Evan K. Franseen

Pinning points: a method providing quantitative constraints on relative sea-level history

Abstract Quantitative constraints on the history of relative sea level allow for a better understanding of the controls on depositional sequence development. The constraints are provided by “pinning point curves”, plots of ancient relative sea-level elevations (pinning points) versus time. Constructing a pinning point curve requires identification of ancient stratigraphic positions of sea level through interpretation of facies and surfaces formed at sea level. Then, their ancient relative elevations are determined through reconstructing aspects of ancient paleotopography. Upper Miocene strata from Las Negras, southeastern Spain preserve paleotopography, contain ancient surfaces of subaerial exposure, and contain facies deposited near sea level. The pinning point curve illustrates a complex relative sea-level history with large- and small-scale relative sea-level fluctuations defined by 31 pinning points.

Vertical resolution of a seismic survey in stratigraphic sequences less than 100 m deep in southeastern Kansas

A 400-m long, 12‐fold high‐resolution common depth point (CDP) reflection seismic profile was acquired across shallow converging Pennsylvanian strata in the Independence area of southeastern Kansas. One of the principal objectives was to determine practical vertical resolution limits in an excellent shallow seismic‐data area with borehole control. The dominant frequency of the CDP stacked data is in excess of 150 Hz based on peak‐to‐peak measurements. Interference phenomena observed on stacked seismic data incorporated with models derived from log and drill‐hole information suggest a practical vertical resolution limit of about 7 m, or one‐third of the dominant wavelength. This practical resolution is slightly less than the predicted (theoretical) resolution limit of 5 m based on the generally accepted one‐quarter wavelength axiom. These data suggest conventional rules of thumb describing resolution potential are not accurate when reflectors on shallow, narrow bandwidth data converge rapidly across horizo...

Constraining controls on carbonate sequences with high-resolution chronostratigraphy: Upper Miocene, Cabo de Gata region, SE Spain

Abstract A high-resolution chronostratigraphy has been developed for Miocene shallow-water carbonate strata in the Cabo de Gata region of SE Spain for evaluation of local, regional and global factors that controlled platform architecture prior to and during the Messinian salinity crisis. Paleomagnetic data were collected from strata at three localities. Mean natural remanent magnetization (NRM) ranges between 1.53×10 −8 and 5.2×10 −3 Am 2 /kg. Incremental thermal and alternating field demagnetization isolated the characteristic remanent magnetization (ChRM). Rock magnetic studies show that the dominant magnetic mineral is magnetite, but mixtures of magnetite and hematite occur. A composite chronostratigraphy was derived from five stratigraphic sections. Regional stratigraphic data, biostratigraphic data, and an 40 Ar/ 39 Ar date of 8.5±0.1 Ma, for an interbedded volcanic flow, place the strata in geomagnetic polarity Chrons C4r to C3r. Sequence-stratigraphic and diagenetic evidence indicate a major unconformity at the base of depositional sequence (DS)3 that contains a prograding reef complex, suggesting that approximately 250 000 yr of record (Subchrons C3Br.2r to 3Br.1r) are missing near the Messinian–Tortonian boundary. Correlation to the GPTS shows that the studied strata represent five third- to fourth-order DSs. Basal units are temperate to subtropical ramps (DS1A, DS1B, DS2); these are overlain by subtropical to tropical reefal platforms (DS3), which are capped by subtropical to tropical cyclic carbonates (Terminal Carbonate Complex, TCC). Correlation of the Cabo de Gata record to the Melilla area of Morocco, and the Sorbas basin of Spain indicate that early–Late Tortonian ramp strata from these areas are partially time-equivalent. Similar strata are extensively developed in the Western Mediterranean and likely were influenced by a cool climate or influx of nutrients during an overall rise in global sea-level. After ramp deposition, a sequence boundary (SB3) in Cabo de Gata correlates with a sequence boundary in Morocco and a published third-order eustatic fall suggesting at least a partial eustatic control for the sequence boundary. Coral reefs began to develop earlier in Cabo de Gata than at Melilla or Sorbas, arguing for local factors affecting this major environmental transition. Later Messinian reefs (DS3) from all areas are time-equivalent, suggesting a regional or global control on their formation. Some Halimeda -rich horizons in the Western Mediterranean are not time-equivalent event strata as hypothesized by others. Correlation of the relative sea-level curve for the fringing reef complex (DS3) with a published eustatic curve suggests at least a partial third-order global eustatic control for the highstand part of the sequence. Downstepping DS3 reefs and initial subaerial exposure of earlier DS3 reef strata approximately correlate with initiation of a series of subaerial unconformities in the South Pacific. The longer-term relative fall in sea-level during DS3 downstepping reef progradation does not correlate with a published third-order eustatic fall. Eustatic sea-level fluctuations may have been associated with initiation of the Mediterranean Messinian salinity crisis, but the longer-term fall may have been linked to tectonic uplift in the Mediterranean region. Widespread distribution of ‘TCC-style’ cycles of approximately the same age suggests a regional (Western Mediterranean) or global control on sea-level change responsible for TCC cycles. In addition, four subaerial exposure-capped TCC cycles may correlate with similar subaerial unconformities in the South Pacific, suggesting at least a partial eustatic control on TCC cyclicity. The high rates of relative sea-level change needed to generate a minimum of 25–30 m sea-level changes associated with each cycle are consistent with glacio–eustacy along with rapid evaporitic drawdown in the Mediterranean.

Improving ground-penetrating radar data in sedimentary rocks using deterministic deconvolution

Abstract Resolution is key to confidently identifying unique geologic features using ground-penetrating radar (GPR) data. Source wavelet “ringing” (related to bandwidth) in a GPR section limits resolution because of wavelet interference, and can smear reflections in time and/or space. The resultant potential for misinterpretation limits the usefulness of GPR. Deconvolution offers the ability to compress the source wavelet and improve temporal resolution. Unlike statistical deconvolution, deterministic deconvolution is mathematically simple and stable while providing the highest possible resolution because it uses the source wavelet unique to the specific radar equipment. Source wavelets generated in, transmitted through and acquired from air allow successful application of deterministic approaches to wavelet suppression. We demonstrate the validity of using a source wavelet acquired in air as the operator for deterministic deconvolution in a field application using “400-MHz” antennas at a quarry site characterized by interbedded carbonates with shale partings. We collected GPR data on a bench adjacent to cleanly exposed quarry faces in which we placed conductive rods to provide conclusive groundtruth for this approach to deconvolution. The best deconvolution results, which are confirmed by the conductive rods for the 400-MHz antenna tests, were observed for wavelets acquired when the transmitter and receiver were separated by 0.3 m. Applying deterministic deconvolution to GPR data collected in sedimentary strata at our study site resulted in an improvement in resolution (50%) and improved spatial location (0.10–0.15 m) of geologic features compared to the same data processed without deterministic deconvolution. The effectiveness of deterministic deconvolution for increased resolution and spatial accuracy of specific geologic features is further demonstrated by comparing results of deconvolved data with nondeconvolved data acquired along a 30-m transect immediately adjacent to a fresh quarry face. The results at this site support using deterministic deconvolution, which incorporates the GPR instruments unique source wavelet, as a standard part of routine GPR data processing.

Diagenesis associated with subaerial exposure of Miocene strata, southeastern Spain: Implications for sea-level change and preservation of low-temperature fluid inclusions in calcite cement

Abstract Many ancient carbonate rocks contain calcite cements that precipitated from shallow, fresh groundwater that entered strata during events of subaerial exposure. Such low-temperature cementation may be difficult to interpret from fluid inclusion studies because some of the inclusions may reequilibrate during later thermal events. Miocene rocks of southeast Spain provide an example of the utility of fluid inclusion studies in rocks that have not been subjected to significant heating. In the Mesa Roldan area, one type of calcite cement occurs exclusively below a regional stratigraphic surface of enigmatic origin. The cement has petrographic characteristics indicative of cementation in the vadose zone (generally thought to be a zone of oxidation) but has cathodoluminescent bands containing reduced manganese and iron. Primary fluid inclusions contain mostly fresh water, have variable ratios of vapor to liquid, and are at one atmosphere of pressure. Our observations indicate that calcite precipitated from a freshwater vadose zone, which was subjected to local or repetitive saturation, and minor brackish water. The fluid inclusion data indicate that low-temperature fluid inclusions can be preserved in ancient sequences despite a later history of different pore fluids. This indication of subaerial diagenesis of distal slope deposits suggests a relative sea-level drop of at least 50–55 m during the Late Miocene. Similar petrographic and fluid inclusion observations can be used to interpret sea-level changes in other areas.

Differential compaction of Winnipegosis reefs: A seismic perspective

Winnipegosis Formation reefs in southern Saskatchewan are typically encased in the thick, apparently incompressible salts of the Prairie Evaporite. These reefs are characterized by raised rims and reverse drape along the top of the salt. Both features, clearly visible on seismic data, are primarily due to postdepositional compaction. The rims developed principally as a result of differential compaction within the different reef environments; the structural low at the Prairie Evaporite level is attributed to differential compaction between the reef and the encasing salt. If these salts are effectively incompressible, the rim and lagoonal facies are estimated to have been compacted by at least 30 and 44 percent, respectively. This paper illustrates the usefulness of seismic data to separate postdepositional compaction features from primary features to determine the primary morphology of a reef better and to determine the relative amounts of postdepositional compaction within the different reef environments....

Reservoir analog model for oolite-microbialite sequences, Miocene terminal carbonate complex, Spain

Static three-dimensional (3-D) reservoir analog models were constructed for the Miocene terminal carbonate complex (TCC) in southeastern Spain. The models used field data collected from two areas containing exceptional 3-D exposures (La Molata; La Rellana-Ricardillo). Four TCC sequences in each area are composed of oolite, microbialite (thrombolites, stromatolites), bioclastic sands, and coralgal reefs deposited over paleotopographic relief of 33–76 m (108–249 ft). The models integrate field, laboratory, and petrophysical data with results providing a workflow and reservoir analogs useful in evaluating oolilte and microbialite reservoir characteristics in relation to paleotopography and sea level change. Results from this study reveal favorable reservoir-quality values with the potential for substantial hydrocarbon storage for many lithofacies. Flow and baffle facies were distinguished for the models based on thickness, lateral distribution, porosity, and permeability values. Trough cross-bedded ooid grainstone is volumetrically the most abundant lithofacies within both models, is laterally extensive across the entirety of sequences, has large storage capacities with good permeability, and has good connectivity with other flow facies. This facies represents the best reservoir-quality facies and would be the primary target for hydrocarbon exploitation. Microbialites act both as reservoir and baffle facies. Thrombolites, in particular, are concentrated downslope and in a more restricted embayment but vary between porous and nonporous facies. Stromatolites and fenestral ooid grainstones are concentrated at sequence boundaries and would create laterally extensive baffles with significant thicknesses at the sequence boundaries. Sea level interacting with both paleotopography and paleogeography were identified as the main controls on sequence development and reservoir heterogeneity. An understanding of these controls can aid in exploitation and identification of oolite-microbialite sequences.

Geochemical record of fluid flow and dolomitization of carbonate platforms: ascending freshwater–mesohaline mixing, Miocene of Spain

Abstract This paper constrains fluid flow and chemistry in Miocene dolomites of Spain, where dolomitization has been ascribed to ascending freshwater–mesohaline mixing. End-Miocene dolomite formed as replacement and cement with the same widespread cathodoluminescence. Fluid inclusion final melting temperatures of ice (Tm ice: −0.2 to −2.3 °C) indicate mixing of freshwater and evaporated seawater. δ18O and δ13C data mostly show positive covariation, and only some have variable δ13C and invariant δ18O, arguing that mixing was more important than sulphate reduction. Data range from +0.9 to +6.0‰ for δ18O and from −4.5 to +3.0‰ for δ13C (VPDB (Vienna Pee Dee Belemnite)). Lower stratigraphic units are more depleted isotopically than upper units, suggesting upwards flow of freshwater. 87Sr/86Sr values (0.70866–0.70904) range from less than to greater than late Miocene seawater. δ18O, δ13C and Sr analyses show that freshwater interacted with basement, confirming injection of freshwater from below. Upwards flow of freshwater, driven by low density and hydraulic head, created fluid mixing and CO2 degassing. Comparison of La Molata dolomite to other dolomites of the western Mediterranean suggests that ascending freshwater–mesohaline mixing may be widespread, and that local composition of basement is not the primary driver of dolomitization. The model is broadly applicable to carbonates adjacent to highs, where freshwater discharged into slightly evaporated seawater.

Workflows for incorporating stratigraphic and diagenetic relationships into a reservoir-analogue model from outcrops of Miocene carbonates in SE Spain

A workflow is presented for constructing an outcrop-based static geological model for Miocene heterozoan, photozoan, oolitic and microbial carbonates in SE Spain. Fieldwork and LiDAR (Light Detection And Ranging) data were integrated to create a photorealistic virtual outcrop. Field-based data are depicted on the virtual outcrop, and used to identify and correlate horizons. Multivariate regression is used to extend stratigraphic horizons away from the outcrop and to create realistic time-equivalent model layers. Mapping and measured stratigraphic sections are used to assign depofacies within model layers. Pinning points are used to reconstruct sea-level history, and palaeotopography is used to define palaeobathymetry. Geospatial analysis of depofacies occurrence is used to extrapolate facies while also enforcing palaeobathymetric controls on depofacies distribution. Dolomitization and meteoric calcite cementation are dominant diagenetic products affecting porosity and permeability. Their distribution was mapped in the field and amounts were quantified in the laboratory, and these were used to populate diagenetic products into geomodel cells. Six scenario models were built to represent different stages and combinations of diagenetic effects on porosity and permeability. Construction of the static geological models required the development of new methods. These include: (1) a new workflow to extend clinoform surfaces across the model area by fitting equations to horizon picks made on the outcrop; (2) a new workflow relating relative sea level to model layers to allow calculation of palaeo-water depth and relate that to facies probability; and; (3) an experimental application to predict porosity and permeability from objective visual descriptions of carbonate samples.

Topographic and sea level controls on oolite-microbialite-coralgal reef sequences: The terminal carbonate complex of southeast Spain

The terminal carbonate complex of southeast Spain is a Miocene (Messinian) unit of oolite, microbialite, and coralgal reefs deposited in association with glacioeustasy and evaporitic drawdown. The relationship between paleotopography and sea level history is useful for prediction of microbialite and oolite reservoir facies in the subsurface. Four sequences record sea level change with minimum amplitudes of 32–77 m (105–253 ft). Sequences commonly have local basal stromatolites overlain by local thrombolites, ooid grainstone, volcaniclastic-rich planar-bedded ooid grainstone, and fenestral ooid grainstone. At low substrate positions, thrombolite boundstones are thicker and laterally more continuous than at higher positions. At intermediate substrate positions (relative to sea level history), sequences have a build-and-fill architecture, characterized by a relief-building phase and a relief-filling phase, with thin sequences draping paleotopography. Microbialites dominate during the relative sea level rises and build topographic relief. Oolites dominate during the relative sea level falls and fill topographic relief. At higher substrate positions, close to highstand, sequences thicken and yield stratigraphic character that is inconsistent with a build-and-fill model. Apparently, the build-and-fill model requires an intermediate-elevation substrate position and nonoptimal carbonate productivity during rapid sea level change. Sequences progressively show increasing diversity and more normal marine organisms, possibly caused by decreasing aridity. Lithofacies of the La Molata area show evidence of more restricted conditions compared to the La Rellana-Ricardillo area lithofacies, likely because La Molata was in an embayment. These results show that distribution of oolite, microbialite, and reef facies are predictable given known interaction among sea level, paleotopography of the depositional surface (substrate), and coastline configuration.