Jan Stafleu

Seismic models of a prograding carbonate platform: Vercors, south-east France

Abstract The southern Vercors area (south-east France) displays superb exposures of a Lower Cretaceous prograding carbonate platform. These exposures allow a geometrical sequence analysis to be combined with detailed outcrop observations. Two-dimensional seismic modelling techniques were used to determine the seismic response of three of these exposures: the Montagnette, the Archiane Valley and the Rocher de Combau. The outcrops display prograding and retreating series of clinoforms interfingering with fine-grained basinal sediments. Photographs from different angles of view, combined with detailed field observations, were used to construct lithological profiles with a resolution of f few metres. P-wave velocities and bulk densities were measured from cored hand specimens and selected values were assigned to lithostratigraphic units in each profile. The vertical incidence modelling technique was then used to compute migrated time sections of reflectivity. These were convolved with source wavelets of different frequencies to produce the final synthetic seismic profiles. None of the modelled outcrop geometries is correctly portrayed seismically at 25–50 Hz frequencies. Not only do the synthetic seismic profiles show less detail, they also show misleading geometries. In most seismic models, for instance, pseudo-unconformities are present that correspond to rapid changes in dip and in facies in outcrop. Furthermore, the seismic tool is not able to resolve between stratal patterns punctuated by unconformities and those with gradual shifts in facies belts accompanied by complex interfingering relationships. This implies that an unconformity recognized on a real seismic line should not automatically be assumed to represent a true stratal unconformity. Large outcrops are often used to show the validity of sequence stratigraphy, a concept mainly based on the analysis of seismic lines. This study, however, shows that there is no one to one relationship between geometries observed in outcrop and in seismic lines. Seismic modelling of outcrops therefore provides a means for evaluating outcrop geometries in terms of seismic stratigraphy.

Seismic Modeling of an Early Jurassic, Drowned Carbonate Platform: Djebel Bou Dahar, High Atlas, Morocco (1)

Along the southern margin of the High Atlas of Morocco, Early Jurassic carbonate platforms developed in a transtensional, rifted basin. One of these platforms was the Djebel Bou Dahar platform, which grew to a vertical relief of 500 m, drowned, and was onlapped by deep-marine sediments. Exceptional outcrops allowed detailed lithologic models of the platform to be made with a resolution of a few meters. We constructed a geometric model from building blocks comprising particular rock types. We estimated P-wave velocities and bulk densities of these lithologies from hand specimens, and we assumed these properties to be homogeneous and invariant throughout the building blocks. We constructed time sections of reflectivity from vertical incidence and image-ray migration techniques. These time sections were convolved with 25 to 200-Hz Ricker wavelets to produce synthetic seismic sections. Large-scale features, such as the platform and the drowning unconformity, were resolved regardless of the frequencies and modeling techniques used. Broad, low-frequency wavelets resulted in profiles with apparent truncation of bedding and basinward shifts in the point of onlap. Smaller scale features, such as reef knolls and sand bodies at the platform margin, were poorly resolved at frequencies up to 50 Hz. However, we inferred the existence and location of these smaller features from the disturbance and distortion of other reflections. This distortion is most obvious in image-ray migration modeling because of the refraction of rays through the small-scale features. Geometric effects caused lateral amplitude variations due to wavelet interference along the unconformity reflection where no petrophysical changes were modeled. The seismic models of the Djebel Bou Dahar in its present stage may be good analogs for seismic lines of deeply buried carbonate platforms. To make a comparison with real seismic lines of less deeply buried platforms possible, we reconstructed both the geometry and petrophysical properties, and the subsequent seismic modeling of the Djebel Bou Dahar carbonate platform as it was prior to the onset of deep burial. At this stage, impedance contrasts were at their maximum. This study clearly demonstrates the significance of modeling the seismic response of a detailed, outcrop-based, stratigraphic model, and shows that seismic modeling of specific sequence stratigraphy can reveal what features are likely to be resolved under ideal conditions and what pitfalls may await the interpreter.

Seismic Models of a Shelf-margin Depositional Sequence: Upper San Andres Formation, Last Chance Canyon, New Mexico

ABSTRACT The seismic resolution of stratal geometries and facies distributions observed in San Andres Formation (Permian) outcrops in Last Chance Canyon, Guadalupe Mountains, New Mexico, is studied by seismic modeling of a published, detailed stratigraphic cross section. The outcrops in Last Chance Canyon are composed of two fourth-order depositional sequences: an aggrading carbonate bank (upper San Andres 3; uSA3) followed by a strongly progradational, offlapping mixed carbonate-siliciclastic succession (upper San Andres 4; uSA4). Each sequence comprises a number of subsidiary high-frequency sequences (fifth-order). Two alternative impedance models were used: Model A, in which all facies transitions are reflecting boundaries, and Model B, in which only time-significant surfaces act as reflectors and lateral facies transitions are represented by horizontal velocity gradients. The vertical-incidence modeling technique was used to compute perfectly migrated time and depth sections with different frequencies. Using a low-frequency wavelet (25 Hz), the sequence boundary separating the two fourth-order cycles (uSA3 and uSA4) is poorly imaged. Instead, one is tempted to incorrectly interpret an onlap pattern generated by a high-frequency cycle within uSA4 as this major sequence boundary. In addition, the 25 Hz runs show toplap and downlap lap-out patterns in an overly oblique fashion, obscuring true asymptotic stratal relationships. Both at 35 Hz and 50 Hz, profiles based on Model B image the genetic structure of both uSA3 and uSA4 relatively well. At 50 Hz, Model A incorrectly shows a transition from a ramp to a rimmed margin within uSA4. The 35 Hz models are qualitatively compared with a published Exxon Production Research Co. seismic line, located approximately 50 km along depositional str ke to the northeast. Model A shows an unexpected good match with the Exxon seismic line, whereas Model B comes much closer to the depositional anatomy observed in outcrop. Our results show that the resolution of stratal geometries and facies distributions in Last Chance Canyon is strongly related to carbonate-sandstone alternations and the way impedance contrasts at carbonate-sandstone transitions are represented.

Using 14C-Dated Peat Beds for Reconstructing Subsidence by Compression in the Holland Coastal Plain of the Netherlands

ABSTRACT Koster, K.; Cohen, K.M.; Stafleu, J., and Stouthamer, E., 2018. Using 14C-dated peat beds for reconstructing subsidence by compression in the Holland coastal plain of the Netherlands. Subsidence in the Holland coastal plain of the Netherlands was reconstructed from the vertical displacement of Holocene peat layers below their reference groundwater levels at the time of peat formation. This quantifies the part of subsidence that is due to compression processes and allows specification of the current state of peat compression in a map. 14C-dating of peat layers found intercalated in the Holocene sequence were used in the reconstruction. This dataset was combined with results from a recent coastal-deltaic plain wide three-dimensional (3D) interpolation of reference palaeo-groundwater levels, at which the intercalated peats are thought to have formed before they were buried, compressed, and vertically displaced. Empiric relations between reconstructed displacement and the thickness of overburden were determined and deployed in a national 3D geological subsurface model to establish a subsidence map with continuous cover of the coastal plain. The resulting maps show compressed peat layers under urbanized areas with 1 to 8 m of natural and anthropogenic overburden have subsided 1 to 5 m below the original level of formation. In the agricultural area of the coastal plain, where overburden is merely decimetres thick, consisting of fluvial flood- and sea-ingression deposits, peat generally experienced less than 1 m subsidence. The reference-level reconstruction method is deployable over large coastal plain areas to reconstruct subsidence caused by postdepositional vertical displacement of intercalated peat layers. It could therefore serve as an alternative approach for methods based on soil mechanics, which require input often not available for coastal plains on regional scales.