Sequence III is the Szolnok. Lets hope it's a massive sandstone unit and not slumping :)

Also of interest is the mention of the coarse-grained sediments in sequence I.

SEISMIC STRATIGRAPHY AT HOD-I WELL

The seismic profile can be divided into five seismic sequences based on the geometric relationship of reflections to sequence boundaries and on the internal configuration of reflections within sequences. A top to bottom description of these sequences follows:

Sequence V

(0.0-1.74 sec; 0-2003 m.) Within this sequence, reflectors are concordant to the bottom sequence boundary. Individual reflectors vary from parallel to wavy. Although reflections are strong (high amplitude), few, if any, can be traced for any distance without interruption.

Correlation with core data (described previously) indicates that seismic sequence V represents delta plain facies; depositional environments varied from shallow lake, fluvial, and marsh to possible terrestrial soils. At the Hod-I well site, the bottom boundary of sequence V is placed at 2003 m, about 514 m higher than the base of delta plain facies as determined from core analyses (Figure 8). This difference in unit boundaries is a good example of the differences that result from seismic sequence analysis in contrast to lithologic analysis. Although delta plain facies lie above and below the bottom boundary of sequence V, it will be shown later that the bottom boundary of sequence V represents an unconformity, probably related to more widespread lake conditions and shoaling of the Pan onian Lake.

Sequence IV

(1.74-2.44 sec; 2003-3250 m.) In general, the internal reflectors form an irregular sigmoid pattern. The relation between reflections and sequence boundaries is one of toplap at the upper boundary and downlap at the lower boundary.

Sequence IV can be subdivided into subsequences IVA and IVB at the Hod-I well site. The subdivision is placed at 2.06 sec (2517 m). Reflectors above 2.06 sec lie relatively flat, whereas those below 2.06 sec show a definite pattern of progradation to the southeast.

At the Hod-I well, subsequence IVB is inferred to represent delta plain facies; to the southeast, where the reflectors of subsequence IVB exhibit a strong progradational pattern with a dip of about 4.5°, the facies probably grade to delta front and prodelta. The base of seismic sequence IVB is placed at a depth of 2517 m, in agreement with the base of the delta plain facies as determined from lithologic analysis.

Subsequence IVA is inferred to represent delta front facies at the Hod-I well site. Southeast of the Hod-I well, where the reflection pattern in subsequence IVA suggests lesser dips, the facies probably grade to prodelta.

In general, seismic sequence IV represents prodelta, delta front, and delta plain facies. It will be shown, on seismic records from other parts of the basin, that sequence IV is part of a supersequence that represents a system of stacked deltas built during a late depositional stage. The term "late depositional stage" is used to distinguish this supersequence from an earlier and more widespread supersequence that represents delta construction during a period when the lake was deeper.

Sequence III (Szolnok)

(2.44-3.00 sec; 3250-4285 m.) The boundary between sequences IV and III is placed immediately below the basalmost reflectors that exhibit the progradational pattern characteristic of sequence IV. Reflections in this sequence are concordant to the upper and lower sequence boundaries. The sequence is characterized internally by weak, highly discontinuous, wavy reflections.

The absence of strong, continuous reflections, which usually represent alternating beds with different reflection coefficients, together with the preponderance of sandstone in the cores from the Hod-I well, indicate that this sequence may represent a relatively massive sandstone unit. However, the absence of strong continuous reflections also may be caused by extensively slumped strata, and evidence of slumping was present in some of the cores (Figure 8). The vertical position of sequence III in the stratigraphic column, between a delta (sequence IV) and a deep-basin facies (sequence II), suggests that the sands could have been deposited in front of an advancing sand-prone delta or deltas.

At the Hod-I well site, seismic sequence III lies at depths of between 3250 and 4285 m. At this site, therefore, sequence III is equivalent to the lithologic unit inferred to represent prodelta facies (Figure 8). Based on core analyses, the prodelta facies is divided into subfacies B (slumped strata) and subfacies A (parallel-bedded strata). Such a division cannot be made from the seismic data, and it would appear that the lithologic data makes a finer distinction in comparison to the seismic data. As will be shown later, however, seismic evidence from other parts of the Pannonian basin suggests that updip from the Hod-I well site, at least part of seismic sequence III represents distal deposits associated with an early system of deep-water deltas and that deep-water delta constructio may have been followed by a short-lived destructive phase. The lower part of seismic sequence III, therefore, apparently represents prodelta facies of the older, deep-water delta system, and the upper part of sequence III (slumped strata) may represent sands deposited in front of an advancing sand-prone delta system or, perhaps, it represents deposition during a destructive phase.

Sequence II

(3.00-3.69 sec; 4285-5576 m.) Reflections within this sequence are strong and, in general, can be traced across the entire record section or until they terminate to the southeast in an onlap configuration against underlying reflections or reflections that represent the basement surface. Onlapping reflectors alternate with concordant reflectors. The concordant reflectors are parallel with the basal sequence boundary and have the appearance of being draped over the basement surface southeast at the Hod-I well.

The alternation of concordant and onlapping reflectors suggests that layers of deep basin marl alternate with turbidite deposits. Sediments precipitated from the water column would tend to drape over basements highs. The sediments associated with turbidite deposits, however, would have been carried through canyons into the deeper parts of the basin near the water-sediment interface and, therefore, would be expected to show an onlap relation to underlying horizons.

Although seismic sequence II correlates with deep basin facies at the Hod-I well site, the sequence can be traced seismically updip (northward) where it grades to prodelta, delta front, and delta plain facies. In other sections of this chapter, it is shown that sequence II represents a deep water, lateral equivalent (in time) of a supersequence of stacked deep water deltas built during an early depositional stage.

Sequence I

(3.69-4.05 sec; 5576-6150 m.) The pattern of reflections within this sequence is similar to that noted for sequence II, except that most reflections appear to terminate against the basal boundary (basement surface) in an onlap configuration. At the Hod-I site, seismic sequence I correlates with the basal facies unit (Figure 8). The seismic character (onlap pattern) and the core descriptions indicate that sequence I represents, for the most part, periodic influxes of coarse-grained sediments into a deep basin where the precipitation of CaCO3 and the deposition of mud from suspension occurred. Some of the coarse-grained sediments were transported by turbidite flows from source areas far to the northwest. In addition, coarse-grained sediments probably were derived from erosion of local basement highs, especially the conglomerate layer penetrated in the Hod-I well at a depth of 5450 m.

As the delta wedges advanced further southeastward, the basement high was overwhelmed, and sediments were deposited near the base of the previous slope by slumping subsequent to a shoaling of the lake. Evidence for this conclusion comes from seismic profile 13. Part of the coarser sediment fraction was transported basinward by turbidite flows or directly downslope by slumping, and some of it reached the central part of the basin in the vicinity of the Hod-I well.