Geological Digressions

9. Stratigraphic surfaces in outcrop – baselevel fall

10. A subaerial unconformity (dashed line) outlines this paleotopographic low
where drainage eroded into Pleistocene coastal dunes during falling sea level.
The valley was later filled by a younger dune complex as sea level rose and sand
on the exposed shelf was moved landward in concert with the migrating
shoreline . Kariotahi, New Zealand.

13. Subaerial unconformities
Criteria commonly used to identify subaerial unconformities in
outcrop include (some of these also apply to core):
Evidence of subaerial exposure (e.g., paleosols, regoliths, caliche,
karst, and other vadose zone diagenetic features).
Erosion and truncation of underlying strata.
Paleotopography.
Onlap of overlying strata.
Map extent.
A significant hiatus (one that has regional or basin-wide extent).

14. Estuarine deposits, paleosols, blocky regoliths and karst paleotopography (Expedition Fm, Eureka Sound Group) developed on
deformed Ordovician to Devonian carbonates and siliciclastics, heralding renewed subsidence and stratigraphic onlap across Sverdrup
Basin.
The angular unconformity between Ordovician carbonates and Early Paleocene estuarine mudrocks is underlain by regolith. Fractured
limestone is arranged in jig-saw fit patterns; there is no evidence of reworking – shown in the image following. The unconformity is
overlain by estuarine mudstone, sandstone and thin coals, and locally tidal channel sandstones. Expedition Fm. Mount Moore,
Ellesmere Island.

15. A closer look at the regolith developed on Ordovician limestone. The surfaces of fracture blocks are
pitted and embayed by dissolution. Fracturing and dissolution decrease downwards from the upper
contact. Elsewhere there is evidence for karstification. Same location as above.

16. A more expansive view of the Ordovician – Paleocene subaerial unconformity showing the paleotopography developed by
karstification during protracted exposure of the Ordovician carbonates (dashed line). The karst surface contains regoliths,
dissolution collapse pits, and in places collapse breccias. Lower Paleocene estuarine, tidal channel, sand spit and bar deposits
onlap the unconformity from right to left. Same location as above.

17. The Middle Eocene Buchanan Lake Fm. is a syntectonic conglomerate derived from Mesozoic
and Paleozoic rocks carried in the hanging wall of crustal-scale thrusts during the Eurekan
Orogeny. The deformation resulted in dismemberment of Sverdrup Basin. The conglomerates
were deposited in alluvial fans and more distal fluvial systems. The subaerial unconformity at
this locality (Stang Bay) records an abrupt change from Triassic marine sandstone intruded by
diabase sills to coarse, diabase-rich conglomerate. Axel Heiberg Island.

18. The paleotopographic expression of the unconformity between Late Precambrian Moine schists and Devonian Old Red
Sandstone is nicely presented at Portskerra, north Scotland. Basal fluvial deposits that drape the paleotopographic highs overlie
remnants of a regolith (see following image), with channel-like crossbedded sandstones filling the topographic lows. The paleo-
relief above and below the shore platform is projected to be about 5-10 m.

19. Regolithic breccia at the Moine-Old Red Sandstone unconformity contains broken fragments of
schist and pelite. The fragments are mostly angular and unsorted; some are fitted. Interstices
between fragments are filled mostly with grit and sand. The regolith is overlain by fluvial
sandstone of the Devonian Old Red Sandstone (shown above). Portskerra, north Scotland.

20. The examples of RSME surfaces are from the Paleocene Stand Fiord Formation on Axel Heiberg Island (Arctic Canada), part of a third-
order sequence. The transition from normal regression (left side, labelled HST) to forced regression is recorded by shelf mudrocks
interleaved with abrupt-based, tabular-bedded sandstone units; the base of each sandstone unit is an RSME. The top of each unit is
relatively abrupt but conformable. There are four main periods of RSME development that in this view are the packages of resistant
beds (labelled 1 to 4); each package may represent a higher-order cyclicity that is superposed on the 3rd order sequence.
In outcrop, the sandstones have blocky profiles (see image below) compared with the more recessive weathering mudstones. Each
sandstone unit represents a period of shoreface progradation where sediment supply briefly kept pace with falling baselevel. The
overall shoreline trajectory during baselevel fall was gentle seaward downstepping.

21. An along-strike view of three forced regressive sandstone wedges (package #2 in the
image above). The base of each is an abrupt, locally scoured surface of marine erosion
(RSME) that overlies discordantly shelf mudrocks, and in turn are overlain
conformable by shelf deposits. The sandstones contain planar and trough crossbeds
that formed as subaqueous dunes that migrated across a shallow, sandy shoreface-
beach. Each sandstone wedge prograded as the shoreline down-stepped seaward
during baselevel fall.

22. A different view of the abrupt erosional base of a forced regressive sandstone wedge
(RSME) – this event is part of package #1. The Crossbedded sandstone is underlain
by shale and thin bedded fine-grained sandstone.

23. The base of forced regressive sandstone units have shallow scours like this example (at the
hammer head) and discordantly overlie grey shale. Sandstones are medium to fine grained and
well sorted. The basal sandstone here has shallow trough crossbeds (the lowest unit in package
#2).

24. Sedimentary structures: Shallow marine

39. Sedimentary structures: Alluvial fans

54. Sedimentary structures: Turbidites

57. Graded bedding is one of the diagnostic structures of turbidites. This schematic view of graded
bedding, shows the coarsest grains at the base, with finer grains toward the top of the bed. The
matrix is commonly muddy (clay plus silt particles).

75. Sedimentary structures: coarse-grained fluvial

87. Sedimentary structures: Fine-grained fluvial

97. Sedimentary structures: Mass Transport Deposits