Robert L. Brenner

Meteoric sphaerosiderite lines and their use for paleohydrology and paleoclimatology

Sphaerosiderite, a morphologically distinct millimeter-scale spherulitic siderite (FeCO 3 ), forms predominantly in wetland soils and sediments, and is common in the geologic record. Ancient sphaerosiderites are found in paleosol horizons within coal-bearing stratigraphic intervals and, like their modern counterparts, are interpreted as having formed in water-saturated environments. Here we report on sphaerosiderites from four different stratigraphic units, each of which has highly variable 13 C and relatively stable 18 O compositions. The unique isotopic trends are analogous to well-documented meteoric calcite lines, which we define here as meteoric sphaerosiderite lines. Meteoric sphaerosiderite lines provide a new means of constraining ground-water δ 18 O and thus allow evaluation of paleohydrology and paleoclimate in humid

The mid-cretaceous water bearer: Isotope mass balance quantification of the Albian hydrologic cycle

A latitudinal gradient in meteoric N 18 O compositions compiled from paleosol sphaerosiderites throughout the Cretaceous Western Interior Basin (KWIB) (34^75‡N paleolatitude) exhibits a steeper, more depleted trend than modern (predicted) values (3.0x [34‡N latitude] to 9.7x [75‡N] lighter). Furthermore, the sphaerosiderite meteoric N 18 O latitudinal gradient is significantly steeper and more depleted (5.8x [34‡N] to 13.8x [75‡N] lighter) than a predicted gradient for the warm mid-Cretaceous using modern empirical temperature^N 18 O precipitation relationships. We have suggested that the steeper and more depleted (relative to the modern theoretical gradient) meteoric sphaerosiderite N 18 O latitudinal gradient resulted from increased air mass rainout effects in coastal areas of the KWIB during the mid-Cretaceous. The sphaerosiderite isotopic data have been used to constrain a mass balance model of the hydrologic cycle in the northern hemisphere and to quantify precipitation rates of the equable ‘greenhouse’ Albian Stage in the KWIB. The mass balance model tracks the evolving isotopic composition of an air mass and its precipitation, and is driven by latitudinal temperature gradients. Our simulations indicate that significant increases in Albian precipitation (34^52%) and evaporation fluxes (76^96%) are required to reproduce the difference between modern and Albian meteoric siderite N 18 O latitudinal gradients. Calculations of precipitation rates from model outputs suggest mid^high latitude precipitation rates greatly exceeded modern rates (156^220% greater in mid latitudes [2600^3300 mm/yr], 99% greater at high latitudes [550 mm/yr]). The calculated precipitation rates are significantly different from the precipitation rates predicted by some recent general circulation models (GCMs) for the warm Cretaceous, particularly in the mid to high latitudes. Our mass balance model by no means replaces GCMs. However, it is a simple and effective means of obtaining quantitative data regarding the mid-Cretaceous hydrologic cycle in the KWIB. Our goal is to encourage the incorporation of isotopic tracers into GCM simulations of the midCretaceous, and to show how our empirical data and mass balance model estimates help constrain the boundary conditions.

Black Sea-Marmara Sea Quaternary connections: new data from the Bosphorus, Istanbul, Turkey

Abstract Previous studies concluded that the Bosphorus Strait was formed during the Quaternary by fluvial incision of a valley between the Black Sea, to the north, and the Marmara Sea in the south. Hitherto, however, few details of the evolution of this connection have been elucidated from the sediments deposited within the Bosphorus itself. We report here details of sedimentological and palaeontological evidence relating to this history, obtained from five boreholes drilled into the unconsolidated sediment fill in the north-central sector of the Bosphorus, together with nearby geophysical profiles. The Quaternary fill of this part of the Bosphorus comprises two major facies associations. Yellow arkosic sands dominate the lower Facies Association A: these are assigned a Middle to Late Pleistocene age and the contained faunas have a lagoonal to lacustrine character and a Black Sea provenance (Paratethyan affinities). The abruptly succeeding units of Facies Association B comprise fining and coarsening upwards units of coarse to fine shelly and clayey sands that alternate with shell-bearing green clays. These sediments were formed in a range of marine and coastal settings and biostratigraphic evidence and absolute dating demonstrate the Mid–Late Holocene age of this upper unit. Initially brackish faunal assemblages in this upper unit show an upward increase in marine and Mediterranean affinities. Integrating these new data with previously published observations from coeval deposits in the southern Bosphorus and Izmit Bay (NE Marmara Sea) we conclude that during the Late Pleistocene and Early Holocene a topographic barrier existed in the south-central sector of the Bosphorus, on both sides of which estuarine and lagoonal sediments accumulated, with distinctive Black Sea and Mediterranean faunas. During a significant rise in sea level, between 7000 and 5300 years ago, this barrier was finally submerged, permitting interchange of marine waters between the Mediterranean and the Black Sea and creating the present oceanographic situation. This evolution conflicts with the cataclysmic role of the Bosphorus in the early Holocene as postulated in the ‘Catastrophic Flood’ hypothesis of Ryan et al. [Mar. Geol. 138 (1997) 119–126; Annu. Rev. Earth Planet. Sci. 31 (2003) 525–554]. It also contrasts with the history recorded from the Gulf of Izmit, where intermittent connection between these two bodies of water throughout much of the Quaternary is evident.

GEOHISTORY ANALYSIS AND PETROLEUM RESERVOIR CHARACTERISTICS OF LOWER CRETACEOUS (NEOCOMIAN) SANDSTONES, EASTERN KOPET-DAGH BASIN, NORTHEASTERN IRAN

The eastern part of the Kopet-Dagh basin of northeastern Iran contains over 4000 m of Upper Jurassic through Tertiary strata deposited in a variety of shallow-marine and terrestrial environments. Geohistory diagrams from well and outcrop data provide a useful mechanism with which to relate the stratigraphic framework of this part of the basin to the tectonic history of the region. During some episodes of regional tectonic uplift (e.g., episodes occurring 99-95 Ma, 74-70 Ma, and 63-54 Ma), sediment accommodation space continued to be created in the basin due to sediment loading and compaction of increased amounts of fine-grained sediments, in some cases concomitant with eustatic sea level rises. Much of the post-Jurassic subsidence in this part of the Kopet-Dagh basin was aused by sediment loading rather than tectonism. The effects of basin geohistory on petroleum reservoir properties were studied using the Lower Cretaceous (Neocomian) Shurijeh Formation as an example. Detailed petrologic, sedimentologic, and geohistory analyses done on this formation show that the petroleum reservoir properties of Shurijeh sandstones were affected by their depositional settings and the subsequent subsidence of these units through meteoric and compactional hydrogeologic regimes in this part of the Kopet-Dagh basin. These rocks consist mostly of sublitharenitic red beds deposited during a regressive phase of sedimentation dominated by rapid siliciclastic sediment supply. The lower and middle parts of the interval studied were deposited in low-sinuosity braided fluvial systems, and the upper part was deposited in high- inuosity meandering systems. By relating the paragenetic sequence of the Shurijeh sandstones to the geohistory of this formation, we determined the timing of both porosity-destroying and porosity-enhancing diagenetic processes and related these processes to the timing of petroleum generation.

Oxfordian Sedimentation in Western Interior United States

Upper Jurassic (Oxfordian) sediments of the western interior United States were deposited in part of an epicontinental seaway which extended south from the present arctic. This seaway contained a complex of tectonic elements which influenced significantly the geographic distribution of depositional environments. Sedimentation was divided into an initial transgressive phase followed by a regressive phase. Four distinct facies were developed during the transgressive-regressive sequence. (1) The nearshore sand facies, restricted to the western part of the seaway, is characterized by medium- to coarse-grained chert-rich sandstone, chert conglomerate, and wood fragments. The composition, thickness, and geometry of this facies were influenced by the rising westerly source and a preexisting high (the Belt Island trend), as well as by lateral migrations of sediment depocenters. (2) The mud facies was immediately east of the nearshore marine facies. Clay shale, silty shale, siltstone, and argillaceous limestone dominate the rocks of this facies. (3) The mixed carbonate-clay facies developed only in areas removed from the westerly source of sand-size detritus. In such areas, east of the mud facies, sedimentation included both detrital (clay and silt) and chemical (calcium carbonate) components. As a result, this facies consists of highly calcareous clay shale with minor sandy limestone. (4) The marine bar-sand facies developed only during the regressive phase of sedimentation. It consists of a sheet of well-sorted, orthoquartzitic sandstone which was shaped into a eries of submarine sand bars separated by muddy interbar areas. Individual bars were several kilometers in length, and accumulated to a maximum preserved thickness of 15 m. The bars are considered to be of polygenetic origin, resulting from the interplay of storms, tidal currents, and regional circulation currents in the shallow, regressing Oxfordian sea.

Sequence stratigraphy and sea level history of the upper Paleocene strata in the Kopet-Dagh basin, northeastern Iran

The intracontinental Kopet-Dagh basin formed after the Middle Triassic orogeny in northeastern Iran and southwestern Turkmenistan. This underexplored basin could provide significant petroleum reserves in the 21st century. The upper Paleocene Chehel-Kaman Formation is exposed along northwest-southeast-trending folds and is composed of carbonate strata and minor amounts of siliciclastic and evaporite beds. Six stratigraphic sections in the central and eastern parts of the basin have been used to divide the upper Paleocene (Thanetian) carbonate supersequence into four major carbonate lithofacies, each having multiple subfacies. These lithofacies represent open-marine, shoal, semirestricted lagoon, upper intertidal, and tidal-flat subenvironments that formed on a shallow carbonate ramp. In addition, there are two siliciclastic lithofacies consisting of calcareous shale (marl) and calcite-cemented sandstone. The upper Paleocene interval consists of three depositional sequences (DS1, DS2, and DS3), bounded by type 2 (within the top of the underlying Pestehleigh Formation), type 2, and type 1 sequence boundaries, respectively. Sea level changes during the Thanetian in the Kopet-Dagh basin are similar to global changes proposed by Haq et al. (1988), with differences related to local and regional geological events. Upper Paleocene strata were deposited in about 4 m.y., within the range of second-order cycles. Each depositional sequence was developed as a third-order cycle composed of several shallowing-upward parasequences (fourth- to fifth-order cycles). We estimate that sea level fluctuations in the study area were between 5 and 11 m during development of parasequences. (Begin page 840)

Distinguishing base-level change and climate signals in a Cretaceous alluvial sequence

We present the results of oxygen isotope and electron- microprobe analyses of sphaerosiderites obtained from Cretaceous paleosols in Iowa. The sphaerosiderite d 18 O values record Creta- ceous meteoric groundwater chemistry and an overall waning of brackish groundwater inundation during alluvial-plain aggrada- tion and soil genesis. We focus on horizons that precipitated from freshwater, in which d 18 O values ranging from 23.3‰ to 26.8‰ relative to the Peedee belemnite standard are interpreted to record variations in the Cretaceous atmospheric hydrologic cycle. During relative sea-level highstands, moisture was derived from the Cre- taceous Western Interior Seaway, whereas during lowstands, when the seaway narrowed and occasionally withdrew from the Midcon- tinent, the dominance of hemispheric-scale atmospheric moisture transport initiated in the tropical Tethys Ocean led to decreased precipitation rates. These processes did not operate like a switch, but rather as a continuum of competing moisture sources and mechanisms of transport between the nearby epicontinental sea and the distant tropics. The sphaerosiderite data demonstrate (1) temporal variation in the intensity of hemispheric-scale atmospher- ic moisture transport and (2) long-term amplification of the global hydrologic cycle marked by extreme 18 O depletion at the Albian- Cenomanian boundary.

Construction of Process-Response Models for Ancient Epicontinental Seaway Depositional Systems Using Partial Analogs

Epicontinental seaways, such as those that existed during the Jurassic through the Cretaceous in North America, do not have modern counterparts. Thus, exact analogs are not available for use as sedimentologic models of ancient epicontinental seaway depositional systems. As a result, well-documented modern shoreline features traditionally have been used as physical analogs to describe most paleodepositional features. During the past decade, marine geologists have accumulated a sufficient amount of data concerning modern continental-shelf sedimentologic processes and resulting features to relate these to similar features recognized in epicontinental deposits. For example, studies along the Atlantic epicontinental margin of North America and the North Sea between Great Britain and Europe reveal the dynamics of sand movement in response to storm-generated and tidal currents. These processes interact with sediment-covered sea bottoms to form transverse and longitudinal bed forms. Transverse bed forms, which include sand waves and ripples, form in response to the opposing current couplets created by tidal circulation systems. Longitudinal bed forms, which include sand ribbons, longitudinal furrows, and large-scale sand ridges, are formed predominantly on storm-dominated sand-covered shelves. The mechanisms which create and maintain these features are poorly understood but sand ridges and related longitudinal features apparently may form in response to cellular flow structure in storm-generated currents. Similar processes probably operated within ancient epicontinental seaways. Using present-day continental-shelf environments as partial analogs, we can construct process-response models to explain the development of some epicontinental paleodepositional systems rather than relying solely upon shoreline physical models. This method of model construction consists of an empirical, or data-gathering phase, followed by an interpretive phase. The objective is to match sedimentary sequences to process elements, and then combine these elements with information concerning the regional geologic setting to make paleoenvironmental interpretations. In this way, interpretations are based upon possible processes within particular geologic settings rather than the physical comparison of stratigraphic sections and well-documented present-day depositional systems. The process- esponse approach to sedimentary models frees us from the constraints imposed by the lack of well-documented sedimentologic studies of offshore depositional systems. This method of model building has been used to recognize and delineate offshore depositional systems in the Jurassic and Cretaceous epicontinental seaways of North America. It is postulated that currents generated by tides, storms, and regional circulatory mechanisms played major roles in creating and distributing continental-shelflike depositional systems within the offshore portions of these and other ancient epicontinental seaways.

Lower Cretaceous (Neocomian) fluvial deposits in eastern Kopet- Dagh Basin, northeastern Iran

The Kopet-Dagh intracontinental basin of northeastern Iran, formed after Middle Triassic orogeny, was a site of relatively continuous sedimentation from Jurassic through Miocene time. An epicontinental sea covered the eastern portion of the basin from near the end of the Jurassic through the Early Cretaceous, at which time the sea regressed toward the northwest. The resulting regressive depositional phase is represented in the stratigraphic record of the basin by a thick interval of cyclic redbeds that were deposited in fluvial environments. Sedimentological analysis of these regressive sediments indicate that the coarser-grained sediments of the lower and middle parts of this interval were deposited as sheet-like bodies in a low sinuosity, coarse-grained braided-stream system. The upper part of the interval consists primarily of finer-grained sediments that were deposited in a lower-gradient meandering stream system. Paleocurrent analysis of cross-beds and the thinning of coarser-grained sediment toward the northeast indicate that the source of these Neocomian siliciclastics were to the south and southwest of the Kopet-Dagh basin. Because the lower part of this interval is a gas-producing reservoir in the Khangiran gas field, northeast Iran, the results of this study are applicable to future exploration efforts in the Kopet-Dagh Basin and similar basins elsewhere.

Sussex Sandstone of Wyoming--Example of Cretaceous Offshore Sedimentation

The Sussex sandstone was deposited as part of an extensive muddy sheet which prograded southward and eastward into the central part of the Late Cretaceous Campanian seaway of western North America. Outcrop, core, and well-log data from the Wyoming portion of the Powder River basin show that the sandstone units of the Sussex interval are concentrated into large-scale northwest-trending bodies which have lobate lateral edges. A process-response depositional model has been developed to account for these sandstones by relating modern sedimentary processes to the physical characteristics of the lithologies, and then placing them within the proper stratigraphic and tectonic setting. The model invokes storm-generated and tidal currents to create and shape a series of sand-bar or ridge complexes. These bars were periodically breached by intense storm-generated currents, cutting channels which may subsequently have acted as tidal passes. The mouths of those channels are marked by washover fans and/or tidal deltas which give the overall complexes their lobate edges. The eastern and southern termini of the Sussex sheet are marked by an abrupt depositional slope as indicated by physical and biostratigraphic studies, and may be observed visually on seismic profiles. Petroleum accumulated along the updip pinchouts of the more porous bar and channel lithologies.