The Ancient Environment and Modern Analogues

 

Sawmill geyser, Yellowstone National Park

Above: Sawmill geyser in eruption, Yellowstone National Park.

 

Introduction

Modern Analogues

 

Introduction

By combining all the aspects of the Rhynie geology from the structural setting and sedimentology to the palaeontology and palaeobotany we can interpret the environment in the Rhynie area some 400 million years ago. As we have seen previously, the cherts and associated sediments were deposited in a narrow northeast-southwest trending basin or half-graben with a hinterland of Dalradian metamorphic and Ordovician basic igneous rocks (see Geology and Setting). The basin sediments and rocks indicate a continental setting; generally an alluvial plain with an axial river system, floodplains and localised ponds and lakes. There appears to have been contemporaneous volcanic activity with the deposition of tuffs and the localised extrusion of andesitic lavas. We know, beyond a doubt, that the chert beds represent the surface expression of an active hydrothermal system which appears to have been fed by conduits along an active fault zone bounding the north western edge of the basin. From the suite of textures and the differing biota within individual chert beds it appears that many different sub-environments were present during the deposition of the chert, but how do we deduce these sub-environments? 

We do not base our conclusions of the fossil environments on observations of the Rhynie chert alone. To help us better understand the palaeoenvironments represented by the cherts at Rhynie, modern analogues or comparative environments are particularly instructive.

 

Modern Analogues

We are fortunate in that there are a number of hydrothermally active areas in virtually all of the worlds volcanic regions that can be studied as possible analogues to the Rhynie chert, though only a few have the necessary requirements for geyser activity. The better known areas where geysers are seen include Rotorua in New Zealand and Yellowstone National Park in the USA. Although the structural and volcanic setting is rather different in both these areas compared with that of Rhynie, not to mention the differences in contemporary biotas, in terms of the actual surface processes and sub-environments Rotorua and particularly Yellowstone provide quite good modern analogues. By studying the sub-environments associated with these areas, the type of biota and the sediments and their textures, we can compare and contrast with similar features seen in the Rhynie chert and thus begin to interpret the ancient palaeoenvironments (e.g. Trewin et al. 2003).

For the purposes of this resource, we can look at these modern analogues in two main ways; in terms of 'large-scale features' such as the spatial arrangements of biota, their associations and style of preservation with respect to the hot springs and their deposits, and also in terms of 'micro-features' such as the textures formed in the modern sinters as observed under a microscope: