Miocene Unconformity Research Articles

A New Zealand Late Miocene magnetostratigraphy: glacioeustatic and biostratigraphic correlations

Magnetostratigraphy is described for a sequence of predominantly hemipelagic marine mudstone, 2200 m thick, representing the Late Miocene and earliest Pliocene from the Mangapoike River section, eastern North Island, New Zealand. Together with the early Pliocene magnetostratigraphy previously described from Mangapoike, it now provides a 3000 m thick magnetostratigraphic reference section for New Zealand, ranging in age from 10.5 Ma to 3.5 Ma. Thirteen magnetozones are recognised in the 2200 m described here, and all but the highest are confidently correlated with the polarity timescale from the base of Chron 11 to Chron 7. Correlation of the highest (normal) magnetozone to the timescale is ambiguous, and two possible interpretations are presented, both of which imply a previously unrecognised latest Miocene unconformity. The inferred unconformity is supported by biostratigraphy. It is shown that the integration of biostratigraphy, lithostratigraphy and magnetostratigraphy can discern diastems of the order of 10 4 years. Magnetostratigraphy andδ 13C data also indicates a diastem between 6.30 Ma to < 5.41 Ma at the Miocene-Pliocene angular unconformity at Mangapoike. These two unconformities and a distinct lithological change at the base of the Late Miocene are synchronous with deep-sea benthicδ 18O records and unconformities expected from postulated eustatic sea-level changes. A coherent correlation model for New Zealand on-shore magnetostratigraphic, biostratigraphic and isotopic data conflicts with the two magnetostratigraphic interpretations proposed for DSDP Site 594, 150 km east of New Zealand, both of which are probably incorrect.

Miocene Unconformities, Chronostratigraphy, and Sea Level Cyclicity: Fine-Tuning Early Neogene Relative Coastal Onlap Curve for North Carolina Continental Margin: ABSTRACT

Miocene Unconformities, Chronostratigraphy, and Sea Level Cyclicity: Fine-Tuning Early Neogene Relative Coastal Onlap Curve for North Carolina Continental Margin: ABSTRACT

Geology of the Dangerous Grounds, South China Sea, and the Continental Margin off Southwest Palawan: Results of SONNE cruises SO-23 and SO-27

Multichannel reflection-seismic, magnetic, and gravity data were recorded along 51 profiles in the southeastern part of the South China Sea, predominantly in the Dangerous Grounds and in the Palawan Trough, during SONNE cruises SO-23 in 1982 and SO-27 in 1983. These investigations were combined with geological sampling. Five unconformities of regional extent were recognized in the study area: a Miocene-Pliocene unconformity, a Middle Miocene unconformity which coincides with the end of seafloor spreading in the South China Sea basin, a Lower Miocene unconformity which often marks the top of a carbonate platform in the South China Sea, a Middle to Upper Oligocene unconformity representing the transition from the rift to the drift phase in the South China Sea and a Cretaceous-Paleocene unconformity interpreted as the onset of rifting. The Dangerous Grounds and possibly also parts of the Palawan Trough are underlain by a stretched crust of continental origin. The oldest rocks sampled in the Dangerous Grounds are of Mesozoic age (Upper Triassic to Jurassic). Marine geophysical data indicate the existence of an Upper Oligocene to Lower Miocene carbonate platform that extends from the Dangerous Grounds eastward beneath the Palawan Trough and beneath the central and south Palawan shelf. Hence central and south Palawan are interpreted as being part of a unique microcontinent which incorporates the Dangerous Grounds/Reed Bank, north Palawan, and the Calamian block. The eastern edge of this carbonate platform is overlain by a wedge of chaotically deformed allochthonous sediments which have been overthrust from the south onto the carbonate platform. These results imply that the Palawan Trough does not represent the location of an ancient subduction zone but is an expression of the elastic downwarp of the crust due to isostatic compensation for the thick overthrust allochthonous wedge.

Geology of Canadian Arctic Basin: ABSTRACT

The Canadian Beaufort Sea is underlain by up to 12,000 m of Upper Cretaceous to Quaternary sediments. These sediments were deposited in a series of prograding depositional complexes containing fluviodeltaic, shelf, slope, and basinal clastics. At least 10 major regional unconformities within the Upper Cretaceous to Quaternary section are recognized. The 2 most pronounced unconformities developed in late Eocene and late Miocene times. In the Beaufort-Mackenzie basin, the Upper Cretaceous to upper Miocene sediments were deformed primarily by gravity tectonics into large-scale diapiric anticlines and rotated listric fault-bounded blocks. Some reverse faulting, associated with mud diapirism, is evident in the western part of the basin. The late Miocene unconformity separates the deformed sediments below from the essentially undeformed, overlying Pliocene-Pleistocene strata. Beneath the eastern Beaufort Sea, seaward of Amundsen Gulf and Banks Island, the Upper Cretaceous and younger strata thicken abruptly basinward across a hinge line developed at the edge of the Arctic platform (mostly composed of pre-Mesozoic strata). The Upper Cretaceous to Quaternary sedimentary prism along this part of the continental margin is not disrupted by diapirism or listric faulting; in fact it is relatively undeformed, with only minor normal faulting in the older part of the prism. End_of_Article - Last_Page 250------------

Seismic-stratigraphic framework of the forearc basin off central Sumatra, Sunda Arc

New multichannel seismic reflection data provide information on the stratigraphic framework and geologic history of the forearc basin west of central Sumatra. We recognize six seismic-stratigraphic sequences that reflect the Cenozoic history and development of the outer continental shelf and forearc basin southeast of Nias Island. These sequences indicate several episodes of uplift of the subduction complex and filling of the forearc basin. Early in the development of this margin, Paleogene slope deposits prograded onto the adjacent basin floor. Onlapping this assemblage are two units interpreted as younger Paleogene(?) trough deposits. Uplift associated with rejuvenation of subduction in the late Oligocene led to erosion of the Sumatra shelf and formation of a regional unconformity. The early Miocene was a period of significant progradation. A Miocene limestone unit partly downlaps and partly onlaps the older Paleogene deposits. It is characterized by shallow shelf and oblique progradational facies passing into basin floor facies. A buried reef zone occurs near the shelf edge. The cutting of an erosional unconformity on the shelf and slope in late Miocene/early Pliocene time culminated this episode of deposition. In the late Pliocene, a large flexure developed at the western boundary of the basin, displacing the outer-arc ridge upward relative to the basin. Over 1 km of Pliocene to Recent sediment was deposited as a wedge in the deep western portion of the basin landward of the outer-arc ridge. These deposits are characterized by flat-lying, high-amplitude, continuous reflections that overstep the late Miocene unconformity. Up to 800 m of shallow-water limestone have been deposited on the shelf since mid-Pliocene time.

A modern oceanic hardground on the Carnegie Ridge in the eastern Equatorial Pacific

ABSTRACTFossil surfaces of erosion and non‐deposition are common in limestone sequences from the Mesozoic in western Europe and in the Tethys and have been described under the name ‘hardgrounds’. They are of shallow water as well as of oceanic origin. A modern example in the Pacific is described in this paper. The Carnegie Ridge, an east‐west trending shallow ridge between South America and the Galapagos Islands, has a central, deeper saddle where erosion has removed most of the sediment cover down to a hard chalk and chert bed (acoustic basement), and has cut intricate channel patterns on the south flank and two deep canyons on the north. The erosion has produced a karst‐like relief of steep‐walled channels, cliffs, and corroded chalk remnants. The floors of the channels are covered with ferromanganese oxide crusts or crust fragments over which loose sediment is being transported. In the two canyons on the north flank, this sediment consists of foraminiferal sand travelling downslope in the form of barchan dunes. All sediment down to acoustic basement has been stripped from the Carnegie Ridge crest except where it is protected behind basement ridges and pinnacles. Surface features of the eroded chalk are strongly reminiscent of features observed in Mesozoic hardgrounds. Current measurements over several days indicate a net northward movement, slow but possibly adequate to keep the sea floor free of fresh deposits. The rates, however, seem inadequate to explain the formation of the deep channels, and there is no evidence for the southward flow which is implied by the southern channel system. A process of combined carbonate dissolution and removal by the current of fresh sediment and dissolution residues can account for the required erosion in about two million years. Regional unconformities identifiable in seismic reflection profiles and dated in cores are of middle to late Pliocene age, suggesting that the formation of the erosion surface began 2‐3 million years ago. Buried Miocene unconformities of local extent show that the present erosion period had minor precursors possibly related to short‐lived increases in current action and carbonate dissolution.

Structure of Upper Cenozoic Strata Beneath Kodiak Shelf, Alaska

The structure of upper Cenozoic strata beneath the Kodiak Shelf has been determined from an interpretation of 3,500 km of 24-fold seismic reflection data. The shelf southeast of Kodiak Island is divided into three segments that are defined by depth to a regional unconformity at the base of upper Miocene or Pliocene strata. Two of the segments are basins, the third is a high-standing area between them. Albatross basin, under the southwestern end of the shelf, contains as much as 5 km of undeformed to moderately deformed fill. The seaward limit of the basin is Albatross Bank, which began to grow in the late Miocene or Pliocene. The Dangerous Cape high makes up the central shelf segment and separates Albatross basin from Stevenson basin, which is on the northeast flank of the high. The basal late Miocene or Pliocene unconformity is generally at shallow depth (1 to 2 km) in the area of the high and is nearly flat, except in an area midway between Kodiak Island and the shelf break where numerous reverse faults disrupt the unconformity. Stevenson basin is the northeasternmost shelf segment. Portlock anticline strikes northwest and divides Stevenson basin into a northeast and a southwest subbasin. The northeast subbasin is larger and contains as much as 5 to 7 km of undeformed to gently deformed fill. The southwest subbasin contains as much as 3.5 km of fill. A large submarine channel once cut into the fill in the southwest subbasin. Growth of structures that form the local shelf break dammed the channel, causing it to fill. Strata that overlie a regional unconformity may be of late Miocene and younger age. A lens of strata in Albatross basin may be older than late Miocene. The unconformity at the base of upper Miocene or Pliocene strata may be a subaerial or shallow submarine erosion surface. Prior to the late Miocene, when the present shelf began to submerge, it was at least partly emergent. Local uplift of large shelf-break structures began in the late Miocene or Pliocene. At Albatross Bank the uplift occurred at a minimum rate of 300 m/m.y.

Paleocurrents in the eastern Caribbean: Geologic evidence and implications

Late Cretaceous and Cenozoic sedimentary strata in the southern Venezuela Basin thin to less than 200 m across an east—west crescent-shaped depositional minimum which is 50–100 km wide and more than 400 km long. Drilling has revealed the presence of Santonian-to-late Paleocene and early Eocene-to-early Miocene unconformities along the axis of thinnest sediments. Sediments also thin over the crests of the Beata and Aves ridges, where unconformities in the late Cretaceous-to-Miocene time frame have been revealed or inferred by drilling. The relationship of sediments and structure on the Aves and Beata ridges suggests that these ridges were structural prominences for the duration of time represented by the unconformities. Apparently the Venezuela Basin unconformities are a record of long periods of nondeposition in a regime of sediment transport by currents. Renewed deposition across the unconformity in Miocene time correlates with renewed uplift and volcanism in Panama and the Lesser Antilles, which probably decreased water depth and cross-section to the point of restricting circulation. Renewed deposition in the Miocene also correlates with subsidence of the Aves Ridge, and this could have been another factor in the alteration of the current regime. We suggest that westward flowing currents swept the major crests of the Aves and Beata ridges and the unconformity area of the Venezuela Basin before closure of the Atlantic-Pacific marine connection through the Caribbean.

Miocene Unconformity, Pamlico River Area, North Carolina

A mid-Miocene angular unconformity appears on records from a high resolution “Boomer” survey of the Pamlico River, North Carolina. This unconformity is within the Yorktown Formation, and it is interpreted as indicating a shift in basinal axes during the deposition of the Yorktown Formation.

Cyclic Steam Project in a Virgin Tar Reservoir

Abstract This article discusses the production techniques and results obtained by the first two cyclic steam injection wells in the virgin Vaca Tar sand. The two wells drilled by American Petrofina Co. of Texas and partners are located 1,700 ft apart. One well was partially successful before being shut in due to mechanical problems. The second well is now in its fifth cycle and has recovered 43,900 bbl of the 5 deg. API oil in 18 months. This project illustrates a situation where thermal recovery has provided an effective production technique for a reservoir which previously had been considered nonproductive. Introduction The Vaca Tar sand is in the Oxnard field, Ventura County, Calif. The field lies just east of the city limits of Oxnard which is located on the Pacific coast 40 miles north- west of Los Angeles. The reservoir. defined by deeper wells in the Oxnard field, covers approximately 1,700 acres. Several early completion attempts in the Vaca Tar sand were unsuccessful due to sand influx resulting from the combination of a viscous crude and an unconsolidated sand. A cyclic steam project was initiated to determine if the viscous crude could be produced by thermal techniques. When the viscosity of the crude around the wellbore was lowered by steam injection, the wells could produce the crude through gravel-packed completions without having sand entry. Production results show that the reservoir has prolific production capacity under cyclic steam operations. History The Vaca Tar sand was discovered in Jan., 1937, by Vaca Oil Exploration Co.'s Well 1 located in the south-west corner of what is now American Petrofina's Vacca Transamerica lease. The well established a production rate of 50 B/D but was abandoned because production could not be maintained due to sand influx. In May, 1937, a second well in the area discovered low-gravity oil in a fractured shale reservoir 500 ft below the base of the Vaca Tar sand. Subsequent drilling was to reach the low-gravity oil in the fractured shale, and the Vaca Tar sand was not exploited. Attempts were made to recomplete three such wells in the sand as salvage projects before abandonment. The zone was gun perforated, but sand influx resulted. In 1953, oil was discovered in the Sespe zone at 6,000 ft, and wells drilled to this formation provided additional geologic control for the Vaca Tar sand. The Vaca Tar sand remained in a virgin state until Dec., 1964, when American Petrofina initiated the thermal recovery project by drilling and steaming Vacca Transamerica Well 203. In Feb., 1965, three core holes were drilled on the Hunsucker tract, a northeast offset to the Vacca TransAmerica lease, to gain more reservoir data and establish the continuity of the sand in this area. In May, 1965, Well 203 was shut in due to down-hole mechanical problems. Vacca Transamerica Well 702 was completed in June, 1965, and has been produced by cyclic steam injection since that time. In partnership on the project with American Petrofina are Helmerich and Payne, Inc, and Kewanee Oil Co. Geology The Vaca Tar sand is Pliocene in age and overlies a Miocene unconformity in most areas.* The sand is fine- to medium-grained with scattered coarse grains and pebbles. Cores show the sand to be totally unconsolidated with sand grains held together by the viscous tar. The structure is a northeast-southwest trending homocline, dipping about 151 to the northwest. The Vaca Tar sand has a productive area of approximately 1,700 acres.* Average depth to the top of the pay in American Petrofina's project, located in the northeast portion of the reservoir, is 1,870 ft. Of the five wells drilled by American Petrofina, one did not penetrate the entire pay section and the other four had gross pay thicknesses ranging from 103 to 480 ft with corresponding net pay thicknesses ranging from 76 to 435 ft. Fig. 1 is a map of the net pay. Fresh water lenses exist in the upper portion of the sand in some wells, the most prominent of which is in the Vacca Transamerica Well 203 (Fig. 2). JPT P. 585ˆ