**6. Case studies**

• **Weathered crust** is a retiring wordage still in use in some geological schools of the World; it is applied to different scale features from thick lateritic mantles with bauxites to alteration rinds on pebbles; mantle-scale weathered crusts are equivalent to *in situ* regoloths *sensu* [62].

• **Meteoric vadose and phreatic zones.** Vadose zone applies to the diagenetic environment lying below the land surface and above the zone of saturation or water table where pore space contains both water and air (soil gas); rainfall waters percolate downward developing vadose patterns of dissolution, reprecipitation and alteration of host sediment. Meteoric phreatic zone applies to aquifer of permanent saturation below the water table and is divided from the vadose zone by a capillary fringe [63]. Concept of vadose and phreatic zones is employed to describe carbonate aquifers, subaerial alterations, and karst systems.

• **Aquifer** is a rock formation saturated with groundwater that is porous and permeable enough for sufficient debit to wells and springs; related are *aquicludes* and *aquitards.* Aquiclude is saturated but do not transmit groundwater; *aquitard* is a low-permeability or

• **Critical zone** is a young concept referring to near-surface environment in which complex interactions involving rock, soil, water, air, and biota regulate the natural habitat and determine availability of life sustaining resources [64]; near-surface terrestrial environment in which resource availability is determined by interactions between the biosphere, geosphere, and atmosphere [51]; includes regolith, within- and below-regolith aquifers, fluvial

Under different climate and hydrologic regimes, soil-forming processes create diverse soil profiles described by national and international soil classifications. The North American soil taxonomy [66] is the one that has earned greatest recognition in paleopedologic studies [47, 51]. Diagnostic criteria for pre-Quaternary paleosols and instrumental proxies for landscape

Diagnostics of subaerial exposure profiles and paleokarst systems in carbonate rocks (**Figure 3B**, **C**) were developed by sedimentary geologists as a parallel story to paleopedology, which was driven by economic importance of karst as (1) hydrocarbon reservoir-making factor and (2) a host for bauxite [34] and rare metal accumulation [69]. This move has generated very practical terminology focused on horizons of high preservation potential, first of all caliches (calcretes) and paleokarsts, with various degree of reconciliation with the soil science lexicon [48, 69-71].

*Drowning unconformities* are "maximum flooding surfaces" (= drowning surfaces *sensu*

In the subsurface, drowning unconformities usually make good seismic reflectors with basinal strata onlapping carbonate slopes and platform tops [72]. On the outcrop or core face, these contacts are characterized by condensed sections (e.g., shell concentrates) and non-deposition

impermeable rock formation, usually strata, that confine water flow.

108 Seismic and Sequence Stratigraphy and Integrated Stratigraphy - New Insights and Contributions

systems, soils, and vegetation up to tree canopy [65].

and climate reconstructions are reviewed in [47, 51, 54, 67, 68].

Posamentier and Allen [17]) specific for carbonate platforms.

**5. Drowning unconformities**

#### **6.1. Permo-Pennsylvanian of Sverdrup Basin, Canadian Arctic Archipelago**

Eight subaerial unconformities define major sequences in the Pennsylvanian and Permian strata of the Sverdrup Basin [87–89]. These unconformities are considered to be subaerial surfaces of long duration (>1 My) bounding thick (100–1000 m) third-order sequences [90]. They are correlated across the basin in outcrops of the basin-margin facies belt. Five of these unconformities and their correlative surfaces were traced in the subsurface of Prince Patrick Island [90]. Recent re-examination of cores has confirmed the presence of subaerial exposure surfaces [91]. Subaerial profiles in cores are mostly decapitated by erosion (transgressive ravinements) but preserve features such as calcretic and ferric replacive deposits, *Microcodium*, root traces, and high-chroma ("red") mottling, which provide a clue for their interpretation (**Figure 4**). Of 49 short (<18.5 m) cores totaling 388 m of recovery, signatures of subaerial alteration were encountered in 8 (**Table 1**). Four of these cores intersect disconformity surfaces and one core penetrated the sub-Pennsylvanian angular unconformity into the Ellesmerian basement (Depot Island C-44, 2458.2 m MD).

**Figure 4.** Permian disconformities of Sverdrup Basin in cores: (A–C) disconformity with a thick paleosol breccia at 3061.3 m, Graham C-52 well; (A) lithology and matched borehole logs; (B) core face photo of laminar calcrete crust (cc) interfingering with a claystone of probably upper paleosol horizon (cl); (C) calcretized and argillated breccia; locations of B and C are indicated on lithology. (D) Red mottled calcareous mudrock, Upper Pennsylvanian or basal Permian, Jameson Bay C-31, 2406.70 m; note dense Microcodium penetrations (mm-scale features); the inset shows typical Microcodium aggregates zoom with binocular microscope. (E) Red mottled bioturbated shale and siltstone, same age, Depot Island C-44, 1662–1663 m.


are correlated across the basin in outcrops of the basin-margin facies belt. Five of these unconformities and their correlative surfaces were traced in the subsurface of Prince Patrick Island [90]. Recent re-examination of cores has confirmed the presence of subaerial exposure surfaces [91]. Subaerial profiles in cores are mostly decapitated by erosion (transgressive ravinements) but preserve features such as calcretic and ferric replacive deposits, *Microcodium*, root traces, and high-chroma ("red") mottling, which provide a clue for their interpretation (**Figure 4**). Of 49 short (<18.5 m) cores totaling 388 m of recovery, signatures of subaerial alteration were encountered in 8 (**Table 1**). Four of these cores intersect disconformity surfaces and one core penetrated the sub-Pennsylvanian angular unconformity into the Ellesmerian basement

110 Seismic and Sequence Stratigraphy and Integrated Stratigraphy - New Insights and Contributions

**Figure 4.** Permian disconformities of Sverdrup Basin in cores: (A–C) disconformity with a thick paleosol breccia at 3061.3 m, Graham C-52 well; (A) lithology and matched borehole logs; (B) core face photo of laminar calcrete crust (cc) interfingering with a claystone of probably upper paleosol horizon (cl); (C) calcretized and argillated breccia; locations of B and C are indicated on lithology. (D) Red mottled calcareous mudrock, Upper Pennsylvanian or basal Permian, Jameson Bay C-31, 2406.70 m; note dense Microcodium penetrations (mm-scale features); the inset shows typical Microcodium aggregates zoom with binocular microscope. (E) Red mottled bioturbated shale and siltstone, same age,

(Depot Island C-44, 2458.2 m MD).

Depot Island C-44, 1662–1663 m.

**Table1.** Cores with subaerial exposure profiles from the Upper Paleozoic of Sverdrup Basin, based on Ref. [91].

The thickest paleosol was encountered at 3061.3 m of Graham C-52 well (**Figure 4A**–**C**). The weak low-GR excursion just above the core top may record an upper clayey horizon of this paleosol or a transgressive deposit. This subaerial exposure profile may correlate to the unconformable contact of the Upper Nansen and Raanes formations (Asselian-Sakmarian boundary) of the basin margin zone [90]. Other disconformities from this core inventory occur stray within the defined third-order sequences, but may be assigned to higher frequency sea level fluctuations, as in the case of 9.1-m-thick core from the Belcher Channel Formation (lower Cisuralian) of Jameson Bay C-31 well described by Beauchamp et al. [90]. As stated in [90], these thinner (meter-scale) sequences or cyclothems are quite numerous in the Pennsylvanian— Lower Cisuralian (over 100 counted) but cannot be correlated between sections. Similarly, thin sequences in the Guadalupian part of the succession were traced based on well logs [84], but it is impossible to confirm subaerial nature of alleged sequence boundaries as no cores are available.

As the scanty core coverage in old exploration wells would not offer a chance to capture all stratigraphically meaningful disconformities, it is important to identify zones flushed by meteoric waters percolated from overlying subaerial surfaces. For example, in zones of meteoric oxidation, iron releases from decomposing synsedimentary sulfides and reprecipitates as ferric oxides and hydroxides. Seasonal waterlogging causes patchy reduction of iron into gley, and wettingdrying cycles usually imprint in characteristic red-gley mottling. Occurrence of oxidized basinal shales and siltstones with such mottling (**Figure 4D**, **E**) indicates fairly deep base level falls consistent with glacio-eustasy of the Late Paleozoic ice age [92]. Another feature indicative of paleovadose environment is *Microcodium* (**Figure 4D**), an aerobic microbially induced fabric abundant in Ca-rich subterranean environments of Pennsylvanian-Permian and late Cretaceous-Tertiary times but with no confirmed presence in rocks of other ages (**Figure 4D**; [93]).

#### **6.2. Carboniferous of Moscow Basin, Russia**

The Middle-Upper Mississippian and Pennsylvanian strata of the Moscow epicontinental basin of the central East European Craton (EEC) contain two cyclothemic successions dominated by shallow-marine carbonates and separated by a major Mississippian/Pennsylvanian unconformity [94]. The Upper Mississippian is a type succession for the Serpukhovian Stage, and Pennsylvanian strata host historical type sections for the Moscovian, Kasimovian, and Gzhelian stages of the Geological Time Scale [95]. The Mississippian/Pennsylvanian diastem (MPD) accounts for at least 10 My of late Serpukhovian-Bashkirian lowstand during which thick paleosols and deep (>110 m) fluvial incisions formed. Sequence stratigraphy of the two successions was developed based primarily on outcrops and disconformities which were used as main correlative horizons [96–99].

#### *6.2.1. Middle-Upper Pennsylvanian*

Similar to coeval classical cyclothems of North America [19, 86], Middle Pennsylvanian strata of the Moscow Basin have recorded a forced sea level control with drowning of seafloor into subphotic basinal environment on peaks of highstands and deep base level falls leading to subaerial exposure. These lowstands are thought to be the far-field response to expansion and shrinking of the Late Paleozoic ice dominantly from the Gondwanan icesheet [92]. Disconformities are scoured by transgressive erosion to various degrees, but some are onlapped with quiet-water facies with negligible truncation, preserving delicate features of former solums ("topclays"; **Figure 5A**). Fluvial facies or incised valleys are unknown. Topclays are palygorskitic, in some areas sepiolitic, indicating arid, well-drained pedogenic environments. A shift to montmorillonitic-illitic toplays recorded in the upper part of the studied succession flags the transition to slightly more humid climate. Other features are rare although deeply penetrating rhizoliths, shallow soil carbonate, low alumina/bases and Ba/Sr ratios, enhanced Mn and Sr, presence of soil gypsum and opal, and a characteristic peak in magnetic susceptibility, all suggesting a semiarid to arid pedogenic environment. The palygorskite clay of this paleo-pedon retains 1.1–1.5% of connate organic matter which is fulvate-dominated resembling organic matter from present-day aridisols [100]. The succession seems to be flushed throughout with meteoric fluids and repeatedly exposed to vadose environment, which left the penetrative systems of small solution vugs and oxidized organic matter and pyrites in basinal and siliciclastics-rich units.

#### *6.2.2. Middle-Upper Mississippian*

The thickest paleosol was encountered at 3061.3 m of Graham C-52 well (**Figure 4A**–**C**). The weak low-GR excursion just above the core top may record an upper clayey horizon of this paleosol or a transgressive deposit. This subaerial exposure profile may correlate to the unconformable contact of the Upper Nansen and Raanes formations (Asselian-Sakmarian boundary) of the basin margin zone [90]. Other disconformities from this core inventory occur stray within the defined third-order sequences, but may be assigned to higher frequency sea level fluctuations, as in the case of 9.1-m-thick core from the Belcher Channel Formation (lower Cisuralian) of Jameson Bay C-31 well described by Beauchamp et al. [90]. As stated in [90], these thinner (meter-scale) sequences or cyclothems are quite numerous in the Pennsylvanian— Lower Cisuralian (over 100 counted) but cannot be correlated between sections. Similarly, thin sequences in the Guadalupian part of the succession were traced based on well logs [84], but it is impossible to confirm subaerial nature of alleged sequence boundaries as no cores are available. As the scanty core coverage in old exploration wells would not offer a chance to capture all stratigraphically meaningful disconformities, it is important to identify zones flushed by meteoric waters percolated from overlying subaerial surfaces. For example, in zones of meteoric oxidation, iron releases from decomposing synsedimentary sulfides and reprecipitates as ferric oxides and hydroxides. Seasonal waterlogging causes patchy reduction of iron into gley, and wettingdrying cycles usually imprint in characteristic red-gley mottling. Occurrence of oxidized basinal shales and siltstones with such mottling (**Figure 4D**, **E**) indicates fairly deep base level falls consistent with glacio-eustasy of the Late Paleozoic ice age [92]. Another feature indicative of paleovadose environment is *Microcodium* (**Figure 4D**), an aerobic microbially induced fabric abundant in Ca-rich subterranean environments of Pennsylvanian-Permian and late Cretaceous-Tertiary

112 Seismic and Sequence Stratigraphy and Integrated Stratigraphy - New Insights and Contributions

times but with no confirmed presence in rocks of other ages (**Figure 4D**; [93]).

The Middle-Upper Mississippian and Pennsylvanian strata of the Moscow epicontinental basin of the central East European Craton (EEC) contain two cyclothemic successions dominated by shallow-marine carbonates and separated by a major Mississippian/Pennsylvanian unconformity [94]. The Upper Mississippian is a type succession for the Serpukhovian Stage, and Pennsylvanian strata host historical type sections for the Moscovian, Kasimovian, and Gzhelian stages of the Geological Time Scale [95]. The Mississippian/Pennsylvanian diastem (MPD) accounts for at least 10 My of late Serpukhovian-Bashkirian lowstand during which thick paleosols and deep (>110 m) fluvial incisions formed. Sequence stratigraphy of the two successions was developed based primarily on outcrops and disconformities which were

Similar to coeval classical cyclothems of North America [19, 86], Middle Pennsylvanian strata of the Moscow Basin have recorded a forced sea level control with drowning of seafloor into subphotic basinal environment on peaks of highstands and deep base level falls leading to subaerial exposure. These lowstands are thought to be the far-field response to expansion and shrinking of the Late Paleozoic ice dominantly from the Gondwanan icesheet [92]. Disconformities are scoured by transgressive erosion to various degrees, but some are

**6.2. Carboniferous of Moscow Basin, Russia**

used as main correlative horizons [96–99].

*6.2.1. Middle-Upper Pennsylvanian*

This ~90-m-thick shallow-marine succession deposited during Late Viséan and lowermiddle Serpukhovian (~16 My) is composed of shallow-marine limestone-dominated units bounded by six main disconformities and even more weakly developed subaerial surfaces that could not be traced between outcrops [99]. Fluvial and deltaic floodplain siliciclastics wedge between Viséan limestone units from southwest. The Viséan strata show a number of unusual sedimentary features, such as a lack of high-energy facies, shallow-subtidal marine sediments penetrated by *Stigmaria,* and beds of palustrine marls (*sensu* [101]) composed of a mixture of authigenic saponite, beidellite, and micritic calcite with strong negative offset of δ<sup>13</sup>C. Disconformities range in expression from undercoal solution-collapse horizons of only a few cm thick to deep paleokarsts. Incised fluvial channels are reported at two stratigraphic levels to the west and north of the study area. The deepest incisions (>15 m) developed from the Kholm disconformity, and this stratigraphic break is also marked with the deepest paleokarst profile (**Figure 4B**). All paleosol profiles contain evidence of rooting activity with numerous *Stigmaria* (rooting systems of arborescent lycopsids). The uppermost studied paleosol below

**Figure 5.** Paleosols and Paleokarsts at Carboniferous disconformities of Moscow Basin: (A) major elements and variability of upper Middle Pennsylvanian disconformities of Moscow Basin, slightly modified from Kabanov et al. [27]; (B) Kholm disconformity in top of Mikhailovian (KHU) and Akulshino palustrine marl (APB) at Novogurovsky Quarry, slightly modified from Kabanov et al. [99]; yellow clayey paleosol in solution pockets is arrowed.

the MPD is mid-Serpukhovian in age. It is a thin palygorskitic calcrete [99] formed under significantly drier climate than underlying *Stigmaria*-bearing paleosols. Paleosol mineralogy and proxies for pedogenic environments are discussed in [102, 103].

#### *6.2.3. Disconformities in cores*

Paleosols and karstified profiles of Middle-Late Mississippian and Pennsylvanian age are frequently intersected by cores in oil and gas exploration areas of the eastern EEC (**Figure 6A**–**C**). Project geologists usually ignore these surfaces. However, eroded disconformities invisible with geophysical tools may record prolonged hiatuses, as indicated by thick rhizocretions left by perennial plants requiring fairly thick soil cover to root in (**Figure 6C**).

#### **6.3. Lower-Middle Devonian of Mackenzie Corridor, Northwestern Canada**

Devonian strata of the central and northern Mackenzie Corridor located within the limits of ancestral North America are composed of Lower Devonian-Eifelian shallow-marine carbonates, dolostone breccias, and evaporites; Givetian-Frasnian basinal shales of the Horn River Group hosting isolated carbonate platforms (banks) of Ramparts Formation; and the Frasnian-Famennian Imperial Formation composed of fine-grained turbiditic siliciclastics and coarse-grained siliciclastics and chert conglomerates of the Tuttle Formation. The latter straddles the Devonian-Carboniferous boundary (**Figure 7**; [40, 104]).

**Figure 6.** Eroded paleosols on core face of shallow-marine limestones, Bashkirian (Lower Pennsylvanian), southeastern EEC: (A) collapsed karst breccia with thin laminar calcrete crusts (cc); (B) more massive calcrete crust with rootlet channels; ravinement surface is arrowed; (C) rhizolith (*rh*) with thick peripheral alteration zone (*rph*) found in 3.5 m below a disconformity; (s) is anhydrite fill of karst voids; scale bar in centimeters.

Stratigraphic Unconformities: Review of the Concept and Examples from the Middle-Upper Paleozoic http://dx.doi.org/10.5772/intechopen.70373 115

the MPD is mid-Serpukhovian in age. It is a thin palygorskitic calcrete [99] formed under significantly drier climate than underlying *Stigmaria*-bearing paleosols. Paleosol mineralogy and

Paleosols and karstified profiles of Middle-Late Mississippian and Pennsylvanian age are frequently intersected by cores in oil and gas exploration areas of the eastern EEC (**Figure 6A**–**C**). Project geologists usually ignore these surfaces. However, eroded disconformities invisible with geophysical tools may record prolonged hiatuses, as indicated by thick rhizocretions left by perennial plants requiring fairly thick soil cover to root in

Devonian strata of the central and northern Mackenzie Corridor located within the limits of ancestral North America are composed of Lower Devonian-Eifelian shallow-marine carbonates, dolostone breccias, and evaporites; Givetian-Frasnian basinal shales of the Horn River Group hosting isolated carbonate platforms (banks) of Ramparts Formation; and the Frasnian-Famennian Imperial Formation composed of fine-grained turbiditic siliciclastics and coarse-grained siliciclastics and chert conglomerates of the Tuttle Formation. The latter

**Figure 6.** Eroded paleosols on core face of shallow-marine limestones, Bashkirian (Lower Pennsylvanian), southeastern EEC: (A) collapsed karst breccia with thin laminar calcrete crusts (cc); (B) more massive calcrete crust with rootlet channels; ravinement surface is arrowed; (C) rhizolith (*rh*) with thick peripheral alteration zone (*rph*) found in 3.5 m

below a disconformity; (s) is anhydrite fill of karst voids; scale bar in centimeters.

**6.3. Lower-Middle Devonian of Mackenzie Corridor, Northwestern Canada**

straddles the Devonian-Carboniferous boundary (**Figure 7**; [40, 104]).

proxies for pedogenic environments are discussed in [102, 103].

114 Seismic and Sequence Stratigraphy and Integrated Stratigraphy - New Insights and Contributions

*6.2.3. Disconformities in cores*

(**Figure 6C**).

**Figure 7.** Devonian succession of central and northern Mackenzie Corridor on a cross-section A-A′ anchored on mid-Devonian drowning unconformity (arrowed). TR is Trail River outcrop section; wells from left to right: Cranswick YT A-42, Cranswick A-22, S. Ramparts I-77, N. Ramparts A-59, Ramparts River F-46, Hume River I-66, Hume River D-53, Carcajou L-24, Maida Creek F-57, Hoosier F-27, NWB is Norman Wells oilfield, Little Bear N-09, Bluefish A-49, and Bracket Lake C-21. Stratigraphic units: (Dbfc) Bear Rock, Fort Norman, and Camsell fms.; (Dpta) Peel, Tatsieta, and Arnica formations; (Dl) Landry Fm.; (SDrr) Road River Group; (Dhs) Headless Mbr. of Hume Fm; (Dhm) Hare Indian Fm.; (Dbf) Bluefish Mbr.; (Dbc) Bell Creek Mbr.; (Dfc) Francis Creek Mbr.; (Dpc) Prohibition Creek Mbr.; (Dr) Ramparts Fm.; (Dc) Canol Fm.; Imperial Fm. undivided (Di); (Dml) Mirror Lake Mbr.; (Dlc) Loon Creek Mbr.; (DCt) Tuttle Fm.; ("Cf") informal unit *Cf*. Inset map shows wide occurrence of the Horn River Group between 64 and 68 parallel in (1) outcrops, (2) subsurface, and (3) patchy presence in erosional outliers; (4) Tintina Fault Zone (thick) and smaller scale main faults in the Mackenzie Foldbelt (thin); (5) Canol Formation dips beneath thick siliciclastic wedge of Imperial and Tuttle formations; (6) paleogeographic offshore limits of thick Hare Indian siliciclastics (Bell Creek Mbr.) and overlying Ramparts Limestone. The eastern limit of Laramide deformation front is approximated by Norman Range thrust fault (NRTF).

A shallow-marine peritidal succession of Emsian age measured in the nearly continuous core of Kugaluk N-02 well (**Figure 7**) contains 86 disconformities that bear distinct signatures of subaerial exposure (rank 0, 1, and 2 discontinuities in **Figure 8**). Of these, 43 surfaces are marked with thick (>1 m) paleokarst profiles and 3 surfaces by thick rubbly paleosols and several meters of karstified rock below [105, 106]. This 440 m thick succession deposited over a period of 15–18 My, assuming that the top of Landry Formation approximates to the base of Eifelian [105, 107] and Delorme/Arnica contact is found in the Lochkovian or Pragian [108]. However, only seven subaerial exposure profiles have been identified in the Arnica—lower Landry part of this succession in the outcrop section measured at Rumbly Creek West Ridge, including one deep profile with thick paleosol [109]. Given very similar shallow-water facies assemblage of this outcrop and Kugaluk N-02 core, small number of disconformities appears to be an artifact of poor preservation of the weathered section and limited time spent on it by the examiner.

Subaerial exposure profiles of similar character are very common in Lower and basal Middle Devonian cores over the broad expanse of Mackenzie Corridor. Some thick profiles show signature of prolonged exposure and multiphase pedogenic overprinting resulted in complete loss of sedimentary fabrics, as exemplified by a mature paleosol profile at 600.25–603.5 m of Ebbutt D-50 well (**Figure 9B**–**D**). One interesting feature is the absence of root penetrations that are characteristic of younger Phanerozoic paleosols (**Figure 6**), which is interpreted as an evolutionary imprint of prevascular plant landscape. Small (<1 mm in diameter) rhizocretions occur only in thin marshland beds (palustrine facies; [101]) occupying incursive and transgressive positions in peritidal sequences of Landry Formation [99]. This "palustrine facies" has been also identified in outcrop [109]. Like in described above Late Paleozoic examples, none of available geophysical logs can be relied upon to trace even thickest paleosols of this type in the subsurface (**Figure 9**).

#### **6.4. Mid-Devonian drowning unconformity of Mackenzie Corridor**

Bioturbated and richly fossiliferous benthic limestones of Hume Formation containing a diverse benthic fauna are onlapped by black calcareous laminated shales of the Bluefish Member. The onlap surface is a strong seismic reflector commonly used as stratigraphic datum (**Figure 7**). In the project area (**Figure 7**), the surface appears table flat on outcrop scale, if not tectonically displaced, but in the southern Mackenzie Corridor it is outgrown by pinnacleshaped carbonate buildups referred to as Horn Plateau reefs [110, 111].

The Hume/Bluefish contact has been measured in three cores from Canol Shale exploration wells and accessed in three outcrops of the Norman Range and northern Mackenzie Mountains [45, 109]. The coral-stromatoporoid facies composing the main part of the upper Hume Formation occurs in direct contact with the Bluefish shale in two of six sections, and in both cases, it shows a rugged corroded top with deep (8 cm in core) solution pockets filled with black shale from the overlying anoxic facies. The upper few decimeters below the top are chertified and also very pyritic in core or rusty in outcrops. Phosphatic crusts

A shallow-marine peritidal succession of Emsian age measured in the nearly continuous core of Kugaluk N-02 well (**Figure 7**) contains 86 disconformities that bear distinct signatures of subaerial exposure (rank 0, 1, and 2 discontinuities in **Figure 8**). Of these, 43 surfaces are marked with thick (>1 m) paleokarst profiles and 3 surfaces by thick rubbly paleosols and several meters of karstified rock below [105, 106]. This 440 m thick succession deposited over a period of 15–18 My, assuming that the top of Landry Formation approximates to the base of Eifelian [105, 107] and Delorme/Arnica contact is found in the Lochkovian or Pragian [108]. However, only seven subaerial exposure profiles have been identified in the Arnica—lower Landry part of this succession in the outcrop section measured at Rumbly Creek West Ridge, including one deep profile with thick paleosol [109]. Given very similar shallow-water facies assemblage of this outcrop and Kugaluk N-02 core, small number of disconformities appears to be an artifact of poor preservation of the weathered section and limited time spent on it by

116 Seismic and Sequence Stratigraphy and Integrated Stratigraphy - New Insights and Contributions

Subaerial exposure profiles of similar character are very common in Lower and basal Middle Devonian cores over the broad expanse of Mackenzie Corridor. Some thick profiles show signature of prolonged exposure and multiphase pedogenic overprinting resulted in complete loss of sedimentary fabrics, as exemplified by a mature paleosol profile at 600.25–603.5 m of Ebbutt D-50 well (**Figure 9B**–**D**). One interesting feature is the absence of root penetrations that are characteristic of younger Phanerozoic paleosols (**Figure 6**), which is interpreted as an evolutionary imprint of prevascular plant landscape. Small (<1 mm in diameter) rhizocretions occur only in thin marshland beds (palustrine facies; [101]) occupying incursive and transgressive positions in peritidal sequences of Landry Formation [99]. This "palustrine facies" has been also identified in outcrop [109]. Like in described above Late Paleozoic examples, none of available geophysical logs can be relied upon to trace even thickest paleosols of this type in

Bioturbated and richly fossiliferous benthic limestones of Hume Formation containing a diverse benthic fauna are onlapped by black calcareous laminated shales of the Bluefish Member. The onlap surface is a strong seismic reflector commonly used as stratigraphic datum (**Figure 7**). In the project area (**Figure 7**), the surface appears table flat on outcrop scale, if not tectonically displaced, but in the southern Mackenzie Corridor it is outgrown by pinnacle-

The Hume/Bluefish contact has been measured in three cores from Canol Shale exploration wells and accessed in three outcrops of the Norman Range and northern Mackenzie Mountains [45, 109]. The coral-stromatoporoid facies composing the main part of the upper Hume Formation occurs in direct contact with the Bluefish shale in two of six sections, and in both cases, it shows a rugged corroded top with deep (8 cm in core) solution pockets filled with black shale from the overlying anoxic facies. The upper few decimeters below the top are chertified and also very pyritic in core or rusty in outcrops. Phosphatic crusts

**6.4. Mid-Devonian drowning unconformity of Mackenzie Corridor**

shaped carbonate buildups referred to as Horn Plateau reefs [110, 111].

the examiner.

the subsurface (**Figure 9**).

**Figure 8.** Lithofacies log for the Arnica-Landry succession in core of Kugaluk N-02 well with ranked disconformities, modified from Kabanov [105, 106]. Each facies point represents a mid-point of the descriptive interval. "No information" gaps in joint line indicate dolostones with obliterated sedimentary fabrics or "lost cores" from fractured zones. Black hollow arrows point at thick highstand intervals with offshore lithofacies and no disconformities. Orange arrows point at thickest subaerial exposure profiles with preserved paleosols.

characteristic of hardgrounds at other drowning unconformities did not develop, which is explained by overall phosphorus-lean sedimentary system [45]. Four other sections show 0.5–2.6-m-thick transitional interval of argillaceous bioturbated micritic limestones and shales. This transitional interval contains smooth discontinuity surfaces but no rugged hardgrounds. This transitional limestone contains brachiopod banks but no stromatoporoids. Pelagic tentaculitids appear in this unit and become rock-forming in base of Bluefish Member. The top of this transitional unit is usually smooth and probably storm-scoured. The basal few cm of the Bluefish Member characteristically contain lag concentrate of imbricated brachiopod shells mixed with diverse tentaculitids, sometimes dominated by tentaculitids with rare disintegrated brachiopod valves. Bioturbation in this basal Bluefish

**Figure 9.** A shallow-marine peritidal succession measured in core of Ebbutt D-50 well (southern Mackenzie Corridor): (A) Striplog showing lack of well log response at multiple paleosols. (B–D) Polished and etched core face with details of paleosol at 600.25 m; (B) box view with pedogenic claystone-to-calcrete at 600.25–602.0 m (1969.3–1975.0 ft) and intense alteration down to at least 603.5 m (1980.0 ft); black arrow points at hydrothermal dolostone vein. (C) Top of paleosol profile composed of multiphase clayey calcrete; (D) float breccia at 600.9 m with residual clasts of marine limestone (*cl*). Sticky marks are ED-XRF reading points.

bed drops abruptly to BI ≈ 2 and right above this bed declines to zero. Enrichment in chalcophyle trace metals in enrichment factor notation (EFV and EFMo) grows gradually from moderate in the base of the Bluefish Member to a traceable spike of high values in 2.0 m above the base, indicating a gradual spread of anoxia.
