1 - 3 Book Cliffs Flashcards
How are open marine settings identified in Book Cliffs?
MANCOS SHALES.
Blue-grey fine-grained siltstone and claystone. Contains fossils of ammonites, belemnites.
How do we recognise river-dominated deltas in Book Cliffs?
PANTHER TONGUE MEMBER.
Marine siltstone passes upwards through interbedded sandstone and siltstone into a sand-dominated upper interval. This is upwardly-shallowing, therefore progradational.
Sandstone beds have sharp, erosive bases and grade upwards through planar-lamination into current-lamination, representing waning of current flow. The sandstones represent pulses of sediment, potentially related to periods of high discharge through the fluvial system.
Sedimentary structures consistent with deposition from turbidity currents. Beds form clinoforms dipping basinwards at 3-5 degrees. These are interpreted as delta-front turbidites, recording the original deposition dip on the front of the mouth bar.
Parasequences are up to 25m thick, far thicker than those of the Blackhawk Formation. They are thicker because there was no significant reworking of sand above the storm-wave base, owing to low wave-energy at the time.
Laterally, parasequences are lobe-shaped, 5-10km wide. This reflects the lobate shape of river-dominated delta (e.g. Mississippi today).
The heterolithic nature of individual beds, preservation of original deltaic deposition features (e.g. turbidites and delta-front dip) due to lack of reworking, and lobate shape all point to a river-dominated delta.
How do we recognise a wave-dominated coastline in the Book Cliffs?
e.g. SPRING CANYON MEMBER at GENTILE WASH.
Blue-grey siltstone grades upwards through interbedded siltstone and HCS sandstones and into amalgamated HCS beds. Above this, intensely bioturbated sandstones with extensive burrowing; then well-sorted, trough cross-stratified, medium-grained sandstones; finally planar cross-stratified sandstones, with laminations dipping seawards
Upwards shallowing from offshore to foreshore, therefore progradational.
Wave-formed sedimentary structures (e.g. HCS, TCS) indicate reworking by waves, and so we have a wave-dominated coastline.
How do we see barrier island and lagoon system in the Book Cliffs?
e.g. SPRING CANYON SC5 traced along SPRING CANYON.
It is here possible to trace the parasequences laterally. Moving proximally:
Trough cross-stratified sandstone of the upper shoreface lie adjacent to landward-dipping, rippled and planar-laminated sandstones (washover fans), interbedded with unbioturbated dark-coloured mudstones.
The lagoonal deposits are capped with coal, which represent the filling of the lagoon and form the parasequence boundary.
How do we interpret the coal seams of the Book Cliffs?
Peat requires high water table, and so coal seams are associated with flooding surfaces.
Seams are up to 5m thick. Peat compacts to as much as one tenth its original thickness. Here, 5m of coal is thought to represent 35m of original sediment.
Coal has a LOW-ASH content (4-7%). Low ash content suggests isolation from siliclastic input, indicating a raised mire (rather than a swamp, which would form high ash coal due to siliclastic input).
How do we see estuaries in the Book Cliffs?
Two examples, both from the SUNNYSIDE MEMBER.
- Incised valley (FSST) into underlying HST. Valley system is 3.5km wide and consists of three forks. The main channel is 20m deep. 4m thick bed of coarse-grained, trough cross-stratified sandstones at the base. No marine body or trace fossils. Above this, a variety of facies, including subtidal bars, large-scale inclined heterolithic stratification, tidal flat deposits, thin coals, shell beds and intra-estuary deltas.
Fluvial deposits overlain by tidal deposits indicate that the conditions became increasingly marine (TST). Distribution of facies indicates several discrete base levels suggesting that it was filled in a stepped fashion.
- 20m-thick tidally influenced sandbody cut into non-marine deposit, containing fossil wood with TEREDOLITES borings (Teredolites is a marine bivalve). The marine conditions indicate an estuary.
How do we identify parasequence flooding surfaces in the Book Cliffs?
In GRASSY MEMBER between COAL CANYON and HORSE CANYON, we can see four parasequences along the depositional dip profile.
Distally, the flooding surface is visible as offshore shale overlying offshore transition deposits.
Proximally, the flooding surface is much more difficult to see. In other locations, we can see flooding proximally through coal deposits or carbonate cements.
Where do we see sequence boundaries in the Book Cliffs?
- The base of the CASTLEGATE SANDSTONES is interpreted as a sequence boundary; we have erosion into the underlying Blackhawk deposits, and evidence of falling sea-level above.
- KENILWORTH MEMBER
Lower shoreface deposits are unconformably overlain by an extensive network of valleys (16m deep and containing both fluvial and heterolithic tidal deposits) and 30km-wide interfluves (showing subaerial exposure and palaeosol formation).
Where do we see FSST deposits in the Book Cliffs?
Aberdeen A2 is prograded further seaward and lies further down the depositional profile than A1. It has no coastal plain deposits.
Where do we see LST deposits in the Book Cliffs?
- The CASTLEGATE SANDSTONES are braided fluvial deposits. The shift from meandering to braided fluvial system suggests increased gradient. This is related to a sea-level fall, although VON WAGONER interprets the deposits as LST.
- PRAIRIE CANYON MEMBER
Heterolithic coarsening-upwards sandstone bodies within the Mancos Shale, approximately 40km basinwards of the preceding highstand shorelines.
These contain shallow-water sedimentary structures, trace fossil assemblages, tidal cross-stratification, fluvial channels and rooted interfluves. These are not offshore deposits, they are probably deltaic mouth bars and channels.
These are interpreted as FSST and LST deposits which were laid down while the sequence boundaries in the Blackhawk Formation were being formed and sand was bypassed through incised valleys to a point further into the basin where accommodation space existed.
Where do we see TST deposits in the Book Cliffs?
KENILWORTH K6. Shoreface overlying incised valleys. Parasequence shows backstepping.
Where do we see HST deposits in the Book Cliffs?
The majority of parasequences in the Blackhawk Formation (HSS) are HST. Progradational wave-dominated shoreline.
The Book Cliffs are 300km long, up to 300m high, and cut into by canyons. How does the extensive exposure of sediments help to illustrate the concepts of sequence stratigraphy?
- We can view both marine and non-marine sections of parasequences: e.g. GRASSY MEMBER between HORSE CANYON and COAL CANYON. This allows us to construct models of parasequence formation based around the concept of volumetric partitioning / sediment balance.
- We can see how depositional environments interact, for example river-dominated delta and barrier islands, meandering fluvial and estuary.
The Blackhawk Formation is a Highstand Sequence Set. How does this affect its depositional architecture?
Almost all of the tracts are HST. There are very few FSST or TST tracts present.
What mechanisms are responsible for the cycles we see in the Book Cliffs?
Von Wagoner (1995) partially correlated the Blackhawk Deposits with the MESOZOIC - CAINOZOIC CYCLE CHART through correlation of ammonite biostratigraphy. However, there remain some anomalies: the chart predicts that the Desert Member sequence boundary should be stronger than the Castlegate sequence boundary.
It is therefore likely that, although eustatic sea-level was influential, there are other controls such as local tectonic activity.
