Logging Summary

Modern Carbonate Mounds: Porcupine Basin Drilling

Expedition 307 Scientific Party

Figure 1. General view of Modern Carbonate Mounds - IODP Expedition 307 Porcupine drilling area (UCD Unnithan).

Figure 2. Location map of the Challenger Mound drilled and logged sites.

Figure 3. B Seismic line PO10515 showing sigmoidal unit at Site U1316 and semitransparent seismic layer at Site U1317.

During Expedition 307, a downslope suite of three sites was drilled and logged on the eastern slope of Porcupine Seabight, west of Ireland (Fig. 1). The sites are centered on Challenger mound, a 170 m high, partly buried carbonate mound in the Belgica mound province, topped by dead coldwater coral rubble (Fig.2).

The Belgica mound province belongs to the best documented carbonate mound provinces worldwide. Very high resolution seismic profiling, multibeam bathymetry, and side-scan sonar imaging have shed light on the stratigraphic, structural, and morphological setting. The mounds are rooting on a strongly erosive unconformity and are seated partly on an enigmatic sequence of sigmoidal units and partly on a semitransparent layer (Fig. 3).

The objectives of Expedition 307 can be summarized into four major hypotheses:

• Gas seeps act as a prime trigger for mound genesis - a case for Geosphere-Biosphere coupling


• Mound "events" frame into a palaeoenvironmental plot - prominent erosional surfaces reflect global oceanographic events


• Mounds are high-resolution palaeoenvironmental recorders


• The Porcupine mounds are present-day analogues for Phanerozoic reef mounds and mud mounds


Collection of continuous downhole logging measurements was critically important to the scientific objectives of Expedition 307, as the main contribution of acquisition of in situ, continuous multi-parameter logging data are:


(1) To assess the physical, chemical and structural characteristics of the formation, and to provide the baseline for depth matching the core-derived composite depth (mcd) scale; and


(2) To conduct a seismic integration (time/depth model and synthetic seismogram) allowing identification and dating of seismic reflectors at a regional scale.

Tools and Logging Operations

Figure 4. Schematic illustration of the toolstring configurations used during Expedition 307.

Logging operations utilized the standard IODP tool strings: the triple combo with the addition of the Temperature/Acceleration/Pressure Tool (TAP) the Formation MicroScanner (FMS)-sonic and the Well Seismic Tool (WST) (Fig. 4):


- the triple combo tool string (Fig. 4a) consisting of resistivity (phasor dual induction tool [DIT]), bulk density (hostile environment litho-density sonde [HLDS]), gamma ray (hostile environment natural gamma sonde [HNGS]), and porosity (accelerator porosity sonde [APS]) components, with one additional LDEO tool that measured high-resolution temperature/acceleration/pressure (TAP tool);

- the FMS-sonic toolstring (Fig. 4b) consisting of microresistivity (FMS), dipole sonic imager (DSI), gamma ray (scintillation gamma ray tool [SGT]), and orientation/acceleration (general purpose inclinometer tool [GPIT]) components;

- the WST (Fig. 4c) consists of a single geophone, pressed against the borehole wall that is used to record the acoustic waves generated by an air gun located near the sea surface, offset from the ship.

The logging plan set out in the pre-cruise prospectus was successfully completed (Table 1). In a general manner collected data, including sonic logs necessary to seismic modelling are of excellent quality.

IODP Hole	Location	Water depth (m)	Total Depth (mbsf)	Toolstring deployed	Interval logged (mbsf)
U1316C	51° 22.56' N	965	143	Triple-Combo	60 - 140
	11° 43.81'W			FMS-sonic	41 - 140
U1317D	51º 22.8' N	805	270	Triple-Combo + TAP	87 - 246
	11° 43.1' W			WST	87 - 250 (13 stations)
				FMS-sonic	77 - 246
U1318B	51° 26.16' N	423	255	Triple-Combo	81 - 255
	11° 33.0'W			FMS-sonic	71 - 255

Table 1. Details of logging operations completed during Expedition 307.

 

Summary and Highlights

Figure 5. Log stratigraphy at Hole U1316C. (a) Hole shape, total gamma ray (HSGR) and Potassium content (HFK), (b) thorium (HTHO) and uranium (HURA) contributions to natural radioactivity, (c) deep (IDPH), intermediate (IMPH) and shallow (SFLU) resistivities, (d) porosity (APLC) and formation density (RHOM), (e) capture cross-section (SIGF) and photoelectric factor (PEFL), (f) downhole compressional velocities (VP), and (g) log sub-units.

Figure 6. Core-log integration at Site U1316 gamma-ray, porosity, density, velocity. Post-cruise correlation between features recorded in these logs (especially acoustic, density and gamma-ray) offer potential to provide in-situ ground truth for core data. Note that in the velocity plots, the range of velocity is similar for all datasets (1400 m/s) but absolute values between cores and logs differ by 300 m/s. The core values are about 300 m/s lower than logging data, probably due to expansion of core now under atmospheric pressure.

Site U1316


Site U1316 (965 m water depth, 51° 22.56' N, 11° 43.81'W) is located in the downslope sediment deposits approximately 700 meters to the southwest of Challenger Mound. Sediments recovered from Site U1316 located basinward of the Challenger Mound contain a sedimentary suite of post-, syn- and pre-mound growth phases that correspond to three lithological Units. The uppermost Unit 1 is 52-58 m thick, and mainly composed of grayish-brown silty clay. Unit 2 is a coral bearing facies of 10-13 m in thickness and underlies Unit 1 with an erosional surface. The age of this unit is mostly early-middle Pleistocene, which corresponds to the age of the thick coral mound at the Site 1317. This unit rests on the Unit 3 with a distinct unconformity surface. The Unit 3 consists of the 92-m thick (Hole U1316C) heterogeneous, dark green colored, glauconitic siltstone, and is calcareous in the lower part. Dolomite precipitation formed lithified layers around 72 mbsf.

After an unsuccessful attempt to log Hole U1316A, Triple Combo and FMS sonic downhole logs were acquired between 60 and 140mbsf in Hole U1316C. The density, resistivity, and acoustic velocity logs show a steady downhole increase due to compaction, interrupted by 1-5-m-thick intervals of higher values, indicating the presence of more lithified layers (Figs 5 and 6). The PEF values for these layers indicate they are carbonate-rich. These lithified layers are the cause of several strong reflections in the sigmoidal package in the seismic section at this site.




Figure 7. Log stratigraphy at Hole U1317D. (a) Hole shape, total gamma ray (HSGR) and Potassium content (HFK), (b) thorium (HTHO) and uranium (HURA) contributions to natural radioactivity, (c) deep (IDPH), intermediate (IMPH) and shallow (SFLU) resistivities, (d) porosity (APLC) and formation density (RHOM), (e) capture cross-section (SIGF) and photoelectric factor (PEFL), (f) downhole compressional velocities (VP) and interval velocity determined by the check-shot survey, and (g) log sub-units.

Figure 8. Core-log integration at Hole U1317 gamma-ray, porosity, density, velocity. Post-cruise correlation between features recorded in these logs (especially acoustic, density and gamma-ray) offer potential to provide in-situ ground truth for core data. Initial examination shows that the core depths are offset 4-6 m downwards from the log depths.

Figure 9. Thermal measurements at Site U1317. (a) Drilling mud temperature measurements using the EMS (blue, down logging) and TAP tool (red, down and up logging) compared to ADARA measurement, (b) thermal conductivity measurements on core (dot) and in-situ (ADARA measurement, star).

Site U1317

Site U1317 is located on the northwest shoulder of the Challenger Mound (51º 22.8' N, 11º 43.1' W, in 781 to 815 m water depth). Sediments from the on-mound Site U1317 can be divided into two units; the Pleistocene coral-bearing unit (Unit 1) and the Neogene siltstone (Unit 2). Unit 1 consists mainly of coral (mostly identified as Lophelia pertusa), floatstone, rudstone, wackestone, and packstone, and repeats cyclic color change between light grey and dark green. This coral mound unit rests on Unit 2 with a sharp erosional boundary that appears identical to the boundary between Units 2 and 3 of Site U1316. Unit 2, consists of glauconitic and partly sandy siltstone. It is lithologically correlated with Unit 3 at Site U1316.



Triple Combo, FMD-Sonic downhole logs, and a zero-offset VSP were between 80 and 245 mbsf in Hole U1317D. The density, resistivity, and acoustic velocity logs show a steady downhole increase due to compaction, interrupted by 1-5-m-thick intervals of higher values, indicating the presence of more lithified layers similarly to Hole U1316C. The PEFL values for these layers indicate they are carbonate-rich. These lithified layers are the cause of the high amplitude sigmoidal reflectors observed in the seismic profiles (Figs 7 and 8). Interval velocities were calculated from the checkshot survey (Table 2): they confirm the values of the acoustic velocity logs, but show that the physical property measurements made on the cores significantly underestimate the in-situ velocity.

Stack number	Measured depth	Depth	Measured 1W-TT	Corrected 1W-TT	Interval velocity
	(m)	(mbsf)	(ms)	(ms)	(ms)
13	905	94	583.43	583.86	1767.86
12	915	104	589.07	589.52	1982.16
11	925	114	594.11	594.56	1723.99
10	935	124	599.90	600.36	1938.86
9	945	134	605.04	605.52	1770.69
8	955	144	610.68	611.17	2032.27
7	965	154	615.59	616.09	1901.70
6	980	169	623.52	624.03	2005.52
5	995	184	630.98	631.51	1978.63
4	1010	199	638.49	639.04	1669.09
3	1025	214	646.15	646.71	2201.70
2	1040	229	652.90	653.47	2161.09
1	1056	245	660.29	660.88	-

Table 2. Check-shot survey at Hole U1317D

 

Figure 10. Log stratigraphy at Hole U1316C. (a) Hole shape, total gamma ray (HSGR) and Potassium content (HFK), (b) thorium (HTHO) and uranium (HURA) contributions to natural radioactivity, (c) deep (IDPH), intermediate (IMPH) and shallow (SFLU) resistivities, (d) porosity (APLC) and formation density (RHOM), (e) capture cross-section (SIGF) and photoelectric factor (PEFL), (f) downhole compressional velocities (VP), and (g) log sub-units.

Figure 11. Core-log integration at Site U1316 gamma-ray, porosity, density, velocity. Post-cruise correlation between features recorded in these logs (especially acoustic, density and gamma-ray) offer potential to provide in-situ ground truth for core data.

Site U1318


Site U1318 (423 m water depth, 51° 26.16' N, 11° 33.0'W) is located in on the eastern slope of the Porcupine Seabight on the southwest continental margin of Ireland and is upslope from the Belgica Mound Province, including Challenger Mound. Sediments from the up-slope Site U1318 were divided into three units based on sediment colors, erosional surfaces, and biostratigraphy. The uppermost Unit 1 is 79.9 - 82.0 m thick, and consists of brown-colored silty clay with black motted structure, which is partly laminated and bioturbated. Dropstones are common in this unit. Across a distinct erosional surface, Unit 2 of 4-6 m thick underlies. This unit mainly consists of olive-gray, medium-fine sand interbedded with dark yellowish-brown silty clay. The sand beds are normal graded with sharp lower and upper boundaries. Dropstones, up to 3 cm in diameter, are found in both sand and clay horizons. The base of this unit is a conglomerate resting on a distinct erosional surface. It is 5-10 cm thick, and associated with black-colored apatite nodules. The Unit 3 of 155 m thick (Hole U1318B) consists of dark green siltstone, which frequently intercalates with sandstone layers in the upper and lower horizons. The siltstone tends to become calcareous to downward.

Triple Combo and FMS sonic downhole logs were acquired between 70 and 240 mbsf in Hole U1318B. The downhole logs are characterized by low amplitude variations in lithological subunits 3A and 3B (92-192 mbsf), and by increased velocity and thin lithified layers in subunit 3C (below 192 mbsf) (Figs 10 and 11). The hiatus represented by the oyster bed at the base of unit 2 is rich in uranium (as seen in the natural gamma radiation logs), which tends to accumulate at hiatuses and condensed intervals. Major changes in physical properties were observed at lithological unit boundaries that can be directly related to reflectors in the seismic section. The sand layers, silty clays, dropstones and oyster bed of lithological Unit 2 create a high amplitude reflector in the seismic profiles, and this erosive reflector has been tentatively identified as the upslope continuation of the moundbase reflector. The enigmatic low amplitude seismic package, whose identification was one of the main aims of drilling this site, corresponds to homogeneous calcareous silty clays. Lithostratigraphic Subunit 3C, below 192 mbsf, is characterized by a slight general increase in density in combination with some high density thin beds, and corresponds with high amplitude, high frequency parallel reflectors which can be traced along the seismic profile to the sigmoid unit at Site U1316.

Conclusion

All of the three sites on Expedition 307 were logged, with a checkshot survey undertaken in Site U1317 (Challenger Mound). High quality data was acquired in all of the logged holes due to a combination of good heave compensations and excellent borehole conditions. The logging data will be used for a range of research topics including,

- (1) core-log correlation for refining core composite depth splices and correction of core physical properties for in-situ condition, a necessary step to obtain correct sedimentation and mass-accumulation rate;

- (2) core-log-seismic integration and structural use of FMS images for interpreting the complex seismic stratigraphy of Porcupine Seabight by performing a core - log - seismic integration (seismic modelling) and characterizing azimuth and dip of bedding and fractures identified on FMS images (manual picking);

- (3) high-resolution analysis of micro-resistivity logs (FMS images) for cyclostratigraphic analyses.

Already many results are clear, as detailed in the initial reports. The mound is composed of coral, clay, and coccoliths down to its base at 130-155 mbsf, and at least 10 distinct layers - growth rings of the coral mound - are evident in the lithology and physical properties. Much of the late Pleistocene material has been eroded from the top of the mound, while at the same time siliciclastic sediment is building up in drifts both upslope and downslope: the mound is slowly being buried. The theory that this mound is built from carbonate precipitated by microbes fed by methane seeps has been disproved. The lithology and age of the enigmatic sedimentary packages that underlie the mound, known previously only from seismic lines, have been identified. The mound is rooted on an erosive unconformity that has been identified in all three sites, and directly below the mound a thin layer of early Pliocene sediments overlies a thick early Miocene package of green-grey calcareous siltstones.

Based on data collected at Site U1318 further studies will allow detailed characterization of the semitransparent basement layer, off-mound Site U1316. Data from the basal sequence on-mound Site U1317 will allow investigation on the nature of the sigmoidal units. Detailed studies at both Sites U1316 and U1318 will refine the age of the unconformity and the importance of the hiatus. Finally, data from the on-mound Site U1317 will unveil the environmental record locked in a carbonate mound and will shed light on the processes that may have controlled the genesis of the mound, and assessing the importance of environmental forcing factors.

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Philippe Gaillot: Logging Staff Scientist, Center for Deep Earth Exploration (CDEX), Japan Marine Science and Technology Center, Yokohama Institute for Earth Sciences, 3173-25 Showa-machi, Kanazawa-ku, Yokohama, Kanagawa 236-0001, Japan

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