mid-Cretaceous Unconformity Research Articles

Provenance changes across the mid-Cretaceous unconformity in basins of northeastern China: Evidence for an integrated paleolake system and tectonic transformation

Abstract Detrital zircon U-Pb dating and whole-rock Nd isotopic analyses were carried out on selected stratigraphic horizons across a major unconformity between synrift and postrift stages in the Songliao Basin and Dasanjiang basin group of NE China to constrain the crustal evolution of the source area providing detritus into these basins. The strata underlying the mid-Cretaceous unconformity in the Songliao Basin show regionally distinct detrital zircon age populations and Nd isotopic compositions, which generally are characterized by Phanerozoic age peaks and relatively depleted Nd isotopic compositions, indicating derivation from nearby highlands. In contrast, the overlying strata are dominated by Proterozoic zircon ages and enriched Nd isotopic compositions, which imply that the provenance source region shifted to the northern part of the North China craton. A coeval provenance change also affected the sedimentary architecture in the eastern Dasanjiang basin group, marking the migration of erosion centers from west to east. The contribution from Lesser Xing’an–Zhangguangcai ranges was pronounced during deposition of synrift strata but became negligible afterward in the Songliao and Dasanjiang areas, which is consistent with both basin complexes temporarily forming an extensive lake system during the early Late Cretaceous. This paleolake was likely responsible for transgressive events recorded in the Late Cretaceous strata of basins in NE China. Combining observations from seismic reflection profiles and the stratigraphic record of neighboring intracontinental sedimentary basins as well as widespread contemporaneous exhumation and denudation events, we suggest that the provenance variation in basin strata was controlled by large-scale tectonic transitions in East Asia. The switch from extension to contraction during the mid-Cretaceous is attributed to the docking of the Okhotomorsk block along the East Asian continental margin. The resulting lithospheric buckling might have been responsible for reshaping the basin-and-range configuration in NE Asia.

Paleoenvironments, diagenesis, and sequence stratigraphy of the mid-cretaceous section (albian/cenomanian) in the Main Pass Field area, northeastern Gulf of Mexico

The Main Pass Block 253 Field, northeastern Gulf of Mexico, produces oil from mid-Cretaceous carbonate reservoirs at 8,700 to 8,900 feet. The field is located on Main Pass Blocks 253 and 254 and is situated along the present-day continental shelf margin, which closely approximates the shelfedge during Early Cretaceous time. This field is significant because it demonstrates the oil potential of Lower Cretaceous carbonates in the northeastern Gulf of Mexico. Core and thin-section studies of the Washita reservoir rocks, Late Albian to Early Cenomanian in age, show that these carbonates accumulated as part of a reef-shoal complex in a carbonate rimmed-shelf system. A general progradational trend in the section is evident: shelf-margin reef facies are overlain by back-reef and patch-reef facies represented by middle-to-inner-shelf carbonate deposits. Overall, 10 distinct cycles grading upward from mud-supported bafflestone, wackestone, and packstone to grain-supported rudstone and grainstone are observed in core. These coarsening-upward cycles reflect an increase in energy upsection. The boundaries of the cycles are defined by physical surfaces of erosion or non-deposition, or by abrupt changes in facies. The major constituents in the reef facies are rudists, including caprinids and radiolitids. These organisms are in life position in the lower part of the sequence. A major unconformity is observed in the core. Washita carbonates are overlain by Campanian chalk. The chalk was dated using nanofossil associations. This unconformity is the regional mid-Cretaceous unconformity of the northeastern Gulf of Mexico as recognized on seismic profiles. This unconformity probably contributed to the intense diagenetic alteration of the Washita carbonates observed in core. Diagenetic processes observed in the core include dolomitization, neomorphism, pressure solution, and karstification.

Change of tectono-stratigraphic regime in the Australian plate during the 99 Ma (mid-Cretaceous) and 43 Ma (mid-Eocene) swerves of the Pacific

The clockwise bend at 99 Ma (mid-Cretaceous) in linear volcanic chains in the tropical Pacific coincides with a change from pre-99 Ma head-on Chilean-type subduction of the Pacific plate beneath eastern Gondwana to 99–43 Ma sinistral oblique Mariana-type subduction and strike-slip breakup by simple sea-floor spreading between Australia and Antarctica and by backarc spreading in the southwest Pacific. The 99 Ma breakup of Australia from Antarctica is documented by a mid-Cretaceous unconformity. This tectono-stratigraphic change founded modern Australia, with a mountain chain along an upper plate margin in the east and lowlands on the lower plate margin in the south. The counterclockwise bend at 43 Ma (mid-Eocene)—the Emperor-Hawaiian bend—coincides with the onset of structure in the Challenger Rift of New Zealand, the Eromanga-Cooper basin of central Australia, and the oil-shale grabens of coastal Queensland.

Low-Temperature Thermal History Using Fission Track Dating in Three Wells in Southern Alaska Offshore Basins: Lower Cook Inlet, Shelikof Strait, and Stevenson Trough

Three wells, all offshore, in southern Alaska studied using apatite fission track dating make a transect southward from Lower Cook Inlet to the Kodiak Shelf and include ARCO Lower Cook Inlet COST #1 well (LCI well), Chevron OCS-0248 #1A well (Shelikof Strait), and Kodiak COST KSSD #1 OCS 77-1 well (Stevenson Basin, Kodiak). The ages of deep partially annealed samples from Lower Cook Inlet well suggest that the region cooled between ~100 and 75 Ma and or sometime after. Two scenarios are presented: (1) maximum heating before cooling in Late Creta ceous times and (2) maximum heating before cooling during mid-Tertiary times. Which is better is uncertain from the thermal and age data alone, but mid-Tertiary or later uplift, erosion, and cooling is preferred because data from Shelikof well suggests that the mid-Cretaceous unconformity was minor relative to the mid-Tertiary unconformity. Finally, because of the ~12 C warmer past than present bottom-hole temperature, the base of the LCI well is now ~500 m shallower than during maximum, burial (12 C/24 C/km geothermal gradient). Single-grain apatite fission track ages (20-25 Ma) from deep in the Shelikof well approach the age of the overlying mid-Tertiary (Miocene; ~23 Ma) unconform ity, suggesting significant and rapid exhumation. This suggests that strongly annealed, once deeply buried strata were uplifted and cooled quickly prior to onlap of the unconformity. The Miocene unconformity, therefore, is interpreted to be the major unconformity in the Shelikof well section. In this scenario the section was buried deepest, and was therefore hottest, until the onset of mid-Tertiary erosion. Approximately 665 m of late Tertiary and Quaternary strata have since been deposited in Shelikof Strait and have reburied the Shelikof section to within ~536 m of its original maximum burial depth. Including modern water depth, the Shelikof well section has experienced ~1 km of burial+submergence since ~25 Ma (832 m section+166 m water=998 m). It follows that the depth to the base of the well is now ~290 m shallower than it would have been during maximum burial. Single-grain apatite fission track ages deep in the Kodiak KSSD1 well are as young as 20-25 Ma and approach the age of overlap of a mid-Miocene regional unconformity (<23 Ma). The deepest Eocene samples were exhumed to within 574 m of the Miocene unconformity surface during Miocene time and were reburied by ~1.7 km of late Tertiary strata. The total section before exhumation was ~5 km; this suggests that Oligocene-age deposits may have existed in the Stevenson Basin. Together with the known Eocene strata, such deposits were exhumed during ~4.4 km of uplift and erosion during a short interval culminating in early to middle Miocene times (>25-23 Ma). Unique and anomalous apatite compositions (high F-, low Cl-, moderate OH-) from the Eocene section could provide a chemical tracer for determining their sediment source along the northeast Pacific rim prior to translation and accretion.

Hotspot origin of the Mississippi embayment and its possible impact on contemporary seismicity

Previous authors have related the Late Cretaceous/early Tertiary subsidence of the Mississippi embayment to the opening of the Gulf of Mexico, but the Gulf opened earlier in Triassic/Jurassic time. We offer an alternative hypothesis that development of the embayment was coeval with the passage of the Mississippi Valley graben system over the Bermuda hotspot about 90 Ma. Several lines of evidence of significant uplift of the embayment axis accompanying mid-Cretaceous magmatism and prior to Late Cretaceous subsidence support this proposal. First, reactivation of the Pascola arch in the northern embayment is recorded by flanking deposits of basal Upper Cretaceous gravel. Second, beneath a regional mid-Cretaceous unconformity, subcrops of Jurassic and Early Cretaceous strata define a pronounced southwest-plunging arch in the southern embayment. This arch is collinear with an arch revealed in Paleozoic rocks after restoration to mid-Cretaceous structural geometries. Third, a deep weathering profile on mid-Cretaceous alkalic plutons along the western embayment margin is nonconformably overlain by Paleocene sediments, and rapid mid-Cretaceous cooling of these intrusions has been interpreted from apatite fission tracks. Moreover, exploratory holes along the embayment axis encountered similar weathered alkalic intrusions nonconformably overlain by basal Upper Cretaceous strata. Fourth, there was an anomalous influx of clastic sediment into the northern Gulf of Mexico during mid-Cretaceous time, and subsequent clastic facies patterns suggest the Mississippi River drainage began to enter the Gulf in the Late Cretaceous. Passage of the Mississippi Valley graben over the Bermuda hotspot during elevated hotspot activity of Cretaceous time may have significantly weakened the previously rifted lithosphere. Rifted continental margin at Charleston, South Carolina, also passed over this hotspot in latest Cretaceous time. Similarly, the St. Lawrence rift system passed over the Great Meteor hotspot during the Cretaceous. It is important to note that these rift systems are the principal loci of strong seismicity in eastern North America, and thus weakening by increased Cretaceous hotspot activity may be an important common factor for these seismic rift zones.

Crustal structure and tectonic evolution of the anza rift, northern Kenya

The Anza trough is a Mesozoic rift located in northern Kenya that appears to be the failed third arm of a paleo-triple junction which allowed the separation of Madagascar from Africa during the Jurassic. The rift is oriented NW-SE and its tectonic evolution is related to that of the Mesozoic southern Sudan rift system. We analyzed seismic and gravity data from the southwestern side of the Anza rift including the Chalbi Desert to gain a better understanding of rift structure. Gravity data delineate the main rift basins as well as a small sub-basin on the southwest side of the main rift. Normal faulting evident on the NW end of a 42-km-long, NW-SE oriented Vibroseis® profile, marks the western boundary of the sub-basin. This sub-basin is offset from the trend of the main Anza trough; the western boundary may be a complex fault zone accommodating a change in direction of the main rift trend. Gravity values increase to the NW in the faulted area, suggesting shallowing of basement. A strong NW-dipping reflection from 0.5 s to almost 3 s is interpreted as a pre- to mid-Cretaceous unconformity. The configuration of the unconformity and the normal faulting strongly resembles the half-graben geometry imaged in the East African Rift. Numerous discontinuous reflections can be seen deeper in the section between 6 and 9 s, but a distinct reflection Moho cannot be interpreted with certainty. In addition to seismic and gravity data, regional geologic and well data lead us to conclude that there are probably Jurassic marine sediments in the bottom of the Anza rift.

Depositional bias and environmental change—important factors in sequence stratigraphy

Differences among depositional systems, here called depositional bias, strongly influence sequence patterns. Siliciclastics and shallow-water carbonates, for instance, shed most of their sediment during opposite phases of a sea-level cycle (lowstand shedding and highstand shedding, respectively). Furthermore, the two systems generate their own, system-specific relief on the sea floor, disperse their sediment load along different avenues and differ in the way they are deactivated: reefs and carbonate platforms can be drowned, whereas siliciclastic deposition can be shut off and renewed at any depth. As a consequence of these differences, pronounced unconformities (drowning unconformities) develop where carbonate platforms are terminated and buried by siliciclastics (the siliciclastic-to-carbonate transition tends to be more gradual). Drowned platforms and drowning unconformities appeared world-wide in great abundance in the Miocene, Cretaceous (Valanginian-Turonian), Jurassic (Toarcian) and Devonian (Frasnian/Famennian). Examples of drowning unconformities interpreted as sequence boundaries, include those of the Early Cretaceous platforms off New Jersey and off Morocco, the mid-Cretaceous unconformity in the Gulf of Mexico and Miocene unconformities on top of reefs in the Far East.The eustatic cycles postulated from sequence stratigraphy are a very unlikely cause for the mass drownings of reefs and platforms: either the rates of rise are an order of magnitude lower than the growth potential of platforms, or cycle amplitudes are too small, or the cycles are too short and subsidence too slow to remove platforms permanently from the photic zone. The critical element in the mass drowning seems to be environmental stress that reduces the growth potential of carbonate systems. Thus, drowning unconformities demonstrate the importance of environmental change as a control on sequence development. Other examples of dominantly environment-controlled sequences include slope deposits shaped by shifting currents (e.g. Florida and Blake Plateau) and basin fills that were controlled by changes in sediment input related to tectonic alteration of the drainage in the hinterland (e.g. Gulf of Mexico).Environmental change must be considered as a third, independent factor that competes with eustatic and tectonically driven regional changes of sea-level for control of sequences. The definition of sequences and sequence boundaries should be broad enough to include the possibility of non-sea-level controls.

Seismic stratigraphy of the mid‐Cretaceous unconformity (MCU) in the central Gulf of Mexico basin

A widespread high‐amplitude reflection seen on seismic data throughout the Gulf of Mexico has been called the “Mid‐Cretaceous Unconformity” (MCU). This reflector seems to be a major stratigraphic boundary in the Gulf of Mexico basin. It is believed to correspond to Vail’s type 1 unconformity of mid‐Cenomanian age (94 Ma), which records an eustatic drop in sea level of approximately 60–100 m. The study area includes all of the Gulf of Mexico with the exception of areas of thick and highly deformed salt, where following the MCU with any degree of confidence becomes impossible. The MCU is easy to follow in multichannel seismic reflection profiles collected in the Gulf of Mexico by the Institute for Geophysics of the University of Texas at Austin. In the central, deeper part of the gulf, reflectors above and below the MCU are parallel. In the southern and eastern rims of the Gulf, along the Campeche and Florida escarpments, reflectors are truncated below the MCU and show onlap relationships above it. The MCU, therefore, may be interpreted as representing an unconformity along the southern and eastern rims of the Gulf of Mexico basin. The uncomformity appears to die out and grade into a conformable section toward the central part of the basin. Channeling is common in the MCU along the Campeche and Florida escarpments, which was probably initiated by subaerial exposure of the top of the escarpments during the mid‐Cretaceous lowstand. The downslope channels were cut in deep water.

Leaching of Heavy Minerals Above and Below the Mid-Cretaceous Unconformity in the Ohuriawa Gorge Area of the Waipara River, North Canterbury, New Zealand

ABSTRACT Contrasting heavy mineral populations rich in 1) ilmenite and zircon, and 2) ilmenite, epidote and semi-opaque debris, exist in both indurated Mesozoic (Torlesse) basement sandstones and in overlying transgressive Cretaceous-Tertiary sandstones around Ohuriawa Gorge. The ilmenite-zircon population forms a zone over 100 m thick below the unconformity at the top of the Torlesse on its eastern boundary northeast of the gorge. The great structural complexity of the Torlesse contrasts with the parallelism between the ilmenite-zircon zone and the unconformity and, together with the lack of any change in overall detrital composition paralleling the change in heavy minerals, shows that the population contrast in the Torlesse rocks must result from alteration that post-dates deformation. The i menite-zircon zone in the overlying Cretaceous-Tertiary sediments extends up to near the Paleocene-Eocene boundary. Apatite in coal measures within this zone has been leached early during diagenesis and may be being affected by acidic pore fluids now associated with weathering of pyrite. The nature and distribution of solution effects on epidote grains in Tertiary samples and the absence of the same effects in Torlesse samples show that the change in the Tertiary sediments has not been inherited from the Torlesse but formed soon after deposition by leaching in a dominantly alkaline pore-fluid regime.

Drowning Unconformities--A Peculiar Response of Carbonate Platforms to Rising Sea Level: ABSTRACT

The slopes of modern carbonate platforms are generally steeper than siliciclastic ones. Furthermore, carbonate slopes steepen as they grow higher, whereas siliciclastic slopes do so only at very early stages of upbuilding. These conclusions are drawn from measurements of over 500 slope profiles in the Atlantic and Pacific. A survey of seismic profiles suggests that the greater declivity of carbonate slopes also holds for the more distant geologic past. A consequence of the oversteepening of carbonate slopes is the drowning unconformity. When carbonate platforms of a certain height are drowned and buried by siliciclastics, the clastics will onlap the carbonates simply because they cannot assume the steep slope angle. The resulting unconformity geometrically will resemble a lowstand unconformity even though it is caused by the demise of the platform, not by a drop in sea level. Well-known unconformities for which the interpretation of drowning unconformities (rather than lowstand unconformities) should be considered include the Early Cretaceous unconformity off west Africa and off eastern North America as well as the mid-Cretaceous unconformity in the Gulf of Mexico. Erosional truncation of the platform is common with drowning unconformities. This erosion need not be related to a lowstand of sea level but rathermore » to the upbuilding stage of the platform. As the platform grows higher, slopes steepen and turbidity currents shift from deposition to erosion, turning the upper slopes of high-rising platforms into bare and erosional features.« less

The geochemistry of carbonate cements in the Avalon Sand, Grand Banks of Newfoundland

AbstractCalcite cements from the Lower Cretaceous Avalon zone of the Hibernia field are, in places, extensively recrystallized, which complicates interpretation of the chemical and isotopic data. The oxygen isotopic data are widely scattered with δ18O ranging between +1.6 and −9.2 for calcite and siderite. Siderite has lower δ13C values (−6.6 to −13.2) than calcite +12.4 to −9.8. Typical trace element contents determined by ICP on acid-leached samples, range from 270 to 2100 ppm Sr and 180 to 2200 ppm Zn in calcite.The trace element data indicate that some of the calcite has been precipitated from, or recrystallized by meteoric water. The trace elements show trends related to variations in δ18O in such a way as to imply that not all the spread to low δ18O values can be attributed to meteoric water influence alone. The data are not well enough constrained to calculate meaningful temperatures, but the range of °18O values probably represents an elevated range of temperatures of precipitation or recrystallization.Microprobe analyses show that non-recrystallized fossils have a composition distinctly different from veins, cements, and recrystallized fossils, all of which are similar. The compositions of calcite cements are highly variable, with FeO (for example) ranging from 0.15 to 4.39 wt. %, but show no consistent patterns of zonation. Fossil fragments which show no textural evidence of recrystallization have low FeO contents (0.2 wt. %). Meteoric water, believed to be responsible for at least some of the cementation and recrystallization observed, probably entered the Avalon during and after formation of the mid-Cretaceous unconformity.

Deep Sea Drilling Project, Leg 77, southeastern Gulf of Mexico

In January 1981, R/V Glomar Challenger drilled five holes in the southeastern Gulf of Mexico to provide ground data for extensive seismic surveys and to document the pre-Tertiary history of the Gulf. Holes 535 and 540 were drilled in a basinal terrane for maximum penetration of the Cretaceous-Tertiary sequence. Rhythmic alternations of light bioturbated and dark laminated carbonaceous limestone represent the Early Cretaceous interval. Some of the dark layers are rich but immature oil source rocks. The limestones resemble the Blake-Bahama Formation in the North Atlantic but their stratigraphic age overlaps in part with the Hatteras Shale. Late Cretaceous rocks are almost totally missing in the basin sites and the Cenozoic section consists of chalk and marly carbonate ooze. Holes 536,537, and 538A were drilled on high-standing fault blocks. Hole 537 recovered phyllite that records 40 Ar/ 39 Ar plateau ages of about 500 m.y. and is overlain by an Early Cretaceous deepening sequence of alluvial to littoral elastics and oolitic-oncolitic limestones, capped by a thin sequence of Cretaceous and Cenozoic pelagics. In Hole 538A, basement consists of mylonitic gneiss and amphibolite, intruded by several generations of diabase dikes (that is, “transitional” crust). 40 Ar/ 39 Ar dates of hornblendes and biotite from the regional metamorphic rocks suggest a 500-m.y. (“Pan-African”) age with mild late Paleozoic thermal overprint. 40 Ar/ 39 Ar whole-rock dates from the dikes suggest intrusions between 190 and 160 m.y. ago. Basement is covered by a thin layer of pelagic chalk, followed by Early Cretaceous skeletal-oolitic limestones and, finally, Cretaceous-Tertiary pelagics. The oolitic-oncolitic limestones at both sites represent either parts of a shallow-water carbonate platform or platform talus deposited in deep water. Hole 536 bottomed in shallow-water dolomite (Jurassic or Permian), overlain by middle Cretaceous skeletal limestones with shallow-water biota and intercalations of pelagic chalk, interpreted as Cretaceous talus at the foot of the Campeche Bank. Cretaceous-Tertiary chalk and carbonate ooze cap the sequence. Among the most significant results of the leg are: (1) recovery of “transitional” crust with early Paleozoic (Pan-African) metamorphic rocks, (2) recovery of Early Cretaceous deep-water limestones with immature petroleum source beds, (3) recovery of mid-Cretaceous platform talus resembling the reservoirs in the Poza Rica and probably some of the Reforma fields of Mexico, and (4) discovery of a Late Cretaceous hiatus of 30 m.y. that corresponds approximately to the “mid-Cretaceous unconformity” recognized widely on seismic records in the Gulf of Mexico.