All our county owned parks (Harris County) have prohibited metal detecting. Any city owned parks, are ok to detect in.
About 7 years ago the Houston area club, had full permission from county park officials, to use a county park for group hunts. They had done so for at least 10 or 12 years before that with no problems. A THC employee who happened to see them in the park, didn't like the fact that they were digging (which in fact were seeded coins for a group hunt)...it was said they were digging up trinkets.
The THC employee got with the county park officials,who then got with their lawyers, and a law banning detectors was made. The main reason was,they said that the Indians had used the area way back when, and the THC didn't want metal detector users digging up what they called trinkets.
The stone arrow points and pottery shards that are common in our area are non metallic and couldn't be detected by a metal detector. No metal items have been dug by the archaeologists, in these areas where Indians are have known to have been. And they have done numerous digs.