> Is there a "first bought, first sold" clause in US tax law regarding securities, or is it the average purchase value of your total holdings during a sale that counts?

Neither. It's all specific lots (individual orders). How Coinbase chooses which bitcoin to sell will affect what tax you owe.

With a typical brokerage (e.g. Vanguard), you can configure individual securities to sell FIFO, lowest cost, avgcost, or specific lots. Not sure if Coinbase lets you do that.

Edit: From your example:

> E.g. I buy 10 BTC for $1 and then buy 10 BTC for $50 and then sell 10 BTC for $100:

You could sell the last 10 BTC you bought, in which case you'd owe capital gains tax on the $50 appreciation, or you could sell the first 10 BTC you bought, in which case you'd owe capital gains tax on 9 * $5 + 1 * $9 (= $54, slightly more).