Field Analysis:How Sedimentary and Secondary
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[edit] Sedimentary and Secondary Features
Sedimentary and secondary features provide information about the depositional and burial environment.
[edit] Sedimentary features
Sedimentary features include the finer scale bedding that results from individual depositional events. Obviously the identification and classification of such features depends on the criteria used to define seperate 'contexts'. The distinction between context and bed or laminae is left to the archaeologist, hence sedimentary structure can refer to the internal properties of a single context/bed, or to the presence of multiple beds and/or laminae within a context.
- Beds – Greater than 1 cm thick and may or may not be grouped in bedsets.
- Laminae – Less than 1 cm thick.
Record the presence of sedimentary structures, and the nature of any textural fining sequences associated with them. Also record evidence of disturbance and examine the boundaries of the deposit / context to see if the sedimentary structures faithfully observe this boundary.
Sedimentary structures include:
- Horizontal bedding
- Cross bedding
- Graded bedding
[edit] Secondary features
Secondary features are formed in sediments following deposition. These include:
- Drying (dessication)cracks:
- Liquefaction features:
- Faulting and Microfaulting
- Unconformities
- Burrows
- Frost wedges
Record the presence of secondary structures and the depth within the context where these structures start. Also examine the boundaries of the deposit / context to see if the secodary structures faithfully observe this boundary.
[edit] Way-up Criteria
Way-up criteria are sedimentary and soil features that provide information about whether the deposit is still in its original orientation. These can be very useful for identifying redeposited material where the fabric of the material has been little disturbed, as well as being an indicator of turbation (mixing).
Way-up criteria can include:
- Turf lines – The organic rich A horizon (or turf line), if in-situ, should be underlain by a minerogenic B horizon. The original orientation of turves used in constructions is best analysed under the microscope using soil thin sections.
- Roots and burrows – These should originate from and increase in frequency towards the original surface.
- Horizonation – The sequence of soil horizons should be logical. The organic rich A horizon should be underlain by a minerogenic B horizon. In a podzol, a pale eluvial (albic) horizon should be underlain by a darker iron and/or organic enriched spodic horizon.
- Cross bedding – The upper surface of cross bedded units may be truncated by erosion, and the lower surface of the individual bed should taper outwhere it meets the underlying deposit.
- Graded bedding – Graded bedding should be interpreted with care, usually grain size decreases upwards through a deposit as energy conditions fall. However, in some cases such as debris flows the reverse happens. Also, if previously abandoned channels are reoccupied by flows, coarser lag deposits may overlie finer sands, silts and clays.
- Ice wedges and dessication cracks – Wedges and cracks in the correct orientation are widest at the ground surface, tapering to a point down through the profile.
- Ripple marks – Ripples form either through wind or moving-water deposition. There pointed crests should point up, and their rounded troughs down.
- Scour marks – Scour marks are usually associated with deposition from moving water. They form in the underside of a deposit as casts of depressions formed by erosion of the underlying deposit.
