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Investigations of graptolite argillite and phosphorite in 2020

29. July 2020 - 13:16
rock samples
Graptolite argillite and phosphorite (a collage of samples photographed by M. Isakar and T. Saadre)

The Estonian Parliament adopted on 6 June, 2017 the strategy „General principles of Earth’s crust policy until 2050 “, which sets as one of the priority objectives maintaining the Estonian geological information up-to-date. A problem in Estonia is the outdated dataset regarding deposition and geochemistry of graptolite argillite (found within the Türisalu Formation) and phosphorite (found within the Kallavere Formation). Earlier studies have been carried out more than 30 years ago. The level of accuracy of laboratory analyses during those times is unknown and would require controlling with studies performed using modern laboratory equipment.

Vast majority of drill cores stored in Estonia consist of carbonate rocks. Graptolite argillite and phosphorite strata have been preserved only in a few drill cores. They have been weathered to the extent that e.g. geochemical studies of the phosphorite strata would not be adequate. A large proportion of rocks from the Türisalu and Kallavere Formations that were acquired during earlier exploration and research were used up for carrying out chemical analyses. However, reports dealing with old analysis results do not describe the existence microcomponents (above all rare earth elements) within the phosphorite strata. It needs also to be taken account that the properties of materials staying in storage alter through time — certain minerals weather and re-crystallize, leading to the situation that they cannot be used for complementary studies any more.

Information from 16 earlier drill cores and other cross-section parameters will be verified within the framework of the permits for the studies „geochemical investigation of the Obolus sandstone of the Kallavere Formation in Virumaa“ and „geochemical investigation of graptolite argillite in Virumaa“, issued by the Estonian Environmental Board. The information acquired during the studies in 1979–1989 will be controlled and the geochemical dataset will be essentially extended. The objective of the drilling is to penetrate through the Obolus sandstone beds of the Kallavere Formation, and to get drill cores with maximum output from these deposits and the overburden that lies immediately above. As drilling a borehole is expensive, the requirement is to get as much information as possible from every borehole. Together with information of geological resources, hydrogeological information is desired.

The locations of the boreholes of these investigations have been chosen in such a manner that the impact of the operations on both the environment and the population would be minimal. The drilling operations will take place during the period from 27 July to 15 October. They will be carried out using modern methods, taking into account the specific character of geology in these regions, and without damage to the environment. Most of the boreholes will be closed after drilling and the sites will be tidied. A selection of boreholes will remain open in an appropriate manner for the purposes of long-term hydrogeological investigation.
The drill cores will be delivered to the Arbavere research centre, where the geologists of the Geological Survey of Estonia and other experts can start carrying out geophysical and geochemical investigations.

Altogether, the planned investigations have three principal objectives: 1) to control the earlier research dataset, 2) preparatory work for presenting the Soviet-era research database and information from studies of the new drill cores following internationally accepted research standards, 3) to gather information of the existence of rare and dispersed metals in phosphorite and its overburden. As a result, we wish to reach the situation where the state of Estonia is as precisely as possible informed of its underground resources, together with the opportunities and risks related to them.

Locations of the exploration sites.