata of Djebel Cherichira.

Astronomical dating of a tectonic rotation on Sicily and consequences for the timing and extent of a middle Pliocene deformation phase

A detailed palaeomagnetic study of long and continuous middle Pliocene sections from the Caltanisetta basin on Sicily reveals a differential clockwise rotation occurring around 3.21 Ma. The rotation appears to be a rapid event (80,000–100,000 years) which suggests that the responsible tectonic processes also occur rapidly. Its timing corresponds closely to the transition from the Trubi to the Narbone Formation at 3.19 Ma. This transition marks a major change in sedimentary environment on Sicily and in Calabria, and it is coeval, for instance, with the onset of sapropel formation in the eastern Mediterranean. Apparently it marks a synchronous and central–eastern Mediterranean-wide event. Data from the oldest sediments overlying the Tyrrhenian basement (ODP Leg 107) suggest an acceleration in opening of the Tyrrhenian Sea during the middle Pliocene. We speculate that this acceleration is related to a transpressional event in the Sicilian fold-and-thrust belt and extension which formed troughs in the foreland, the Strait of Sicily. Thrust imbrication accompanying the transpressional event on Sicily induced the middle Pliocene clockwise rotation and resulted in shallowing of the Caltanisetta basin causing the change in sedimentation regime characterised by the Trubi–Narbone transition. Following this middle Pliocene tectonic phase, no rotation took place in the southern Apennines, Calabria and Sicily until the middle Pleistocene (1.0–0.7 Ma).

L'île de Kur: géologie du flanc oriental du bassin de Weber (Indonésie orientale)

New biostratigraphical, geochronological and geochemical data from Kur island and two nearby dredgings allow us to show unknown events on the eastern margin of the Weber basin: an Early Oligocene magmatic arc; a tectono-metamorphic event between 24 and 17 Ma; and an Early Pliocene deformation, related to the collision between the Australian plate and the Banda arc. The Weber basin was created during Pleistocene with an uplift of its eastern margin. These events are briefly replaced within the geodynamic context of the Eastern Indonesia.

The geology and tectonic evolution of Waigeo Island, NE Indonesia

Waigeo occupies a critical position between the Halmahera-Philippine arcs to the northwest and Australia-New Guinea to the southeast. The island consists of a deformed ophiolitic basement of supra-subduction zone type overlain by probable Paleogene forearc sedimentary rocks. The forerarc basement is cut by mylonite shear zones and the basement and sedimentary cover rocks were deformed by southward-directed thrusts and associated folds during the Oligocene. The deformed sequences are intruded by basic dykes, thought to be coeval with basalts and andesites of Late Oligocene age. The entire Paleogene sequence is overlain unconformably by a thick (up to 2000 m) sequence of Miocene limestones which accumulated during a tectonically quiet period. A final phase of deformation occurred during the Pliocene which caused the development of two very large anticlines and an intervening syncline, associated with left-lateral wrench faults. Waigeo is interpreted to have been situated in a forearc position in an intra-oceanic island arc during the early Paleogene, forming part of the East Halmahera-Waigeo forearc terrane. The Waigeo arc terrane collided with a continental block in about the Middle Oligocene, contemporaneous with similar arc-continent collision in northern New Guinea. A period of tectonic quiescence during the Miocene was followed by Pliocene deformation in Waigeo related to a left-lateral wrench faulting on splays of the Sorong Fault in northern New Guinea. The Pliocene deformation is interpreted as resulting from compression on a right-stepping restraining bend in this wrench fault system.

Structural styles, Cepu oil fields, Java, Indonesia

Reinterpretation of 18 local and 7 regional seismic lines in northeast Java, numerous exploration wells and their integration with newly measured stratigraphic sections has enabled a new structural model to be developed for the Cepu oil fields. The generally shallow water limey-clastic sequence developed in a rifting back-arc basin with many northeast-southwest oriented basement faults. Deformation in the early Middle Miocene caused reactivation of the basement faults in the Nglobo-Semanggi area with wrenching and the initial development of flower structures. This deformation caused areally restricted erosion of the main reservoir rocks in this area. Later Pliocene deformation accelerated the development of the flower structures in the Nglobo-Semanggi area which were reflected at the surface as a series of en echelon, hydrocarbon-bearing anticlines. The Tambakromo-Kawengan area underwent minor north-over-south thrusting along east-west oriented listric, reverse faults with detachment at shallow depths and the development of hydrocarbon-bearing anticlines in the cover sequence. It is possible that further hydrocarbon-bearing folds exist in the subsurface north of the Tambakromo-Kawengan structure. These folds would be related to blind imbricate thrusts parallel to the Tambakromo-Kawengan thrust.

Structural development of hydrocarbon traps in the Cepu oil fields, northeast Java, Indonesia

Reinterpretation of 18 local and 7 regional seismic lines in northeast Java, numerous exploration wells and their integration with newly measured stratigraphic sections, has enabled a new structural model to be developed for the Cepu oil fields. The generally shallow-water, limey-clastic sequence developed in a rifting back-arc basin with many northeast-southwest oriented basement faults. Deformation in the early Mid Miocene caused reactivation of the basement faults in the Nglobo-Semanggi area with wrenching and the initial development of flower structures. This deformation caused areally restricted erosion of the main reservoir rocks in this area. Upper Pliocene deformation accelerated the development of the flower structures in the Nglobo-Semanggi area which were reflected at the surface as a series of en echelon, hydrocarbon-bearing anticlines. The Tambakromo-Kawengan area underwent minor north over south thrusting along east-west oriented listric, reverse faults with detachment at shallow depths and the development of hydrocarbon-bearing anticlines in the cover sequence. It is possible that further hydrocarbon-bearing folds exist in the subsurface north of the Tambakromo-Kawengan structure. These folds would be related to blind imbricate thrusts parallel to the Tambakromo-Kawengan thrust. The wrench structures and detached compressional structures forming hydrocarbon-bearing folds in the Cepu oil fields are probably a result of the transfer of stresses due to oblique subduction at the Java Trench during the Neogene to Pleistocene.

Late Miocene and Pliocene Synorogenic Sedimentation in Northern Livermore Basin, California: ABSTRACT

Late Tertiary synorogenic sedimentation in the northern Livermore basin, Contra Costa County, California, has recorded two major changes in provenance. Changes in clast composition of fluvial conglomerates reflect regional tectonic reorganization as the San Andreas fault system began to evolve at this latitude. Shallow marine deposition of Sierran andesitic sediment gave way at approximately 8 Ma to fluvially dominated deposition of sediment from a local, graywacke-rich, Coast Range source. Deposition of reworked andesitic material prior to 5.5 Ma records development of anticlinal uplifts along trends of the present Altamont Hills and Calaveras fault system. By 4 Ma, the areal extent of the subsiding basin had increased; the Altamont Hills continued to be uplifted, but the Calaveras fault region began to subside rapidly. Late Miocene and Pliocene deformation of the Livermore basin area extended over a broad zone east of the developing transform fault system. Structures that developed included broad synclinal basins and gentle anticlinal uplifts that had topographic expression but were not deeply incised. Intense deformation of the basin and uplift of Mount Diablo occurred after 3 Ma.

Pliocene change in Pacific-plate motion

The absolute motion of the Pacific plate has been nearly constant during the past 15 Myr, but Epp1 recognized that some change took place between 15 Myr and the present, and tentatively chose 1 Myr for the time of the change. Cox and Engebretson2 found a change in Pacific absolute motion based on hotspot tracks and a change in Pacific–Antarctic relative motion. The Pliocene deformation in California and the Juan de Fuca rotations led them to postulate a 5-Myr age for the change. The manner and timing of the change are explored more thoroughly here, and we put the time for the change in motion of the Pacific plate at between 5 and 3.2 Myr, basing the younger limit on the correlation of computed hotspot tracks with Pacific and Nazca hotspot traces. The change may explain several observations on the neighbouring continental and oceanic plate boundaries, including direct adjustments at the Pacific spreading ridges. A likely mechanism for the change is a large clockwise torque caused by acceleration of the sinking slab beneath the Kurile trench, where a fossil spreading ridge is inferred to have been subducted.

Petroleum Geology of Cook Inlet Basin -- An Exploration Model

Oil exploration commenced onshore adjacent to lower Cook Inlet on the Iniskin Peninsula in 1900, shifted with considerable success to upper Cook Inlet from 1957 through 1965, then returned to lower Cook Inlet in 1977 with the COST well and Federal OCS sale. Lower Cook Inlet COST No. 1 well, drilled to a total depth of 3,775.6 m, penetrated basinwide unconformities at the tops of Upper Cretaceous, Lower Cretaceous, and Upper Jurassic strata at 797.1, 1,540.8, and 2,112.3 m, respectively. Sandstone of potential reservoir quality is present in the Cretaceous and lower Tertiary rocks. All siltstones and shales analyzed are low (0 to 0.5 wt. %) in oil-prone organic matter, and only coals are high in humic organic matter. At total depth, vitrinite readings reached a maximum ave age reflectance of 0.65. Several indications of hydrocarbons were present. Oil analyses suggest that oils from the major fields of the Cook Inlet region, most of which produce from the Tertiary Hemlock Conglomerate, have a common source. More detailed work on stable carbon isotope ratios and the distribution of gasoline-range and heavy (C12+) hydrocarbons confirms this genetic relation among the major fields. In addition, oils from Jurassic rocks under the Iniskin Peninsula and from the Hemlock Conglomerate at the southwestern tip of the Kenai lowland are members of the same or a very similar oil family. The Middle Jurassic strata of the Iniskin Peninsula are moderately rich in organic carbon (0.5 to 1.5 wt. %) and yield shows of oil and of gas in wells and in surface seeps. Extractable hydrocarbons from this strata are similar in chemi al and isotopic composition to the Cook Inlet oils. Organic matter in Cretaceous and Tertiary rocks is thermally immature in all wells analyzed. Oil reservoirs in the major producing fields are of Tertiary age and unconformably overlie Jurassic rocks; the pre-Tertiary unconformity may be significant in exploration for new oil reserves. The unconformable relation between reservoir rocks and likely Middle Jurassic source rocks also implies a delay in the generation and expulsion of oil from Jurassic until late Tertiary when localized basin subsidence and thick sedimentary fill brought older, deeper rocks to the temperature required for petroleum generation. Reservoir porosities, crude oil properties, the type of oil field traps, and the tectonic framework of the oil fields on the west flank of the basin provide evidence used to reconstruct an oil migration route. The route is inferred to commence deep in the truncated Middle Jur ssic rocks and pass through the porous West Foreland Formation in the McArthur River field area to a stratigraphic trap in the Oligocene Hemlock Conglomerate and the Oligocene part of the Tyonek Formation at the end of Miocene time. Pliocene deformation shut off this route and created localized structural traps, into which the oil moved by secondary migration to form the Middle Ground Shoal, McArthur River, and Trading Bay oil fields. Oil generation continued into the Pliocene, but this higher API gravity oil migrated along a different route to the Granite Point field.

Tertiary Planation in Idaho

Field work by members of the U.S. Geological Survey and others has led to the recognition of an ancient erosion surface or peneplain in parts of Idaho, Montana, and Wyoming. Differences of opinion have arisen regarding the age and character of this peneplain. The problem is regional in character, necessitating more extended field work before a final solution may be reached. The paper gives a discussion of the problem in the light of studies by the author in southeastern Idaho. It is concluded that the idea of post-middle Miocene planation across earlier Tertiary and older rocks, as advocated for Idaho by Blackwelder and Rich, is not proved by the available evidence. There seems a probability that the erosional history of western and northern Idaho and of western Montana was somewhat different from that of southeastern Idaho or of western Wyoming. For southeastern Idaho the evidence seems to favor pre-middle Miocene planation succeeded by: (1) middle Miocene deformation; (2) late Miocene and perhaps early Pliocene erosion; (3) Pliocene deformation, erosion, and deposition; and (4) early Pleistocene deformation or rejuvenation and subsequent erosion.