Return to Purbeckian of Dorset

Dorset Abstracts:

1. IAS Dublin, September, 2000

Edwin J. Anderson
Department of Geology, Temple University, Philadelphia, PA 19122, USA, andy@astro.temple.edu

The Purbeckian (Lower Cretaceous) of Dorset comprises a stacked hierarchy of allocycles. The smallest-scale (fundamental) cycles are interpreted as the products of the Earth’s precessional signal. These rock cycles are the same as punctuated aggradational cycles, PACs (Anderson & Goodwin, 1990) and ‘elementary sequences’ (Strasser & Hillgärtner, 1998). These meter-scale cycles are bundled into sets as the magnitude of the precessional forcing signal is modulated by periodic variation in the degree of eccentricity of the Earth’s orbit at 100 ka and 400 ka (Fischer and Bottjer, 1991). The hierarchic cyclic structure is like that in correlative Purbeckian facies in the Sierra del Pozo section in southern Spain (Anderson, 2000).

In these allocyclic interpretations all cycle boundaries (i.e. at every scale in the hierarchy) are the products of precessional sea-level rises. The boundaries are surfaces where deeper facies abruptly overlie shallower facies or exposed surfaces. Evaluation of patterns in the magnitude of facies change at successive cycle boundaries provides evidence for defining sets or bundles of cycles. The sharp boundaries of cycles are thought to form when rates of sea-level rise exceed a critical value. A consequence of this model, given the fixed time interval of precessional sea-level rise, is that higher amplitude rises result in both longer intervals of discontinuity and larger facies-depth contrasts across cycle boundary surfaces.

In the marginal marine, peritidal or freshwater facies of the Purbeckian of Dorset (see the excellent stratigraphic log of Clements, 1993) sea-level rise events typically lead to the abrupt superposition of carbonate on shale. This is thought to occur because fine grain terrigenous sediments are trapped in aggrading coastal plain environments in response to base-level rises. Coarser, thicker and more fossiliferous limestones are correlated with larger magnitude sea-level rise events. Either at late high stands or consequent to sea-level falls, fine grained terrigenous clastic sediments are deposited abruptly over the carbonate beds. In more landward positions paleosols may be associated with sea-level falls. If the critical rate of sea-level fall is exceeded sea-level fall surfaces marked by abrupt facies change to shallower or more restricted facies may form. The magnitude of facies change at these surfaces corresponds to the amplitude of the fall.

References:

Anderson, E.J. and Goodwin, P.W. (1990) The significance of meter-scale allocycles in the quest for a fundamental stratigraphic unit. J. Geol. Soc., London, 147, 507-518.

Anderson, E.J. (2000) Criteria for recognizing an orbitally forced cyclic hierarchy: The ‘Purbeckian’ at the classic Sierra del Pozo section (Berriasian) Southern Spain. Sediment 2000, Meeting Leoben, Austria.

Clements, R.G. (1993) Type section of the Purbeck Limestone Group, Durlston Bay, Swanage, Dorset. Proc. Dorset Nat. Hist. Archaeol. Soc., 114, 181-206.

Fischer, A.G. and Bottjer, D.J. (1991) Orbital forcing and sedimentary sequences. J. Sed. Petrol., 61, 1063-1069.

Strasser, A. and Hillgärtner, H. (1998) High-frequency sea-level fluctuations recorded on a shallow carbonate platform (Berriasian and Lower Valanginian of Mount Salève, French Jura). Eclogae geol. Helvetica, 92, 375-390.

2.    NEGSA Burlington, March, 2001

Applying the ‘Milankovitch’ orbital-forcing model, the Purbeck Group at the type locality in Durlston Bay is divisible into meter-scale rock cycles and cycle sets (sequences). The Purbeckian, 100 meters of marginal marine, brackish and freshwater facies and soils, is divisible into about fourteen 400 ka, 4^th order, sequences. Each 4^th order sequence comprises a set of four 100 ka, 5^th order, sequences. These in turn consist of from one to five 20 ka, 6^th order, rock cycles (PACs). Sixth order cycles are produced by precession and are grouped into sets by eccentricity modulation of the precessional signal. All levels of the cyclic hierarchy are recognized by asymmetry in their patterns of facies distribution. Larger facies changes occur at the bases of cycles and early in sequences (sets of cycles) while smaller facies changes occur at cycle boundaries higher in sequences of cycles. In the Purbeck Group coarse-grained skeletal limestone is the dominant facies at the base of cycles and low in cyclic sequences while fine carbonates, shale and soils characterize the upper parts of cycles and sequences.

3. SEPM Cyclisti Workshop, Sorrento, May, 2001

Drs. Strasser, Fischer and D'Argenio on field trip, SEPM sponsored Cyclisti Conference, Sorrento

ASYMMETRICAL FACIES PATTERNS IN ORBITALLY FORCED 3^RD,4^TH, 5^TH AND 6^TH ORDER SEQUENCES: THE PURBECKIAN OF DORSET

Anderson, Edwin J., Department of Geology, Temple University, Philadelphia, Pa 19122 andy@astro.temple.edu

The Purbeck Group in Dorset, England is divisible into meter-scale rock cycles and three additional orders of cycle sets (sequences). These cycles and sequences are arranged in a four-tiered hierarchy and are the product of ‘Milankovitch’ orbital-forcing. The Purbeckian at the thickest section in Durlston Bay (100 meters of marginal marine, brackish and freshwater facies and soils) is divisible into two complete 2 ma 3^rd order sequences (plus a partial third sequence) and fourteen 400 ka, 4^th order, sequences. In turn, each 4^th order sequence comprises a set of four 100 ka, 5^th order, sequences and each 5^th order consists of from one to five 20 ka, 6^th order, rock cycles (PACs). Sixth order cycles are produced by precession and are grouped into sets by eccentricity modulation of the precessional signal at the 100 and 400 ka levels. All levels of the four-tiered hierarchy are recognized by asymmetry in their patterns of facies distribution. Larger facies changes occur at the bases of cycles lower in sequences (sets of cycles) while smaller facies changes occur at cycle boundaries higher in sequences of cycles. In the Purbeck Group coarse-grained skeletal limestone is the dominant facies at the base of cycles and low in cyclic sequences while fine carbonates, shale and soils characterize the upper parts of cycles and sequences.

Stratigraphic sections to the west of Durlston Bay (Fig. 1 Dorset Subsidence.ppt) are progressively thinner (80 meters at Worbarrow, 60 meters at Mupe Bay and 45-50 meters at Lulworth). However components of every level of the cyclic hierarchy are preserved at the thinner sections. Key stratigraphic marker beds including The Great Dirt Bed, the Gypsum Bed, the Mammal Bed, the Freshwater Chert Bed, the Cinder Bed, the Scallop Bed and the Broken Shell Limestone provide a basis for correlation of 4^th and 5^th order sequences across the study area. The observed westward decrease in stratigraphic thickness is a result of less subsidence toward the basin margin. Thus average stratigraphic accumulation rates of about 2.0 meters per 100 ka at Durlston Bay decrease to approximately half that rate at Lulworth Cove. Tracing of correlative parts of the four-tiered hierarchy westward along the Dorset coast demonstrates that while most 5^th order sequences are (in part) preserved they often are condensed (or amalgamated) into single limestone shale couplets, less than a meter thick, sometimes topped by soils, near the basin margin.

Key words: Milankovitch, cyclic hierarchy, pedogenesis, stratigraphic correlation

4.    IAS Davos, 21st Meeting, September, 2001

Integration of bed descriptions, molluscan depth zones and faunicycles with an orbitally forced four-tiered hierarchy of lithic allocycles: In the Lower Cretaceous, Purbeckian, of Dorset, England

Anderson, Edwin J., Department of Geology, Temple University, Philadelphia, Pa 19122, andy@astro.temple.edu

Applying the ‘Milankovitch’ orbital-forcing model, the Purbeck Group at coastal sections in Dorset, England is divisible into meter-scale rock cycles and cycle sets (sequences). In the type locality at Durlston Bay, the Purbeckian consists of 100 meters of marginal marine, brackish and freshwater facies and soils. This sequence is divisible into two complete (and part of a third) 2 ma, 3^rd order sequences and twelve 400 ka, 4^th order, sequences. Each 4^th order sequence comprises a set of four 100 ka, 5^th order, sequences that in turn each consist of from one to five 20 ka, 6^th order, rock cycles or PACs (punctuated aggradational cycles). Sixth order cycles are the product of the precessional signal and are grouped into sets by eccentricity modulation of the strength of this signal at both 100 and 400 ka. Each tier of this cyclic hierarchy in the Purbeck Group is recognized by asymmetry in its pattern of facies distribution. Larger facies changes occur at the bases of cycles lower in sequences (sets of cycles) while smaller facies changes occur at cycle boundaries higher in sequences of cycles. Also, coarse-grained skeletal limestone is the dominant facies at the bases of cycles and low in cyclic sequences while fine carbonates, shale and soils characterize the upper parts of cycles and sequences.

Clements (1967, 1993) has described the bio and lithofacies of 241 beds that comprise the Purbeck Group at Durlston Bay. F.W. Anderson (1985) has divided the Purbeck of southern England into 41 ostracod faunicycles (Fig. 1 Faunicycles in the Mammal 4th Order Sequence.ppt). Each faunicycle consists of a lower more-marine S-phase and an upper more-fresh water C-phase assemblage of ostracods. These cycles are traceable laterally and Anderson suggests that they are the product of sea-level fluctuation events. Morter (1984) describes 8 molluscan depth zone assemblages and uses them to interpret patterns of sea-level rise and fall through the Purbeck. Using Clements’s beds as markers, it is possible to to integrate F. W. Anderson’s ostracod faunicycles and Morter’s depth zone assemblages with the lithologically determined four-tiered hierarchy of allocycles (Fig. 2 Morter diagram.jpg). Faunicycles closely approximate lithically defined 5^th order sequences and transgressive events interpreted by Morter occur at either the first and/or the second 5^th order boundaries in 4^th order sequences. These independent biological approaches corroborate the cyclic hierarchy determined by lithofacies analysis based on assumptions of orbital forcing.

Key words: Orbital forcing, Molluscan depth zones, Faunicycles

ANDERSON, Edwin J., PERRY, Lisa L. and STYNCHULA, Jamey A., Department of Geology, Temple University, Philadelphia, Pa 19122, andy@astro.temple.edu

At least six distinctive stratigraphic units representing 100 ka sequences or their lower boundaries can be traced for up to 25 km in coastal sections of the Purbeck Group in Dorset. These include in ascending stratigraphic order: the Great Dirt Bed, quartz sandstone in the Hard Cockle Member, the Mammal Bed unconformity, the Freshwater Chert Bed, the Cinder Bed, the Scallop Bed and the Broken Shell Limestone (Fig. 1 Dorset Subsidence). Within this context of marked lateral stratigraphic continuity other 100 ka sequences up to 3.5 meters in thickness comprising multiple precessional cycles (20 ka rock cycles or PACs) entirely disappear in lateral distances of less than a kilometer. Two examples of such lateral discontinuity of 100 ka sequences occur in the Mammal Bed 4^th order sequence (400 ka) between the north and south outcrops at Durlston Bay, Dorset. The first example occurs immediately above the Mammal Bed unconformity (a 4^th order sequence boundary). Each complete 4^th order sequence (a product of long eccentricity) contains four 100 ka (short eccentricity) sequences labeled A, B, C and D in stratigraphic order. At the north outcrop (Fig. 2 DB north column) the first or ‘A’ 100 ka sequence is 1.6 meters thick and contains parts of four 20 ka rock cycles. The sea-level low-stand phase of the first 20 ka cycle is the Mammal Bed proper. Less than a kilometer to the south, at a second exposure of this stratigraphic interval in Durlston Bay, this 100 ka sequence is entirely missing (Fig. 3 DB south column, Mammal 4th) and the ‘B’ 100 ka sequence sits on the unconformity. The second example of discontinuity occurs at the top of the Mammal Bed 4^th order sequence. This time in the south outcrop at Durlston Bay the ‘D’ 4^th order sequence is well developed where it comprises three 20 ka sequences and is nearly 2 meters thick (Fig. 4 DB south column, Cinder 4th). This ‘D’ sequence and 2 meters of the overlying ‘A’ sequence of the next 4^th order sequence are nearly completely missing at the north outcrop where a few centimeters of clay (soil?) occurs at the 4^th order boundary. A significant result of applying a genetic hierarchic stratigraphic model lies in the ability to recognize loss of specific allocyclic elements and their time implications.

Milankovitch, cyclic hierarchy, unconformity, pedogenesis, correlation of cycles, pedogenesis