Tectonic and sedimentary structures in the northern Chukchi region, Arctic Ocean

Authors


Corresponding author: A. Hegewald, Alfred Wegener Institute for Polar and Marine Research, Post Box 120161, DE-27515 Bremerhaven, Germany. (Anne.Hegewald@awi.de)

Abstract

[1] The interpretation of tectonic and sedimentary structures in the northern Chukchi region, Arctic Ocean, is important to enhance our understanding of the tectonic evolution of this region. Therefore, multichannel seismic lines as well as seismic wide-angle reflection and refraction data were acquired in the northern Chukchi region during the RV Polarstern ARK-XXIII/3 summer expedition in 2008. These data have been processed and interpreted for the three main geological provinces (Chukchi Plateau, Chukchi Abyssal Plain, and Mendeleev Ridge) to describe the sedimentary and basement structures of the northern Chukchi region. Furthermore, using the age control of five exploration wells drilled near the coast of Alaska in combination with additional seismic reflection lines located on the Chukchi Shelf, we were able to date sediment horizons within the research area. In total, six sediment horizons with ages between Barremian/Hauterivian and the Top Miocene were identified. Especially, the Top Oligocene horizon forms a pronounce unconformity on the Chukchi Plateau and on the Mendeleev Ridge flanks. The origin of this unconformity can be associated with the opening of the Fram Strait indicating a significant change in the Arctic Ocean current system.

1 Introduction

[2] The northern Chukchi region—consisting of Chukchi Borderland and Chukchi Abyssal Plain—is part of the Amerasian Basin, Arctic Ocean (Figure 1). The opening of the Amerasian Basin remains controversial [e.g., Lawver and Scotese, 1990; Lane, 1997; Miller et al., 2006] owing to the paucity of available geophysical and geological data. The acquisition of new data is quite difficult because of the limited accessibility due to the perennial ice cover and the short summer season.

Figure 1.

Bathymetric map (IBCAO [Jakobsson et al., 2008]) showing seismic reflection lines, sonobuoys as well as exploration wells within our research area (northern Chukchi region and southern part of the Mendeleev Ridge). Abbreviations: CAP - Chukchi Abyssal Plain, CBL - Chukchi Borderland, CC - Chukchi Cap, CG - Charlie Gap, CP - Chukchi Plateau, CR - Chukchi Rise, CS - Chukchi Shelf, MR - Mendeleev Ridge, NR - Northwind Ridge, WI - Wrangel Island.

[3] During the ARK-XXIII/3 expedition in the summer of 2008, the Alfred Wegener Institute (AWI) acquired the first seismic data set—including seismic reflection lines as well as seismic wide-angle reflection and refraction data—in the northern Chukchi region (Chukchi Plateau as part of the Chukchi Borderland and Chukchi Abyssal Plain) as well as the southern part of the Mendeleev Ridge (Figure 1) [Jokat et al., 2009].

[4] The stratigraphy of the Chukchi Borderland was reconstructed from 15 piston cores and one box core that sampled bedrock outcrops on the Northwind Ridge carried out by three U.S. expeditions in 1988, 1992, and 1993 (Figure 1) [Grantz et al., 1998]. This study reported the presence of Phanerozoic basement (Cambrian, Ordovician, and Carboniferous to Cretaceous shelf-facies strata). The Chukchi Plateau is divided into a southern part (Chukchi Rise) and a northern part (Chukchi Cap, Figure 1) [Shaver and Hunkins, 1964, Jakobsson et al., 2008]. These plateaus rise as much as 3400 m above their surroundings. They are relatively shallow (less than 300 m below sea surface) and have steep flanks. The Chukchi Plateau is bounded to the south by the shallow Chukchi Shelf (Figure 1).

[5] The stratigraphy of sediments deposited in the Chukchi Abyssal Plain (Figure 1) is poorly known, as is the age of its basement. The Chukchi abyssal plain is bound by the Chukchi Plateau, the Mendeleev Ridge, and the Chukchi Shelf (Figure 1). Northward, the Chukchi Abyssal Plain is linked by the narrow Charlie Gap with the Canada Basin (Figure 1) [Shaver and Hunkins, 1964]. The average water depth above the flat Chukchi Abyssal Plain sea floor is 2.3 km [Jakobsson et al., 2008].

[6] The evolution of the Mendeleev Ridge remains controversial, and its suggested age varies from Paleozoic to Early Tertiary [Hall, 1970; Weber and Sweeney, 1990; Lebedeva-Ivanova et al., 2006; Verzhbitsky et al., 2011]. Its origin has been explained, independently of age, as: (1) an oceanic plateau formed at a spreading center [Hall, 1970], (2) a single oceanic plateau created by hot spot volcanism [Forsyth et al., 1986; Lawver and Müller, 1994], or (3) a rifted volcanic continental margin [Lebedeva-Ivanova et al., 2006] underscores its controversial nature. The Mendeleev Ridge separates the Canada Basin from the Makarov Basin (Figure 1). In the research area, the Mendeleev Ridge is characterized by rough topography and rises as much as 2500 m above the surrounding basins. The minimum water depth above the Mendeleev Ridge is about 800 m [Jakobsson et al., 2008].

[7] The interpretation of the tectonic and sedimentary structures in the northern Chukchi region utilized two other data sets: (1) seismic reflection lines from the Chukchi Shelf, the Northwind Ridge, and the northern part of the Mendeleev Ridge (Figure 1) [Grantz et al., 2004; Arrigoni, 2008; Dinkelman et al., 2008; Verzhbitsky et al., 2008; Dove et al., 2010; Drachev et al., 2010; Kumar et al., 2011], and (2) logging/age information from five exploration wells (e.g., depth log, sonic log, gamma ray log, and biostratigraphic ages) drilled on the Chukchi Shelf near the coast of Alaska between 1989 and 1991 (Figure 1) [Sherwood et al., 2002] The maximum depth of these wells varies between 2000 m and 3600 m, and the oldest sediments are of Permian age [Sherwood et al., 2002].

2 Seismic Data—Acquisition and Processing

[8] In the summer of 2008, the AWI collected more than 3300 km of multichannel seismic (MCS) lines as well as seismic wide-angle reflection and refraction data (Figure 1) [Jokat et al., 2009]. For the MCS data acquisition, two different streamers were used, depending on the sea ice coverage during the expedition: (1) a 3000 m long streamer with 240 active channels and a group interval of 12.5 m, and (2) a 600 m long streamer with 96 active channels and a group interval of 6.25 m. The seismic energy was generated with an air gun array of 4 G-Guns (total volume of 32 l, fired with 200 bar every 15 s). Furthermore, using 12 sonobuoys seismic wide-angle reflection and refraction data were recorded in parallel to the MCS data. Their recording time varied between 3 h and 5 h, which resulted in 21 km to 34 km offsets, respectively, between the seismic source and the sonobuoys.

[9] After data acquisition, the MCS data were demultiplexed and CDP sorted (25 m CDP interval). Using a band-pass filter of 10 Hz to 100 Hz, the data were filtered, and a velocity model was determined for each seismic line. Afterwards, the spherical divergence and the normal moveout were corrected. The multiples attenuation was done by f-k filtering. Finally, the seismic data were stacked, and time migrated in the omega-x domain.

[10] The sonobuoy data were filtered with a band-pass filter of 5 Hz to 20 Hz, and an automatic gain control with a time window of 0.7 s was applied. The velocity-depth models (Figure 2) were calculated by 2D ray tracing (Figure 2) [Zelt and Smith, 1992].

Figure 2.

Velocity-depth functions of four sonobuoys used in this study. The average interval velocities are shown in km/s.

[11] Regarding the uncertainties of the seismic data, the vertical and horizontal resolutions are characterized by the Fresnel zone [Militzer and Weber, 1987]. The vertical resolution of our MCS data is about 15 m below the seafloor (using a seismic velocity of 1.8 km/s and a peak frequency of 35 Hz) and decreases to about 100 m at a depth of 8 km (using a seismic velocity of 5.0 km/s and a peak frequency of 18 Hz). The horizontal resolution is about 40 m at a depth of 50 m below the seafloor (using a seismic velocity of 1.8 km/s and a peak frequency of 35 Hz).

3 Age Control for Seismic Data and Age Model

[12] The age control for our MCS data is based on five exploration wells located on the northwest coast of Alaska (Figure 1) and additional seismic reflection lines from the Chukchi Shelf (Figure 1) [Grantz et al., 2004; Arrigoni, 2008; Dinkelman et al., 2008; Verzhbitsky et al., 2008; Dove et al., 2010; Drachev et al., 2010; Kumar et al., 2011]. Using the logging information (sonic log, depth log, gamma ray log, and age control based on biostratigraphy [Sherwood et al., 2002]), we correlated six dated sediment horizons with seismic reflection lines crossing the exploration wells. Following Sherwood et al. [2002], the six marker horizons have ages between Lower Cretaceous (Barremian-Hauterivian) and Top Miocene. Hence, to transfer the age control into the new MCS network (Figure 1), we correlated these marker horizons northward over a distance of more than 200 km through the existing southern seismic network. We extrapolated the horizons over the “data gap” of about 70 km between the southern seismic network and our MCS lines (Figure 1) using seismic interval velocities of the sediments, reflection characteristics of the seismic horizons, and our insight into the unprocessed seismic reflection lines from the U.S. expedition MGL 11–12 which are located within the “data gap” [Coakley et al., 2011]. However, it was not possible to follow each horizon trough the entire new MCS network because of basement highs, faults, unconformities, and variations in sediment thickness. In total, three of the six horizons (Base Tertiary, Top Oligocene, and Top Miocene) could be identified in the entire new MCS network.

[13] Our age model consists of four horizons (Figure 3). The oldest sediment horizon is the Lower Cretaceous unconformity (Barremian-Hauterivian) which is characterized by two high amplitude seismic reflectors within the research area. The Mid Brookian unconformity (Base Tertiary) is a high amplitude seismic reflector in the middle of a thick, transparent seismic reflector band. The Top Oligocene horizon forms the top of a thin, undulated, high amplitude seismic reflector band. Finally, the Top Miocene horizon is a high amplitude seismic reflector in the middle of a mostly transparent, thick, and undisturbed seismic reflector band.

Figure 3.

Section from seismic line 20080045 located in the Chukchi Abyssal Plain showing the reflection characteristics of four interpreted sediment horizons with ages [Gibbard et al., 2010] between Barremian/Hauterivian and Top Miocene. Abbreviations: see previous figures and LCU - Lower Cretaceous Unconformity.

4 Main Geological Framework

[14] We have studied and analyzed seismic data from three main geological provinces (Figure 1): the Chukchi Plateau which is part of the Chukchi Borderland, the Chukchi Abyssal Plain, and the southern part of the Mendeleev Ridge. The subsequent sections include the data, results, and interpretation for each province.

4.1 Chukchi Plateau

4.1.1 Results

[15] Three MCS lines were acquired across the western part of the Chukchi Plateau. Figure 4 shows line 20080050, extending from the Chukchi Shelf to the Chukchi Rise, and line 20080045, crossing the western flank of the Chukchi Rise. Line 20080001 is located on the Chukchi Cap.

Figure 4.

Three interpreted seismic lines showing the deeper structures of the Chukchi Plateau, which is divided into the Chukchi Rise and the Chukchi Cap. The line numbers are abbreviated to the last two digits of the original line numbers. The velocity-depth function of one sonobuoy is incorporated. Abbreviations: see Figure 1 and ESS - East Siberian Shelf, MBU - Mid Brookian Unconformity, SB - Sonobuoy.

[16] Along the three MCS lines (Figure 4), the acoustic basement of the Chukchi Plateau is highly fragmented by numerous normal faults (shown by dissected reflection pattern) forming horst and graben structures. Based on the sonobuoy SB0803 data (Figure 2), the interval velocity of the basement is 5.2 km/s. In total, the Chukchi Rise basement high (Figure 4, line 20080050) and two grabens below the Chukchi Cap (Figure 4, line 20080001) can be seen. Toward the Chukchi Shelf, the basement deepens into the North Chukchi Basin [Drachev et al., 2010] and could not be imaged by the seismic data (Figure 4, southern end of line 20080050). The steep western flank of the Chukchi Rise (Figure 4, line 20080045) is dominated by normal faults which caused a maximum basement offset of 300 m across an individual fault. Several normal faults along the western flank of the Chukchi Rise (line 20080045), the Chukchi Cap (line 20080001) as well as the Northwind Ridge (part of the Chukchi Borderland, Figure 1) [Arrigoni, 2008] affected the overlying sediments, mostly shown as offsets along the faults. Some of these normal faults reach the seafloor (Figure 5a).

Figure 5.

Zoomed part of seismic line 20080001 and 20080050 located on the Chukchi Plateau. For the position of the zoomed parts see Figure 3. (a) Sediment filled channel on the Chukchi Cap and Top Oligocene horizon which presents an erosional surface. (b) Picture detail of line 20080050 showing the Oligocene and Miocene sediments thinning out against the normal faults of the Chukchi Rise basement high. Abbreviations: see previous figures.

[17] The thickness of the sediment cover varies across the Chukchi Plateau. Above the Chukchi Rise basement high, the average sediment thickness is 600 m. In contrast, next to the Chukchi Rise basement high, the average sediment thickness is 1.6 km (Figure 4; line 20080050). To the north (77.5°N), the average sediment thickness decreases to 400 m, and to about 1 km in the two grabens on the Chukchi Cap (Figure 4; line 20080001). The average interval velocity for the sediments is 2.3 km/s (Figure 2; SB0803). In total, three seismic reflectors with ages of Base Tertiary, Top Oligocene, and Top Miocene were correlated within the Chukchi Plateau.

[18] The Mid Brookian unconformity (Base Tertiary) is the oldest of the horizons correlated on the Chukchi Plateau. This horizon is only found at three locations (Figure 4): (1) between the Chukchi Shelf and the Chukchi Rise basement high (southern end of line 20080050), (2) at the western flank of the Chukchi Rise (line 20080045), and (3) tentatively in the deep channel on the Chukchi Cap (Figure 5a). The Mid Brookian unconformity onlaps onto preexisting basement topography.

[19] The Top Oligocene horizon cuts into older sediments on the western slope of the Chukchi Cap (Figure 5a). This horizon and the Top Miocene horizon were found on the entire Chukchi Plateau, excluding the Chukchi Rise basement high area (Figure 4). Both horizons are less affected by basement faulting. At few locations, however, both horizons are dissected by faults (Figures 4a and 4b). The Oligocene and Miocene sediments have a constant thickness of approximately 1.4 km on the Chukchi Rise and about 250 m on the Chukchi Cap (Figure 4). Both sediments thin onto the Chukchi Rise basement high (Figure 5b). The uppermost sediments (above Top Miocene horizon, Figure 4) decrease in thickness from south (75°N; about 3 km) to north (77.5°N; about 300 m). Pliocene and Quaternary sediments cover the Chukchi Rise basement high (Figure 4, line 20080050).

4.1.2 Interpretation

[20] Several studies [Grantz et al., 1979; Hall, 1990; Vogt et al., 1998; Klemperer et al., 2002] suggest that the Chukchi Shelf basement consists of continental crust and evolved in an E-W directed extensional regime. Our results—basement interval velocity of 5.2 km/s (Figure 2), horst and graben structures with large offsets in mainly E-W direction (not involving sediments) as well as sediments older than the Mid Brookian unconformity (Figure 4)—confirm the published interpretation.

[21] The entire Chukchi Plateau is covered by Tertiary sediments which decrease in thickness from south to north. These sediments originated from the Chukchi Shelf as evidenced by the prograding sediment horizons at the southern end of line 20080050 (Figure 4) [Hegewald and Jokat, 2013].

[22] The sediments below the Top Oligocene horizon show undulating seismic reflectors with high impedance contrasts which might be the results of tectonic activity (e.g., opening of the Canada basin, evolution of the Chukchi Borderland) and significant contrast in sediment character (reflecting a generally starved setting with minimal influx of terrigenous clastic sediment [Houseknecht and Bird, 2011]). The Top Oligocene horizon is an erosional surface on the western slope of the Chukchi Cap (Figure 5a), represented by a high amplitude seismic reflector. Above the Top Oligocene horizon, the sediments consist of continuous and undisturbed seismic reflectors with low impedance contrast (reflecting the arrival of the prograding sediment dispersal system into the study area [Houseknecht and Bird, 2011]). Hence, the changing of sedimentation characteristic at the Top Oligocene horizon was probably produced by variation in Arctic Ocean current. Following Hegewald and Jokat [2013], a significant sea level lowering event occurred at about Top Oligocene and is associated with the beginning of the opening of the Fram Strait at that time [Engen et al., 2008].

[23] The Top Miocene horizon marks a high impedance contrast within the otherwise transparent seismic reflector band. This horizon might have been deposited during the flooding of the Bering Strait in the Late Miocene/Pliocene [Gladenkov et al., 2002; Verhoeven et al., 2011]. The flooding of Bering Strait may have been abetted by a global eustatic sea level rise [Haq et al., 1987] and allowed an exchange of north Pacific with Arctic Ocean water. This event probably caused a temporal variation in material and/or ocean current on the Chukchi Shelf and within the northern Chukchi region.

4.2 Chukchi Abyssal Plain

4.2.1 Results

[24] Five MCS lines were acquired across the Chukchi Abyssal Plain. Figure 6 shows lines 20080065, 20080045, and 20080090 crossing the Chukchi Abyssal Plain. Lines 20080010 and 20080005 image the Charlie Gap (Figure 6).

Figure 6.

Five interpreted seismic lines showing the deeper structures of the Chukchi Abyssal Plain. The line numbers are abbreviated to the last two digits of the original line numbers. The velocity-depth functions of two sonobuoys are incorporated. Abbreviations: see previous figures.

[25] Along the five MCS lines, the top of the acoustic basement is characterized by a high amplitude seismic reflector band. Based on the data of two sonobuoys (SB0809, SB0814; Figure 2), the minimum interval velocity of the basement is 5.4 km/s. Furthermore, few sub-basement dipping reflectors are identified in the Chukchi Abyssal Plain (Figure 7a) and along the eastern slope of the Mendeleev Ridge (Figure 7b). Both areas have an average interval velocity of 4.1 km/s (Figure 7b). In general, the acoustic basement is characterized by horst and graben structures (Figure 6). The maximum basement offset across an individual fault is approximately 1 km.

Figure 7.

Zoomed part of seismic line 20080065 and 20080090 located within the Chukchi Abyssal Plain. For the position of the zoomed parts, see Figure 6. (a) Sub-basement dipping reflectors (black) within the Chukchi Abyssal Plain. (b) Sub-basement dipping reflectors (black) at the foot of the eastern slope of the Mendeleev Ridge. Abbreviations: see previous figures.

[26] The sediment thickness in the Chukchi Abyssal Plain varies from 4 km next to the Chukchi Plateau (Figure 6; line 20080045) to 2.5 km next to the Mendeleev Ridge (Figure 6; line 20080065) and 1 km in the Charlie Gap (Figure 6; line 20080010). The oldest of the identified horizons is the Lower Cretaceous unconformity (Figure 3). This horizon is offset by basement faulting in the western part of the Chukchi Abyssal Plain, next to the Mendeleev Ridge (Figure 6, lines 20080065 and 20080090), but not in the eastern part of the Chukchi Abyssal Plain, next to the Chukchi Plateau (Figure 6, line 20080045). Furthermore, the sediments below the Lower Cretaceous unconformity as well as the Mid Brookian unconformity become thinner from east to west and pinch out by onlap against the basement of the Chukchi Plateau and the Mendeleev Ridge (Figure 6, lines 20080045, 20080065, and 20080090). In the Charlie Gap, however, the Lower Cretaceous unconformity could not be identified, and the Mid Brookian unconformity is significantly offset by basement faulting (Figure 6, lines 20080010 and 20080005).

[27] The Top Oligocene horizon is an erosional surface at the flanks of Mendeleev Ridge and Chukchi Plateau, and grades into a correlative conformity in the Chukchi Abyssal Plain (Figure 6). Above this horizon, the sediment package has a constant thickness of about 500 m. The Top Oligocene and Top Miocene horizons are not affected by faulting.

4.2.2 Interpretation

[28] The Chukchi Abyssal Plain evolved in a mainly E-W directed extensional regime indicated by the basement horst and graben structures that formed perpendicular to extension. The basement very likely consists of oceanic crust due to the high impedance contrast between the sediments and the basement with interval velocities of 4 km/s and more than 5.6 km/s, respectively (SB0809; Figure 2), as well as the dipping reflectors which most likely represent submarine basalt flows (Figure 7).

[29] Concerning its age, the Chukchi Abyssal Plain basement is older than the Lower Cretaceous unconformity. If we assume a sedimentation rate of 2 cm/ky to 3 cm/ky, which correspond to the typical sedimentation rates in the Chukchi Abyssal Plain resulting from or interpreted seismic reflection data (Figure 8), the sediments between Lower Cretaceous unconformity and acoustic basement were deposited within 70 Ma to 30 Ma. Therefore, the Chukchi Abyssal Plain basement likely evolved in Jurassic time.

Figure 8.

Stratigraphic correlation between the northern Chukchi region, the southern Mendeleev Ridge, and the northeastern East Siberian Shelf. The lower part is showing the thickness of sediments between correlated horizons and calculated sedimentation rates. The line numbers are abbreviated to the last two digits of the original line numbers. Abbreviations: see previous figures.

[30] Within the eastern part of the Chukchi Abyssal Plain, next to the Chukchi Plateau, the Lower Cretaceous unconformity is not offset by basement faulting (Figure 6). However, in the west, next to the Mendeleev Ridge, the Lower Cretaceous unconformity was offset by basement faulting (Figure 6). These observations lead to the conclusion that the Chukchi Abyssal Plain evolved in parallel with the opening of the Amerasian Basin in Jurassic to Early Cretaceous time [Grantz et al., 1979].

[31] The younger sediments show two main reflection characteristics separated by the Top Oligocene horizon like on the Chukchi Plateau: (1) high impedance contrasts of undulated seismic reflectors below the horizon, and (2) low impedance contrasts of continuous and undisturbed seismic reflectors above the horizon. These results confirm our interpretation of the Chukchi Plateau data, that the Top Oligocene horizon marks a regional event—beginning of the opening of the Fram Strait [Engen et al., 2008]—which caused a significant sea level lowering event in the Arctic Ocean [Hegewald and Jokat, 2013], due to the outflow from the Arctic Ocean water into the North Atlantic Ocean, and therefore, a changing of the Arctic Ocean current system [Jakobsson et al., 2007].

[32] The Top Miocene horizon shows the same high impedance contrast within the otherwise transparent seismic reflector band like on the Chukchi Plateau. Hence, this observation supports our interpretation from the Chukchi Plateau that the Top Miocene horizon represents the flooding of the Bering Strait [Gladenkov et al., 2002; Verhoeven et al., 2011] which probably caused a temporal variation in sediment character and/or ocean current.

4.3 Mendeleev Ridge

4.3.1 Results

[33] Four MCS lines were acquired across the Mendeleev Ridge. Figure 9 shows the location of lines 20080090, 20080030, and 20080010 that cross the Mendeleev Ridge in an E-W direction. Line 20080085 extends in S-N direction from the East Siberian Shelf toward the Mendeleev Ridge.

Figure 9.

Four interpreted seismic lines showing the southern part of the Mendeleev Ridge. The line numbers are abbreviated to the last two digits of the original line numbers. The velocity-depth functions of two sonobuoys are incorporated. Abbreviations: see previous figures.

[34] In the four MCS lines, the acoustic basement is dominated by several normal faults forming horst and graben structures (Figure 9). Based on the interval velocities of two sonobuoys (SB0801, SB0814; Figure 2), the basement can be divided into two subunits. Below the top of acoustic basement, the interval velocities range between 3.6 km/s and 4.1 km/s within a section 2 km thick (Figure 9). This upper subunit contains dipping reflectors at the top of the Mendeleev Ridge (Figure 10a). The lower basement subunit is characterized by an interval velocity of 5.4 km/s (SB0801, SB0814; Figure 2).

Figure 10.

Zoomed parts of seismic line 20080010 showing the southern part of the Mendeleev Ridge. For the position of the zoomed parts, see Figure 9. Both zoomed parts were plotted with different vertical exaggeration. (a) Sub-basement seismic reflectors (black) at the top of the Mendeleev Ridge. (b) The Top Oligocene horizon cuts into older sediments at the steep flanks of the southern Mendeleev Ridge. Abbreviations: see previous figures.

[35] The sediments which cover the Mendeleev Ridge have a thickness of about 1 km which increases to approximately 6 km next to the East Siberian Shelf (Figure 9, line 20080085). The interval velocities of the sediments (1.6 km/s – 2.3 km/s) are similar to those from the Chukchi Plateau and the Chukchi Abyssal Plain (Figure 2). In total, three horizons (Mid Brookian unconformity, Top Oligocene, and Top Miocene) were identified in the seismic network of the southern Mendeleev Ridge (Figure 9). The Mid Brookian unconformity is offset by basement faulting, and the offsets are of similar magnitude to those of the acoustic basement (<800 m). However, the Mid Brookian unconformity is absent across several basement highs (Figure 9, line 20080010). The Top Oligocene and Top Miocene horizons show no faulting. Figure 10b displays the Top Oligocene horizon which cuts into older sediments on the Mendeleev Ridge flanks. This type of erosion is found at all steep flanks of the southern Mendeleev Ridge (Figure 9). The sediments above the Top Oligocene horizon are not affected by faulting. This upper sediment unit consists of continuous, parallel, transparent seismic reflectors with a total thickness of 300 m across the entire southern Mendeleev Ridge region (Figure 9).

4.3.2 Interpretation

[36] The subdivision of the northern Mendeleev Ridge basement into two units was prior observed by Dove et al. [2010] based on four sonobuoys, located at the northeastern flank of the Mendeleev Ridge. Hence, the upper sub-basement unit was modeled with interval velocities ranging from 3.5 km/s to 4.0 km/s. This upper unit most likely represents submarine basalt flows which erupted during the last stage of formation of the Mendeleev Ridge. The lower unit has interval velocities higher than 5 km/s which correspond to our sonobuoy data (Figure 2; SB0801, SB0814) and probably consists of basalt, too [Schön, 2004].

[37] The absence of the Lower Cretaceous unconformity, as well as the similarly faulted Mid Brookian unconformity and basement, suggest that development of the Mendeleev Ridge was completed perhaps during the Late Cretaceous to Early Tertiary, clearly later than opening of the Canada Basin. Moreover, the absence of the Mid Brookian unconformity at the tops of several basement highs leads to the conclusion that erosion occurred at these positions after submarine ridge formation (Figure 9, line 20080010) [Dove et al., 2010].

[38] The flank erosion along the Top Oligocene horizon is found in 500 m to 1000 m water depth such as along the northwestern Chukchi Plateau flank (Figure 5a; line 20080001). Furthermore, the high impedance contrasts below the Top Oligocene horizon, and the low impedance contrast above this horizon lead to the same interpretation as for the Chukchi Plateau and the Chukchi Abyssal Plain. Hence, the opening of the Fram Strait [Engen et al., 2008] with the following significant relative sea level lowering [Hegewald and Jokat, 2013] and the changing of the Arctic Ocean current system [Jakobsson et al., 2007] might have caused the described variations in sedimentation characteristics, because of its regional, simultaneously influence to the sediment deposition/erosion in the Chukchi and Mendeleev Ridge region. The high amplitudes of the Top Miocene reflector within the entire northern Chukchi region were probably caused by the flooding of the Bering Strait and also represent an event with regional influence to the sediment deposition.

5 Discussions

5.1 Quality of Age Control in the Northern Chukchi Region

[39] The five exploration wells, drilled on the Chukchi Shelf near the coast of Alaska, are located more than 200 km southeast of our MCS lines (Figure 1) [Sherwood et al., 2002]. Based on the multiple crossing points of the southern seismic lines, it was possible to identify the six marker horizons with a high accuracy within the southern seismic network. Over the “data gap,” the marker horizons were extrapolated from the southern seismic network into our MCS lines, using seismic interval velocities, reflection characteristics of the seismic reflectors as well as our insight into the unprocessed seismic reflection lines from the U.S. expedition MGL 11–12 crossing the “data gap” [Coakley et al., 2011].

[40] The special type of flank erosion at the southern Mendeleev Ridge (Figure 10b) and the northwestern Chukchi Plateau (Figure 5a) as well as the subsequent covering by younger, undisturbed sediments also were described by Dove et al. [2010] for the northern Mendeleev Ridge and by Bruvoll et al. [2010] for the northwestern Alpha Ridge. However, both authors did not present any age control for the unconformity or any other sediment horizon. Bruvoll et al. [2010] proposed two age models based on the correlation of seismic images from the northern Mendeleev Ridge, the northwestern Alpha Ridge and the central Lomonosov Ridge with the IODP core from the ACEX expedition 302 to the central Lomonosov Ridge [Jakobsson et al., 2007]. Depending on the two models, the unconformity can be older than Early Eocene (>49.7 Ma, model 1) or younger than Mid Eocene (<45.4 Ma, model 2 [Bruvoll et al., 2010]). Our dated unconformity age of Top Oligocene corresponds to model 2 after Bruvoll et al. [2010].

[41] Based on our results, we concluded that the Chukchi Abyssal Plain evolved during the opening of the Canada Basin [Grantz et al., 1998] which is in accordance with Grantz and Hart [2006]. Finally, we suggested that the Mendeleev Ridge evolved in the Early Tertiary, which is in accord with Hall [1970], Forsyth et al. [1986], Lawver and Müller [1994] and Lebedeva-Ivanova et al. [2006]. Therefore, the ages of our dated horizons within the northern Chukchi region are of good accuracy.

5.2 Origin of the Top Oligocene Unconformity

[42] The Top Oligocene horizon is observed in the entire new seismic network covering the northern Chukchi and southern Mendeleev Ridge region. Especially on the Chukchi Plateau and at the flanks of the southern Mendeleev Ridge, the Top Oligocene horizon forms an erosional surface. This type of flank erosion is also reported by Bruvoll et al. [2010] for the Alpha and Lomonosov Ridge as well as by Dove et al. [2010] for the northern Mendeleev Ridge. Furthermore, the IODP core from the ACEX expedition 302 to the central Lomonosov Ridge shows a 26 Ma hiatus due to erosion and/or nondeposition between 44.5 Ma and 18.2 Ma [Jakobsson et al., 2007]. In contrast, following Poirier and Hillaire-Marcel [2011], the 26 Ma gap represents a time interval where the sedimentation regime on the Lomonosov Ridge changed from lake stage to marine stage with very low sedimentation rates (<0.2 cm/ky). In general, this hiatus/gap separates two modes of sedimentation (below - biosiliceous-rich silty clay, above - fossil-poor silty clay with sand lenses and drop stones), which both were deposited in a shallow water regime [O'Regan et al., 2008].

[43] The comparison of the sedimentation rates from Mendeleev Ridge and Chukchi Plateau (Figure 8) for the time intervals Base Tertiary to Top Oligocene (65.5 Ma to 23 Ma; 1 cm/ky – 2 cm/ky), Top Oligocene to Top Miocene (23 Ma to 5.3 Ma; 1 cm/ky), and Top Miocene to present day (5 cm/ky) with sedimentation rates from the central Lomonosov Ridge (ACEX core 302 [Poirier and Hillaire-Marcel, 2011]) for the time intervals 47 Ma to 44.5 Ma (2.5 cm/ky), and 18.2 Ma to present day (0.8 cm/ky – 1.5 cm/ky) show: (1) similar values before the Top Oligocene, and (2) higher values from Mendeleev Ridge and Chukchi Plateau after the Top Oligocene. Following O'Regan et al. [2008], the Lomonosov Ridge rapidly subsided in the Early Miocene. Therefore, the top of the Lomonosov Ridge was in shallow water until the Early Miocene. In contrast, Mendeleev Ridge and Chukchi Plateau occupied deeper water positions.

[44] The opening of the Fram Strait in the Early Miocene [Engen et al., 2008] produced a significant lowering of the arctic relative sea level [Hegewald and Jokat, 2013] and a pronounced change in the ventilation regime of the Arctic Ocean [Jakobsson et al., 2007]. Due to the massive relative sea level fall, the Lomonosov Ridge sediments were eroded more significantly than these on Mendeleev Ridge and Chukchi Plateau. On the other hand, the changing/intensifying of Arctic Ocean currents lead to sediment erosion at the ridge flanks of Mendeleev Ridge and Chukchi Plateau in deeper water regions, and to variation in modes of sedimentation. The latter can be seen in the different reflection characteristics of the seismic reflectors below and above the Top Oligocene horizon (Figures 4a, 7b, and 10b). Therefore, the opening of the Fram Strait in the Early Miocene and its consequences are the most possible events which provide regional erosion and variation in sedimentation characteristic within the entire Arctic Ocean and are the origin for the Top Oligocene unconformity.

6 Conclusions

[45] In the northern Chukchi region, it was possible for the first time to correlate six sediment horizons, using the age information from five exploration wells and additional seismic reflection lines from the Chukchi Shelf. The six horizons have ages between Barremian/Hauterivian and the Top Miocene. The Top Oligocene horizon, which shows erosion on the Chukchi Plateau and at the flanks of the Mendeleev Ridge, might be associated with the opening of the Fram Strait. The consequences of this tectonic event were a significant relative sea level lowering and a changing of the Arctic Ocean current, which influenced erosion and sedimentation processes.

[46] The interpretation of the seismic lines provides an overview of the tectonic activity and the age of evolution of the three geological provinces within the research area (Chukchi Plateau, Chukchi Abyssal Plain, and Mendeleev Ridge). We interpret the Chukchi Abyssal Plain to have evolved in Jurassic to Early Cretaceous time. The Mendeleev Ridge evolved most likely in the Early Tertiary, after the opening of the Amerasian Basin.

Acknowledgments

[47] We thank the captain and the crew of R/V Polarstern as well as the seismic team during the ARK-XXIII/3 expedition for their excellent job. We acknowledge ION-GX Technology, the USGS, and TGS-NOPEC for the insight in their seismic data to constrain the extrapolation of the logging information into our seismic network. Furthermore, we thank Bernard Coakley, who led expedition MGL 11–12 (2011), for a first view of the MCS data, which are located in the “data gap” between the southern seismic network and the AWI MCS data.

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